20111013 Ver 1_Public Comments_20120113

I- 1013
Mcmillan, Ian
From Heather [nverkeeper @ptrf org]
Sent Friday January 13 2012 12 40 PM
To Mcmillan Ian
Cc Chapman Amy
Subject Martin Marietta
Attachments Comments on 401 for Martin Marietta docx
Hey guys
I ve attached some rough points regarding issues with Martin Marietta proposed mine As you 11 see in the opening
paragraph these comments focus more on what will be assessed re WQC but should also be included in the Corps EA
Not sure how helpful these will be I apologize I do not have anything yet in a more final format
111 be finalizing comments next week and will send those to you
Thanks
Heather Jacobs Deck
Pamlico Tar RIVERKEEPER
Pamlico Tar River Foundation
P 0 Box 1854
Washington NC 27889
[phone] (office)
[phone] (fax)
[phone] (cell)
www ptrf org
Follow us on Facebook http //www facebook com /pamlicotar
Follow us at Twitter www twitter com /ptrfriverkeeper

Comments on 401 for Martin Marietta draft
I am basing these comments on my belief that the 401 WQC must also address the mine dewatering
impact since the mine cannot function independently from the dewatering discharge (not as they have
proposed) and may impact downstream WQS I have little to comment on regarding the direct impact
to 404 wetlands other than 11 mitigation is not sufficient I believe you all have already commented on
the need for sufficient hydrology to ensure adjacent and downstream wetland functions are maintained
so I do not include any of those comments here even though I too will raise that issue But I will be
including all of these comments as well to the Corps for their consideration in their EA
Concerns regarding CZR habitat assessment
Have you all had conversations with the company regarding the pH of the discharge water?
Blounts creek is naturally 4 5 5 at most The NPDES application included quarry water discharge
data and the pH was 7 4 (the source of the sample is not provided other than information that
it is quarry discharge water of a similar mining operation )
Comments from an ECU researcher Dr David Kimmel ECU regarding habitat analysis (comments
in blue)
• What is largely missing is some context of these findings when compared to other
similar streams This is particularly true for the fish The macro invertebrate data is
compared to some state standards but the Biotic Integrity measures are often designed
for a single system and broadly applied
• Regarding the Jaccard index and the Morisita Horn indices both are fairly standard
techniques to measure similarity It might be useful to measure a stream that is similar
but outside of the potential impact range to give a taste of what some of the difference
outside of the system might be For example these systems may already be degraded in
some way or may be exceptional habitat Usually the state has some definition of
minimal quality in relate to some measurements
• The lower natural pH for coastal streams will often predispose the taxa towards lower
diversity
• With regards to this statement in the analysis The Jaccard index indicated that
although UT2 had the most species in common with Blounts Creek ( 0 75) the Morisita
Horn index indicated that UT2 was more similar to UT1 in terms of community overlap
(0 79) (Table 4) on page 8 these comparisons are probably meaningless unless
compared to some known distribution or diversity of these streams
• With regards to the statement in 4 2 fish in page 14 Overall both species richness and
total abundance were relatively low for both impact and control monitoring locations
Compared to what? It is unclear here what is meant by low it has to be qualified by comparison to some
standard or other stream These numbers may appear low but may be natural for these systems
Also the fish numbers reported are absolute numbers and do not incorporate the effort it took to
collect them For example you could collect 9 fish in a few minutes in a healthy system and collected
the same amount over 5 days in an impacted system Need some CPUE analysis

Additional concerns
o 1 day of sampling does not provide sufficient information on downstream impacts In
personal communication with Dr Anthony Overton ECU sampling for young of year
was conducted too early and should have been conducted in June or July
o Seasonal sampling required
o Little to no analysis on downstream impacts from dewatering discharge re flow
chemical changes salinity etc
Concerns regarding hydraulic assessment of receiving streams
These concerns are focused on the discharge point and impacts downstream to the UTs and
blounts creek
From Dr Scott Lecce (ECU)
For 3 of the 4 channels the addition of the 18 6 cfs would produce flows that equal or exceed
the bankfull discharge which could lead to channel instability in the form of widening or
incision This could produce knickpoints that migrate upstream Although the stream power
and shear stress values estimated in their scenarios appear to be fairly low I would also be
concerned that substantially increasing the discharges experienced by these streams even to
flows less than bankfull have the potential to produce channel adjustments I assume these
elevated discharges would be experienced continuously and this could impact vegetation and
the stability of channel banks Bankfull flows normally experienced by these streams may occur
several times each year but only for a few days in duration not continuously throughout the
year The report also does not comment on what happens during storm events when stormflow
is also conveyed by the channel in addition to baseflow and the 18 6 cfs? I assume there would
be some restriction on discharging the 18 6 cfs during such periods
From Dr Mike 0 Driscoll
what could happen if the UT2 segments were to receive too much discharge they may respond
by widening and deepening (especially if new sediment inputs do not increase with the
discharge increases) Usually if the deepening occurs then the slope will increase and a
knickpoint will develop that would result in erosion that occurs and the knickpoint would
migrate upstream just like a waterfall moves upstream over geologic time This has happened a
bit in Greenville where stormwater outfalls have deepened and widened some channels Some
of the literature seems to suggest that these disturbances may take a long time to work through
the system and so it could be decades before the stream channel and its sediment and discharge
regime re equilibrate
One thing that is a bit inaccurate in the report is that they say bankfull flooding should occur
around every 11 13 years those numbers are for streams in Piedmont and Mountain settings
and are too high for the Coastal Plain There is this paper by Sweet and Geratz where they say
that bankfull flooding for similar NC Coastal Plain streams occurs several times a year Also if

streams are channelized they may be able to handle more water than a similar natural channel
but this might put the stream segments further downstream at a greater flood risk since the
floodplain is not really functioning along the channelized reaches If channels are channelized
they may have steeper banks than is normal and if greater discharge is added those banks may
have a tendency to slump and the bank material may add sediment to downstream segments
Additional concerns
• no mention in the application how downstream stability is to be monitored or what
corrective actions would take place should erosion and sedimentation occur if
permitted
• Will a control creek be utilized for monitoring?
• No mention of bank stability downstream impacts water quality standards during
storm events and normal bankfull flows and the added discharge
Impacts to WQ standards and downstream habitat
Impacts of FW discharge and flow changes to brackish system and aquatic habitat Not
evaluated
Impacts of combined groundwater /stormwater discharge including turbidity TSS pH changes
metals other pollutants Not evaluated
MM analysis fails to evaluate impacts beyond the creeks headwaters
What is the design of the treatment systems?
If permitted how does MM plan to mitigate for habitat loss and the loss of other existing uses?
Alternatives
as we know the requirement is to avoid then minimize Avoidance and minimization must also
include the downstream impacts due to the mine dewatering discharge
Alternative analysis should include avoidance or minimization of the discharge via possible
alternatives that may include
o Depressurization wells that could then re infect all or a portion of the groundwater to
avoid or minimize impacts via wastewater discharge and groundwater withdrawal and
drawdown (impacting adjacent private wells and water supply) —this would have to be
wells of only castle hayne water as infection of wastewater is not allowable under NC
rules and statutes This would be an alternative to pumping from the open mine pit
■ This should have been included as well in their engineering analysis for the
NPDES permit application which it was not
• Connection to local water supply
• Other alternatives that would avoid or minimize the discharge of wastewater to Blounts
Creek (or other creek systems)

HYDROLOGICAL PROCESSES
H}drol Process 14 2991 -3010 (2000)
NO kF=@R016P=P
JAN 1 3 2012
QENR WATER QUALITY
ANDS AND STMMMATER BRANCH
Hydrology, geomorphology and vegetation of Coastal
Plain rivers in the south- eastern USA
Cliff R Hupp*
US Geological Surve} Reston Virginia 20192 USA
Abstract
Rivers of the coastal plain of the south eastern USA are characteristically low gradient meandering systems that
develop broad floodplains subjected to frequent and prolonged flooding These floodplains support a relatively
unique forested wetland (bottomland hardwoods) which have received considerable ecological study but
distinctly less hydrogeomorphological study The hydroperiod or annual peno&of inundation largely controls
the development of characteristic fluvial landforms sediment deposition and vegetation distribution patterns
Order of magnitude differences in wetted perimeter width /depth suspended sediment load and hydraulic
roughness may exist between dry in channel seasons and the hydroperiod Substantial sediment (and adsorbed
contaminants) retention and storage through lateral and vertical accretion is common (where not heavily impacted
by flow regulation) along these Coastal Plain rivers The present chapter summarizes our current understanding of
the hydrology fluvial geomorphology general and local sedimentation patterns and related plant ecological
patterns of these Coastal Plain bottomlands
KEY WORDS coastal plain meandering rivers fluvial geomorphology fluvial landforms sediment deposition
woody vegetation
INTRODUCTION
The Coastal Plain physiographic province of the USA (Figure 1) lies almost entirely in the south east It
covers an area of about 1 2 million square kilometres (slightly larger than the combined area of Belgium
France Germany and the UK) This topographically distinct lowland is bounded on the east and south by
the Atlantic Ocean and the Gulf of Mexico respectively and landward on the west and north at some places
less distinctly by the Piedmont and Ozark highlands (Figure 1) Bottomlands typically are broad alluvial
features with low gradients meandering streams most of which terminate downstream in tidal estuaries
These Coastal Plain river systems have received noticeably less hydrological study than higher gradient
Piedmont and montane river systems where such concepts as bankfull discharge and flood return interval
were developed (Leopold et at 1964) The floodplains of Coastal Plain rivers are typically inundated every
year for prolonged (months in some cases) periods The forests (bottomland hardwood systems including
southern deep water swamps) however have received considerable ecological study yet the linkages
between the fluvial geomorphological processes and forest ecology remain poorly understood
Fluvial geomorphology refers to study of the surficial landscape geomorphological forms and physical
processes developed or mediated by the action of flowing water most typically in the form of streamflow
Most geomorphological work in fluvial systems involves the erosion entrainment transport deposition and
storage of sediment At least 90% of all sediment eroded from uplands is trapped in alluvial systems before
reaching saltwater (Meade et at 1990) Detailed spatial and historical analyses of sediment trapping storage
Correspondence to C R Hupp US Geological Survey Reston Virginia 20192 USA
This article is a US government work and is in the public domain in the United States Received 4 June 1999
Accepted 31 August 1999

2992
Fall
Line
r—T�
C R HUPP
Embayed
Section
Atlantic
Coastal Plain
Atlantic Ocean
Figure I The coastal plain of south eastern USA Potential extent of bottomland hardwood forest is shown along major streams Note
that the BLH forests nearly match the inland extent of the Coastal Plain delineated by the Fall Line
and retention time at both large and small scales generally are lacking possibly with the exception of the
lower Mississippi Valley (Saucier 1994) Alluvial processes create and maintain a variety of landforms
including floodplains that support bottomland hardwood (BLH) forest ecosystems in the south eastern
USA These forests interact with hydrological and fluvial geomorphological processes and forms (including
binding alluvium through root development and enhancing deposition by increasing stream roughness) such
that the intrinsic character of each is at least partly the result of the other The purpose of the present paper is
to provide an overview of our current understanding of the form and process linkage between lowland
meandering streams on the Coastal Plain of the south eastern USA and their characteristic BLH forests
Several definitions of BLH have been published recently (Wharton et al 1982 Mitsch and Gosselink
1993 Sharitz and Mitsch 1993 Shepard et al 1998) All definitions confine BLH to the riparian zone thus
associating BLH systems with the bottomland adjacent to streams The riparian zone can be defined as that
part of the landscape supported by and including recent fluvial landforms and inundated or saturated by the
bankfull discharge (Hupp and Osterkamp 1996) Although this definition is not as inclusive of bottomland
features as others (e g Malanson 1993) it is quantitative in that the bankfull discharge typically occurs at
least once every 1 to 3 years (Leopold et al 1964) Bottomland hardwood systems are usually considered
forested wetlands many definitions also require inundation or saturation of the soil at least once annually
during the growing season (Mitsch and Gosselink 1993) The riparian zone in particular the floodplam
reaches its greatest development in the USA along the many low gradient rivers originating on or flowing
across the Coastal Plain physiographic province Thus BLH systems are in large part an extensive and
characteristic feature of south eastern nvers (Figure 1) Here and throughout the text the term vegetation
refers to the woody vegetation of BLH systems it is beyond the scope of the present paper to discuss
herbaceous vegetation although many of the interpretations would apply as well
Since the late 1970s there has been considerable focus on the function and value of wetlands (Greeson et
al 1979 Carter 1986 1996 Landin 1992) summarized recently by Brinson and Rheinhardt (1998)
Riparian wetlands (including BLH) have been cited to be of particular value for several reasons
Biologically the riparian zone is the ecosystem with the greatest biodiversity in most regions of the world
(Nilsson 1992 Naiman et al 1993) Riparian areas also provide critical habitat for many plants and game
and non game species of fish and wildlife Environmentally these wetlands function as an important if not
Hidrol Process 14 2991 -3010 (2000)

COASTAL PLAIN RIVERS 2993
critical natural clement in the maintenance of water quality Properly functioning BLH systems annually
trap and store enormous amounts of sediment nutrients and contaminants (Hupp et al 1993 Kleiss 1996)
Unfortunately BLH systems have decreased tremendously in areal extent principally through conversion
to agriculture (Mitsch and Gosselink 1993 Kress et al 1996) following World War 11 Missouri Arkansas
and Louisiana alone lost 21 000 57 000 and 24 000 ha respectively between 1960 and 1975 (Turner et al
198 1) Their continuing rapid loss is of imminent concern because of concomitant losses of water quality and
habitat functions (Sharitz and Mitsch 1998) In addition the majority of remaining BLH forests have been
modified as a result of lumbering or agricultural usage and /or more recent activities such as highway
construction and channelization (Bazemore et al 1991 Hupp 1992 Lockaby and Walbridge 1998) Many
of these forests particularly those near the Fall Line (Figure l) along the Atlantic Coastal Plain experienced
substantial aggradation in the eighteenth and nineteenth centuries following deforestation and poor
agricultural practices by European settlement (Wolman 1967 Costa 1975 Jacobson and Coleman 1986)
Subsequent reforestation and better agricultural practices substantially reduced sediment loads which has
sled to channel incision leaving floodplains and terraces relatively high and dry along many east coast
streams Heightened post settlement deposition has been documented on at least two Coastal Plain streams
namely along the Congaree River in South Carolina (Patterson et al 1985) and along the Roanoke River in
North Carolina (Hupp et al 1999a) The lower Roanoke River aggraded by as much as 4 in near the Fall
Line this deposit attenuates downstream toward the estuary but nevertheless affects channel depths point
bar development levee formation and vegetation patterns (Hupp et al 1999b)
GEOLOGICAL CONTROLS
The Coastal Plain (including the subaqueous continental shelf) from the Chesapeake Bay to eastern Texas
(Figure 1) is largely the result of sediment deposition both alluvial and marine from the adjacent eroding
mountains and Piedmont since at least the Cretaceous Period This broad plain has been sculpted by
hydrological and fluvial geomorphological processes that vary in their effect in response to changes in sea
level and climate During oceanic regressions (sea level retreat to a low stand) streams on the coastal plain
and adjacent Piedmont tended to entrench into their valley (degradation) owing to higher stream gradients
Oceanic transgressions (sea level rise to a high stand) with lower stream gradients typically led to valley
filling and widening (aggradation) Coupling this rather simple conceptual model with concomitant
variation in climate and tectonics has led to a deceptively complex modern landscape For instance many of
the rivers on the Atlantic Coastal Plain are underfit that is the present channel does not carry sufficient
discharge or sediment to have created the broad alluvial floodplain within which the river now flows This
underfit condition may result from stream capture (loss of drainage area) or perhaps more typically from a
reduction in rainfall Rainfall during a pluvial period 18 000 to 10 000 years ago may have been 18 times
greater than at present (Dury 1977) Melting continental glaciers undoubtedly increased discharge and
sediment load on some of the larger rivers Further Pleistocene frost action and variation in plant cover
probably made available considerable amounts of sediment for transport and deposition
The modern Coastal Plain bottomlands of most south eastern rivers are largely a result of fluvial processes
during the last low stand (about 14 000 to 18 000 years ago) and the subsequent oceanic transgression toward
a new high stand The lower Mississippi River incised into its bottomlands (the largest contiguous area of
BLH) during the last low stand upstream to about the vicinity of Vicksburg Mississippi This relatively
short distance is probably due in part to a concomitant lengthening of the channel (Saucier 1994) With
rising sea level over the past several thousand years the bottomlands rapidly filled widened and formed
large anabranches associated with deltaic processes forming the aggrading Atchafalaya Basin Smaller rivers
of the Gulf Coastal Plain probably did not incise deeply as a result of a high sediment load and relatively low
discharges during the past low stand (Saucier 1994) Many of these streams such as those on the Atlantic
Coastal Plain are now underfit following a pluvial period (18 000 to 10 000 years ago Dury 1977) and
subsequent oceanic transgression and aggradation Additionally several stream captures may have occurred
Hjdrol Process 14 2991 -3010 (2000)

2994 C R HUPP
including the Cache River AR (Bennett and Saucicr 1988) which contributed to the commonly underfit
nature of many BLH systems Similar processes affected streams of the Atlantic Coastal Plain excepting
major tributaries of the Chesapeake Bay All large tributaries of the Chesapeake Bay that arise in the
Appalachian Mountains incised to the Fall Line and remain in an embayed condition today possibly due
largely to the Bay s principal tributary the Susquehanna River The extremely high discharge and ocean
proximity of the Susquehanna River may have lowered the regional base level to such a degree during the last
glacial retreat that aggradation in the other major Bay tributaries has not kept pace with rising sea level
However streams of the region that arise on the piedmont and coastal plain but with substantially less
discharge and erosive capacity than the Susquehanna River did not incise like the major tributaries during
the last low stand and now support BLH systems on relatively broad bottomlands
HYDROLOGIC PROCESSES
Water is of singular importance in BLH systems as in all wetland systems as the medium for biogeochemical
processes (in living and physical systems) as an ecological limiting factor and as the force that controls the
movement and storage of sediment and associated material All wetland functions can be described entirely
or in part by hydrological processes (Nestler and Long 1994) and agreement on the primary influence of
hydrology in wetland ecosystems is pervasive in wetland hydrological literature (Carter 1986 1996 Walton
et a! 1996) Yet our basic understanding of most hydrological processes remains somewhat tenuous owing
in part to the relatively recent emergence of hydrology as a science and because most hydrological processes
are subject to the vagaries of precipitation and climate Slight hydrological changes may result in only loosely
predictable yet often substantial wetland responses (Mitsch and Gosselink 1993) Defining the relation
among hydrological processes and other wetland phenomena in BLH systems is further hampered by the
lack of more than a few rigorous studies in this wetland type (Walton et a! 1996)
A strong seasonal variation in discharge occurs along most medium and large streams on the south
eastern Coastal Plain (Figure 2) Annual variation in evapotranspiration and rainfall produce two distinctly
different hydrological seasons a low flow season from about June through to October when streamflow is
confined largely to a meandering main channel and a high flow season from about November through to
May when large parts of the wooded bottomland may be inundated This period of inundation is referred to
70
60
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P so
a�
20
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FlooValn Inundates!
-- - - - - - -- ---- - - ----
4�
01 ----- Sloughs flooded
— — — ----- — —�
OCT NOV DEC JAN FM MAR APR MAY JUN JUL AUG SEP
MONTH
Figure 2 Mean monthly discharge (1963 to 1977) for the Cache River near Patterson Arkansas The discharge necessary to flood
sloughs (8 4 m3 /s) and to inundate the floodplam (52 5 m3 /s) is indicated Floodplams along this river are typically inundated from late
December until mid April
Hj drot Process 14 2991 3010 (2000)

COASTAL PLAIN RIVERS
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Hidro! Process 14 2991 -3010 (2000)

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COASTAL PLAIN RIVERS 2997
as the hydroperiod (Figure 2) Order of magnitude differences in wetted perimeter width /depth ratio and
roughness may occur seasonally along the same reach This bimodal hydrology leaves an indelible signature
on the biotic and abiotic landscape (Figure 3A and B) and complicates environmental interpretations
Two mayor types of streams classified according to suspended sediment load form the bottomlands that
support BLH systems They are
(1) alluvial rivers that arise in uplands (typically mountainous areas or the Piedmont) and transport
substantial amounts of eroded mineral sediment
(2) black water rivers that arise on the Coastal Plain typically with low gradients that transport relatively
little mineral fines (Table I)
Alluvial rivers can be subdivided further into brown water systems and red water systems The former
usually are large systems with initially high gradients arising in the mountains whereas the latter tend to be
smaller lower gradient systems that arise in the piedmont and derive their red colour from iron oxides that
characterize the Piedmont residuum Black water systems tend to be smaller than either brown or red water
systems principally as a result of their limited potential to develop large watersheds Water in black water
systems with little gradient flows relatively slowly and has limited ability to erode sediment This slow
moving water leaches tannins from the typically highly organic bottomlands or riparian wetlands which
stains the water and lowers the pH ( from 7 to 6 Wharton et al 1982) relative to alluvial systems However
drainage may affect the mineral content of black water streams such that better drained systems develop soils
with a relatively high mineral content The origin of streams flowing on the Coastal Plain also affects the
dissolved load (chemical characteristics) Alluvial rivers have relatively high concentrations of inorganic
ions including several macronutrients and relatively low concentrations of total organic carbon (Table I)
the converse is true for most black water rivers (Wharton and Brinson 1979) Thus pH hardness and
specific conductance tend to be higher in alluvial rivers than in black water streams
Alluvial rivers develop a fairly abrupt reduction in gradient after crossing the Fall Line and contacting the
relatively flat Coastal Plain Coincident with reduction in gradient are greater frequencies of overbank flows
a flatter hydrograph and longer periods of inundation These tendencies are partly due to the relict nature of
the Coastal Plain and the stream regime shift without a reduction in discharge from high energy at least
partly bedrock controlled relatively straight upland reaches to low energy meandering reaches The broad
bottomlands of the Coastal Plain often do not fit empirical hydrological concepts such as bankfull discharge
(Leopold et al 1964) developed for upland streams These floodplains tend to be flooded much more
frequently than every year and a half and for much longer durations During leaf off seasons with high
discharges and little transpiration some BLH systems are regularly inundated for months each year
effectively increasing channel width as much as an order of magnitude during the hydroperiod Streamflow
during the hydroperiod is less meandering and in intimate contact with the riparian zone that supports BLH
forests Many functional attributes of BLH systems including sediment and associated material trapping
(Figure 3) are most prominent during the hydroperiod Unfortunately anthropogenic features such as
nineteenth century agricultural levees may reduce the effective floodplain surface area and reduce residence
time of out of bank flow by increasing velocities
GEOMORPHOLOGICAL FEATURES
Fluvial geomorphological processes create a variety of widely recognized landforms from small channel
bedforms to extensive floodplains The latter typically support BLH systems in the south eastern Coastal
Plain Floodplains like most fluvial landforms are dynamic features almost constantly eroding in some
places while aggrading in others Meandering channel dynamics (typical of most streams in BLH systems)
provide the energy necessary to erode and transport floodplain sediments Meanders typically extend
eroding accreted sediments until they are cut off by an avulsion (channel cut off) leaving an ox bow lake and
a new channel (Figure 4) Entire meander loops additionally tend to migrate downstream Thus over
H}drot Process 14 2991 -3010 (2000)

2998
C R HUPP
P Mbar
y
Internal
Craves" lay
Drainage
Cie plug
11 topography
(r arW swells)
$
H
�
p
i
can
bank
Oxbow
lake slab
F1ovd
i�
vew
x
Figure 4 Generalized fluvial landforms on a coastal plain bottomland Note greater levee development along straight reaches and on
the downvalley side of the stream
geomorphological time nearly all alluvium in BLH systems is in a state of flux even though the transport
distance for eroded alluvium may not extend much past one meander loop downstream (Saucier 1994)
Floodplains on the Coastal Plain tend to have net sediment storage during periods of high or rising sea
level such as the conditions over the past several thousand years Floodplains aggrade in two ways First by
lateral accretion or point bar extension where coarse (sand) material is deposited on the inside bank of
channel bends a corresponding volume is typically eroded on the opposite or cut bank (Figure 4) Second
by the vertical accretion of suspended sediment (typically fines) over the floodplain during overbank flows
Lateral accretion is an episodic process that occurs during high flows building the point bar into a typically
crescent shaped ridge Over time a series of high flow events produce the ridge and swale topography
Transport of sediments and contaminants
Water level from river channel to floodplain
during
hydroperiod e ti y
t s
--- ---- --1 ---------- - L
-- --s ---
Flood al Levee Flooding Vertical accretion
Erosion of
gRbanks Lateral accretion
on point bar
Water level
during low
now
Figure 5 Transport paths of sediment and associated contaminants for both lateral and vertical accretion
Hydrol Process 14 2991 3010 (2000)

COASTAL PLAIN RIVERS 2999
(Figure 4) associated with meander scrolls The establishment of ruderal woody vegetation during
Intervening low flow periods on fresh scroll surfaces creates bands of Increasingly younger vegetation toward
the main channel (McKenney et at 1995) These bands of vegetation may accentuate the ridge and swale
topography by creating distinct microdepositional environments during subsequent high flows but the
hydraulics necessary to produce meander scroll topography and the role of vegetation In Its development are
poorly understood (Nanson 1980 198 1)
Fine sediment deposition Is facilitated by the typically striking reduction in flow velocity as water leaves
the main channel and enters the hydraulically rough floodplain environment (Figure 5) As rising flood
waters overtop the bank the coarse (or heavy) sediment is deposited first and relatively rapidly create natural
levees along the floodplain margin Levees tend to be most pronounced along relatively straight reaches
between meanders and are often the highest ground on the floodplain Levees are sometimes breached by
streamflow resulting In a crevasse splay that may Insert coarse material deep into the otherwise fine grained
floodplain (Figure 4) Levee development and the breaches that form are poorly documented In the
literature yet are critical in the understanding of the surface water hydrology of most coastal plain
bottomlands Levee height and breaches strongly affect the hydroperiod (and thus sedimentation dynamics)
In systems dominated by surface water flow (Patterson et at 1985) The levee surface usually dips gently
away from the channel into the bottomland where the surface may be extremely flat Superimposed on this
flat bottom are Internal drainage networks overflow channels and abandoned main channels or ox bows
that remain wet after the hydroperiod and support the more hydric BLH species Slight differences in
elevation associated with the above floodplain geomorphological features and large woody debris (LWD)
create a complex pattern of microsite velocity regimes during the hydroperiod that ultimately affect intrasite
sedimentation regimes Also highly correlated with these variations in elevation are the distributional
patterns of many BLH plant species (Wharton et at 1982 Sharitz and Mitsch 1993)
WATER QUALITY
Sediment trapping
Suspended sediment may be the most Important water quality concern In the USA today (US EPA 1994)
Increases in suspended sediment directly affects aquatic biota by coating vegetation and clogging gills of
Invertebrates and fishes Indirectly increased suspended sediment changes the habitat from more coarsely
grained aquatic environments to highly silted environments Further increased suspended sediment may
lead to high sediment deposition rates in critical riparian areas thus damaging living resources through
burial and suffocation Perhaps most importantly suspended sediment is the transport medium for
hydrophobic forms of nutrients and pesticides and most trace elements (Horowitz 1991) Mean values of
suspended sediment percentage of suspended sediment finer than 0 062 mm (clay) and dissolved and total
phosphorus nitrogen and carbon are provided in Table I for a selected set of Coastal Plain streams that
support BLH systems along their lower reaches (Alexander et at 1998)
Deposits of fine sediment typically contain large concentrations of adsorbed associated contaminants
(particularly nutrients trace elements and hydrophobic pesticides) from agriculture and urban areas
(Johnston et at 1984 White and Tittlebaum 1985 Phillips 1989a Puckett et at 1993) This sediment and
contaminant trapping function of wetlands is commonly acknowledged (Kadlec and Kadlec 1979
Lowrance et at 1984 Phillips 1989b Brinson 1993 Hupp et at 1993 Lowrance et at 1995 Brinson et at
1995 Kleiss 1996) despite limited understanding of the transport and deposition of sediment and
associated contaminants and the lack of consistent mass balance studies (Boto and Patrick 1979 Winter
1981 Carter 1986 Labaugh 1986 Mitsch and Gosselink 1993) Biogeochemical cycles within forested
wetlands are particularly complex and difficult to study (Walbridge and Lockaby 1994 Lockaby and
Walbridge 1998) Although alluvial material may be considered in transit over geomorphological time most
bottomlands especially those on the Coastal Plain exhibit net aggradation through sediment deposition
Hid of Process 14 2991 -3010 (2000)

3000 C R HUPP
Table 11 Mean sediment deposition rates (mm /year) for Coastal Plain rivers data from dendrogeomorphological
analyses The Cache River was investigated twice in different studies and locations
River
Type
Rate
Reference
Hatchie TN
Alluvial
54
Bazemore et al 1991
Forked Deer TN
Alluvial
3 5
Bazemore et al 1991
Chicahommy VA
Alluvial
30
Hupp et al 1993
Obion TN
Alluvial
30
Bazemore et al 1991
Patuxent MD
Alluvial
29
Schening et al 1999
Cache AR
Alluvial
2 7
Hupp and Schening 1997
Roanoke NC
Alluvial
23
Hupp et al 1999
Cache AR
Alluvial
1 8
Hupp and Morris 1990
Wolf TN
Alluvial
1 8
Bazemore et al 1991
Mattaponi /Pamunkey VA
Alluvial
1 7
Schening et al 1999
Coosawhatchie SC
Blackwater
1 6
Hupp and Schening 1997
Choptank MD
Blackwater
1 5
Schening et al 1999
Pocomoke MD
Blackwater
1 5
Hupp et al 1999
from two initially distinct sources (t) runoff from adjacent uplands (riparian buffer) and (it) streamflow
during inundation of bottomlands (riparian retention) Although the former has received some study
(Brockway 1977 Karr and Schlosser 1978 Peterjohn and Correll 1984 Correll 1986 Johnston et al 1984
Lowrance et al 1984 1986 1998) the latter has received far less (Kletss et al 1989 Hupp and Morris 1990
Hupp and Bazemore 1993 Hupp et al 1993 Kletss 1996)
Geomorphological analyses (Leopold et al 1964 Jacobson and Coleman 1986) verify that riparian
retention of sediment is a common and important fluvial process yet retention time of sediment may be the
most poorly understood generally unquantified aspect of sediment budgets (R B Jacobson written
communication 1996) As of the early 1990s only four published accounts of vertical accretion rates or mass
accumulation for mineral fines in the USA could be located by Johnston (1991) for any type of wetland
Since then vertical accretion rates have been reported for BLH systems in West Tennessee eastern Arkansas
South Carolina North Carolina and along tributaries to the Chesapeake Bay in Maryland and Virginia
07
0.6
Q T OS
p , 0.4
a�0.9
to V
2
01
Hatchie Fiver
Appro3dmete Um of y
initlal charowliratlon
or, all
1
Big Sandy Fiver
1990- 1890- 19M. 1910- 19206 1930• 19W 1950- 1960- 1979
1889 1889 1909 1019 1929 1939 1949 1959 19M 1979
AGE GROUP
Figure 6 Sedimentation rates by age class of trees sampled for rate determination along the Hatchie and Big Sandy Rivers Tennessee
Sedimentation rates are consistently higher along the unchannehzed Hatchie River note sharp increase in deposition beginning around
1950
H} drol Process 14 2991 3010 (2000)

COASTAL PLAIN RIVERS 3001
(Table II) In a rare rairly exhaustive BLH sediment retention study Kleiss (1996) repo ted that the Cache
River Arkansas carries more than 90% of its total annual sediment load during the high flow period and
that more than 14 A (about 800 g /m' /year) of the load is trapped along a 2 -3 km wide 49 river km long
reach The results of the Cache River study (Kletss 1996) support current efforts to rehabilitate /restore BLH
forests In addition to sediment trapping restored systems also improve the water quality of adjacent river
reaches and reduce the sediment burden on existing downstream BLH systems (Kletss 1996) The sediment
and contaminant trapping function in Coastal Plain fluvial systems is especially important because these
floodplam surfaces are the last areas for sediment storage (and biogeochemical cycling) before entering
estuaries and their critical nurseries for marine biological production
Some of the highest concentrations of suspended sediment occur in the Mississippi Embayment because of
channel instability highly erodible uplands ( fine alluvium and loess) and extensive agriculture (Trimble and
Carey 1984 Simon and Hupp 1987) Many streams of this region particularly in West Tennessee south
eastern Missouri and northern Mississippi have been channelized which has led to severe upstream channel
incision with concomitant channel erosion (Simon and Hupp 1987 Simon et al 1996) Channel incision
facilitates runoff and peak flows which reduce the hydroperiod and trapping function in BLH systems that
have been channelized (Hupp in press) ironically in a region where the payoff in water quality benefits of
intact functioning BLH systems would be great A comparison between the unchannelized Hatchie River
and the channelized Big Sandy River in West Tennessee (Hupp and Bazemore 1993) showed that sediment
deposition rates were significantly higher on the unchannelized stream as far back as the time of initial
channelization (Figure 6) particularly after large expanses of the basins and bottomlands were cleared for
agriculture after World War lI (Figure 6)
Like intensive agricultural areas urbanizing areas tend to generate high suspended sediment and
contaminant loads particularly trace elements (White and Tittlebaum 1985) Both deposited and suspended
sediments contain significantly higher concentrations of most trace elements and hydrophobic contaminants
than are dissolved in water (Horowitz 1991) Thus patterns of sediment transport and deposition largely
control the fluvial deposition of most trace elements and many contaminants Water quality concerns in
south eastern Virginia along the Chickahominy River which arises in the urbanizing Richmond area are
high because the river is the source of a water supply reservoir for the densely populated Hampton Roads
area A study of sediment and trace element trapping along Coastal Plain reaches of the Chickahominy
(Hupp et al 1993) has shown that large amounts of sediment and associated trace elements are trapped in
the adjacent BLH system (Table III) Additionally changes in the gradient of the river from relatively steep
Table III Summary of estimated amounts (kilograms) of sediment and trace elements deposited annually at eight
sites along the Coastal Plain reaches of the Chickahommy River between Richmond Virginia and Providence Forge
Virginia site number ascends downstream Area at each site is calculated from forested wetlands delineated from a 2
km reach centred at the site Site 3 (greatest amount of sediment) is located near the confluence of several tributaries
draining urbanizing areas around Richmond site 4 has the highest stream gradient of the study reaches and nearly
the least amounts of sediment and trace elements
Site Sediment Zn Cu Ni Pb Cd Cr Sri
1 670 000 118 6 8 43 < l 23 < 1
2 1 400 000 205 33 17 155 1 60 2
3 7 600 000 1269 76 76 426 2 274 4
4 840 000 235 8 12 41 < 1 26 1
5 2 200 000 446 29 33 130 1 110 2
6 1 600 000 125 5 10 37 < 1 42 < 1
7 2 000 000 259 14 17 54 1 51 1
8 860 000 68 5 7 25 < 1 28 < 1
Total 17 170 000 2725 176 t80 911 5 614 9
Hydrol Process 14 2991 -3010 (2000)

3002
C R HUPP
Figure 7 Relationship between elevation and sediment deposition along the Cache River Arkansas Deposition rates were determined
from dendrogeomorphic analyses (tree ring) and are averages from trees ranging in age from 180 to 25 years
straight channel reaches to low gradient anastomozing reaches strongly control deposition rates of both
sediment (0 7 and 5 7 mm /year respectively) and associated trace elements low stream gradients facilitate
the development of broad bottomlands and long hydropenods which in turn enhance sediment trapping
Local sediment deposition
It may be intuitive that as sediment laden flow leaves the main channel and enters a forested wetland
velocities slow owing to the hydraulically rough nature of a forested bottom (also a dramatic increase in
wetted perimeter) and subsequently sediment deposition occurs ( Kleiss 1996) Yet as previously stated
until quite recently only a handful of attempts have been made to quantify sediment deposition in any
wetland system Thus it should not be surprising that there are even fewer published accounts and
interpretations of factors affecting local variation in deposition rate (Hupp and Schenmg 1997) The amount
of suspended fines available rather obviously pervasively affects deposition potential Variation in local
elevation (Figure 7) across a bottomland and correlated length of hydroperiod also have been cited as
important factors affecting deposition rate (Hupp and Morris 1990 Hupp and Bazemore 1993 Kletss
1993 1996) Several other factors distinct and correlated may play an important role in local sediment
deposition including flow velocity distance in line of flow from main channel hydraulic connection to main
channel internal flow paths ponding (typically in backswamps or behind levees) roughness from standing
vegetation and LWD and beaver activity
The US Geological Survey in association with the US Forest Service and several universities is
conducting research on the functioning of BLH through the Southern Forested Wetlands Initiative (Burke
and Eisenbies 1999) Two sites chosen for this initiative are the Coosawhatchie River SC a black water
H) drol Process 14 2991 3010 (2000)

COASTAL PLAIN RiVERS 3003
stream and the Cache River AR an alluvial stream They have been sampled intensely in order to
investigate several factors that may affect local sediment deposition (Hupp and Schening 1997 Hupp et a!
1999b) The Coosawhatchie and Cache Rivers annually trap substantial amounts of sediment 24 5 kg /ha/
year and 187 6 kg /ha /year respectively reinforcing the water quality functions of both black water and
alluvial forested wetlands Deposition rates were estimated along multiple transects normal to the
downvalley axis using dendrogeomorphological (tree ring) techniques and clay pad marker horizons These
rates were then related to several physical parameters including velocity elevation LWD and hydraulic
connectivity at each sampling station and to dominant woody vegetation The transects were closely spaced
so that the sampling points were arrayed in a grid like fashion permitting a three dimensional analysis of
data Many of the following observations could not have been made using widely spaced transects
(essentially two dimensional analysis)
Mean sediment deposition rates on the black water Coosawhatchie River ranged from 0 02 to 0 20 cm/
year and from 0 20 to 0 36 cm /year on the Cache River The Cache River carries a suspended load of about
100 to 350 mg /1 whereas the Coosawhatchie River carries about 5 to 25 mg /l Thus a greater amount of
' d Flow
II i I
it y'ii i Il iiila� f Io7r1�
� � I I n I f I I I f 11 Iilll If r
I
t I l i t
n it i 111 + f' +1 I l l I
I
h I I �' i
f 1 I f I r ( (1 1111
it I
X I I I II 1
t I I 1
f
II
It
• u, E I I
r Il �L i
15
i Bird
Ill w 'ii 10
i �' '
0
if y �
I
Ill ! ( E
,
I , I ; Ill 5 O E
I+ I �f'' d
�I I O
u 1111 1 1 f III JI �� ii'I II i , i l II i�I +I' + l l
Iii I h 250 II'" 5001 11750 1000 1250 '11500'
l� + I II II [I Ill i' j+ 1
11 i ; J, i I +;+ i Distance from River in meters
Figure 8 Sediment deposition patterns across the Cache River Arkansas bottomland Data shown along grid lines first through fourth
individual points are separated in straight lines by 250 m Deposition rates tend to be greatest along flow paths (A) indicates area of
concentrated large woody debris and relatively high sediment deposition rate (B) indicates a stagnant area that is poorly hydraulically
connected to sediment laden river water and has a relatively low sediment deposition rate
Hydro! Process 14 2991 -3010 (2000)

3004 C R HUPP
sediment deposition on the brown water Cache River was expected and measured Mayor sloughs bifurcating
through the sites affect both study areas Hydraulic connectivity (degree of flow path connections to the
main channel) appears to strongly affect sedimentation rates (Figure 8) with highest deposition occurring
near sloughs and their anabranches with a direct flow path to the river In contrast relatively low deposition
rates occur in stagnant areas poorly connected to the channel or unaffected by sloughs presumably due to
diminished replenishment of suspended sediment during the hydroperod (Figure 8) this occurs despite
nearly complete inundation during the hydroperod at both sites Smaller sloughs associated with crevasse
splay areas near the main channels (Figure 8) similarly experience high deposition rates
Woody vegetation including LWD may also play an integral part in directing and concentrating flow
paths across bottomlands through variation in surface roughness (sloughs tend to be more open)
Sedimentation tends to be high on the upstream faces of ridges (Figure 8) areas that also accumulate
considerable LWD whereas adjacent downstream areas tend to have less sediment accumulation
Deposition rates vary inversely with velocity on the Coosawhatchie River but vary directly with velocity
on the Cache Velocities at both sites are relatively low except in the main channel and mayor sloughs Low
velocities facilitate deposition of fines particularly organic material however relatively moderate velocities
may ensure a continuous supply of sediment laden water particularly mineral fines Fines deposited over the
clay pads on the Coosawhatchie River contained substantial amounts of organic material with a mean of
nearly 40 °/ after loss on ignition as opposed to 22 A on the Cache River Mineral fines were concentrated
largely on the levees and near sloughs
VEGETATION
The likelihood of a given species vigorously growing on a particular landform including the various fluvial
landforms is a function of (i) the suitability of the site for germination and establishment (ecesis) and (n) the
ambient environmental conditions that permit persistence at least until reproductive age (Hupp and
Osterkamp 1996) The distributional pattern may be limited by the tolerance of a species for specific
disturbance or stress regimes as well as by tolerance for other more diffuse interactions including
competition In fluvial systems the distribution of vegetation across landforms may be driven largely by the
tolerance of species to specific geo morphological processes (hydroperod and sedimentation dynamics in
BLH systems) at the severe end of a stress equilibrium gradient and by competition with other bottomland
species at the other end
Variations in hydroperod (and perhaps to a lesser degree sedimentation /erosion) and plant adaptive
strategies largely explain the complex patterns of BLH species distributions (Bedmger 1971 Leitman et al
1984 Wharton et al 1982 Mitsch and Gosselmk 1993 Sharitz and Mitsch 1993) however specific
patterns of BLH species distribution and their quantitative relations with water level and sediment dynamics
remain incompletely understood For example streamflow of varying magnitude and duration and sediment
deposition /erosion dynamics affect vegetation by creating new areas for establishment such as point bars
(lateral accretion) the subsequent ridge and swale topography and by creating hydropenod /sediment size
clast gradients (vertical accretion) across the floodplam Although these fluvial processes are yet
insufficiently understood to allow for reasonably accurate prediction at a specific site even less understood
is the role riparian vegetation plays in affecting fluvial processes (Hupp and Osterkamp 1996) We know that
vegetation increases flow resistance (and thus facilitates deposition) increases bank strength and provides
LWD in the form of log dams in channels and as debris piles (rack) across floodplams (Hickm 1984 Hupp
1992 Hupp and Osterkamp 1996) However separating factors that simultaneously influence both
vegetation patterns and geomorphological processes is difficult because most are distinctly interdependent
and consistent definitions of landform and process generally are lacking within the geomorphological
sciences and particularly between the geomorphological and plant ecological sciences Where conformity
occurs between sciences it is usually in the common belief that hydrological processes control most aspects of
H}drol Process 14 2991 -3010 (2000)

of �'ri : t'r __t, .la�S`..�''`' •/ �y,, --{�`c -
1 .
1'4
1 �' S ;'I1 i �• T _ -... �. s
tt
Ae

3006 C R HUPP
the fluvial BLH ecosystem Indeed only hydrological characteristics provide independent parameters
consistent on all perennial streams
Despite the difficulty to demonstrate quantitative relationships among hydrology geomorphology and
vegetation the striking vegetation zonation across most BLH systems (Figure 9) has tempted several
researchers to develop a classification of vegetation patterns (Kellison et al 1998) Small differences in
elevation often measured in centimetres may lead to pronounced differences in hydroperiod and thus to
community composition (Mitsch and Gosselink 1993) As a result most classification systems infer that
length of hydroperiod is the most influential factor in controlling species patterns most probably due to
anaerobic conditions associated with flooding (Wharton et al 1982) Anaerobic respiration within the roots
of plants leads to the production of toxic byproducts and limits the uptake of nutrients and water Plants
tolerant of varying degrees of flooding have developed physical and /or metabolic adaptations to withstand
inundation and anoxia (Wharton et al 1982) Presumably the degree to which individual species have
adapted to anoxia related stresses controls the distinct and striking changes in vegetation composition across
very short (metres) lateral distances on many floodplains in this region (Huffman and Forsyth 1981)
A zonal classification system (Figure 10) of coastal plain bottomlands described by Clark and Benforado
(1981) and adopted by the National Wetland Technical Council has been the basis for most subsequent
BLH vegetation community classifications This classification is based largely on hydrological regime and is
II I
lilt I,' +I�I1111
I i Iltiil� i' 'III
4 I
II Ill,i I(I +d {i, I I
I I�I IIII i l I ,
i�Pti 111 i II
Zone 11 '' I' illi
t
� Ill �I'' 1 IIII III 1
I�,Illl'ill�l � tI I I
I ' tI'I� II 'III
Na lme
III
II't
Water modifier I
I l
I IIII! ' II 1I
Flooding (frequency
% of years
Flooding duration
% of growing season
I I II I 111
Figure 10 Zonal classification of bottomland hardwood forests showing average hydrological conditions for each zone (after Shantz
and Mitsch 1993 Reproduced by permission of John Wiley & Sons Ltd )
Hydrol Process 14 2991 -3010 (2000)
+ + I
I
I,
Ii
I
I I
I
I
I
III II ll II
I +Ii
Aquatic
t
ecosystem
I' ! I
III
I �� � lj I
Bottomland hardwood eooayetem
I I l i it,
I Bottomland
upland
I
I t
+!
tranaltion
if
I I, 1
'
I f 7( II
pIl�Jil1II
I
t'I I ' 1 1
III
Floodplaln' I ' "
I I i+ I I II'
Nit III' II Iy +II I
' I
I 1fl
Open water
I ! Swamp
l I,
Lower hard -1 1
wood wetland
IIII 'I
I I
Medium hard- I Higher hard
wood wetland i I wood wetland
III 1I
Transition to
upland
Continuously
'flooded
Intermittently
I
Semler rllly
'
I I
Seasonally I I Tempom' d N
II I
Intennitt
Il
e>rposedllj 1
flooded
flooded III flooded
flooded
100
'10011 q
IIII
I
f
l It II
i 1
I
1 10
100
0011 II
I
If II I+
a25
I
12 5-25' 1 I t 2 12 5
If i+ It?
<2 i
Figure 10 Zonal classification of bottomland hardwood forests showing average hydrological conditions for each zone (after Shantz
and Mitsch 1993 Reproduced by permission of John Wiley & Sons Ltd )
Hydrol Process 14 2991 -3010 (2000)

COASTAL PLAIN RIVERS 3007
highly generalized -nuch of the geomorphological detail described earlier is lacking and of limited use at
specific sites However this classification serves as a useful framework for interpreting vegetation patterns as
long as the deceptively complex local hydraulic patterns particularly in plan view (Hupp 1999) are not
ignored With the possible exception of the lower Mississippi (Saucier 1994) hydrogeomorphological
patterns across these bottomlands have received far less study than the vegetation
The species found in BLH systems are remarkably similar throughout the bottomlands of the Coastal
Plain (Sharitz and Mitsch 1993) Forty two tree species occur commonly and 13 of these are ubiquitous
(Kellison et al 1998) A typical pattern of vegetation distribution is shown in Figure 11 Point bars typically
the most recently created surfaces tend to support shade intolerant ruderal species such as Salix nigra
Betula nigra and Populus deltotdes Inward from the channel levee surfaces and scroll ridges frequently
stand in considerable relief relative to the rest of the bottomland and tend to be well drained owing the
typically sandy substrate These fluvial features generally support a mixture of older point bar individuals
and other relatively high and dry species such as Platanus occtdentalts Quercus laurfoha Q phellos Q
nigra Frartnus pennsylvantca and Liquadambar styractflua The often broad floodplam is a mosiac of flats
punctuated by sloughs ox bows and swales which may be only tens of centimetres (or less) lower than the
surrounding flats These lower and thus wetter features support the most hydric species such as Tavodium
distichum and Nvssa aquattca Just outside these areas are slightly less moist surfaces dominated by Quercus
lyrata Cur }a aquattca and Gleditsta aquattca The flats support a diverse forest that may include the levee
species in addition to Quercus mtchauru Q pagoda Ulmus americana Acer negundo A rubrum Celtis
laevtgata Ptnus taeda and Fagus grand foha These species display distributional patterns often in
association with other species (Figure 11) along often virtually imperceptible variations in elevation across
the floodplam Along alluvial rivers the floodplains tend to become increasingly more hydric from the Fall
Bald Cypress Water tupelo
Water elm Buttonbush
River birch
Black willow
Overcup oak,
Red maple
Willow and Pin
oaks Sweetgum
Swamp
chestnut oak
t
Figure I l Cross section of a bottomland hardwood forest showing species distribution relative to a perennial stream and ox bow (A)
is the highest flow elevation (B) is the mean annual high water elevation and (C) is the mean annual low water elevation (after Shantz
and Mitsch 1993 Reproduced by permission of John Wiley & Sons Inc )
H)drot Process 14 2991 -3010 (2000)

3008 C R HUPP
Line to the estuary Extensive tidal BLH systems near estuaries are arely flooded relative to other
floodplains but are wetted daily during wind and /or lunar high tides these systems are largely unstudied
SUMMARY
The Coastal Plain of the south eastern USA is characterized by broad frequently inundated low gradient
fluvial systems that support a characteristic forest ecosystem bottomland hardwoods These systems have
received considerable ecological study but distinctly less hydrogeomorphological study quantitative process
linkages among hydrology geomorphology and ecology remain largely undocumented Although heavily
impacted by land use these floodplains and their bottomland hardwood systems remain a critical landscape
element for the maintenance of water quality by trapping and storing large amounts of sediment and
associated contaminants
Alluvial and black water fluvial systems within the region typically are flooded annually for prolonged
periods creating two distinct hydrological seasons Order of magnitude differences in wetted perimeter
width /depth suspended sediment load and hydraulic roughness exist between the in channel dry season
and the inundated season or hydroperiod Vertical sediment accretion rates may be among the highest of any
fluvial system Channel processes and sedimentation dynamics in these low gradient systems result in the
extensive development of levees point bars and scroll topography avulsion and associated back channels or
sloughs and ox bows and broad extremely flat floodplains Variation in hydroperiod velocity and
suspended sediment may largely control sediment deposition rates These factors are controlled locally by
elevation levee breaches amount of large woody debris and degree of hydraulic connectivity to sediment
laden inundating water Diverse woody vegetation of this ecosystem has adapted to prolonged inundation
(anaerobic conditions) creating unique characteristic riparian vegetation patterns revealed in a mosaic of
associations ranging from relatively mesic levee and high floodplam associations to hydric slough and low
floodplam associations Nearly imperceptible changes in elevation may result in distinct pervasive changes in
species composition and zonation strongly suggesting a rigorous relationship between vegetation and
hydrogeomorphological processes These critical fluvial systems and their attendant forests are ripe for
future multidisciplinary research
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Hyd rl Process 14 2991 -3010 (2000)

COASTAL PLAIN RIVERS 3009
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US Geological Sur ey Water Supplj Paper 21%
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Hydrol Process 14 2991 -3010 (2000)

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
AUGUST AMERICAN WATER RESOURCES ASSOCIATION
2003
JAN 1 3 ZU1Z
DENR WATERQUALITY
V11IMMINANDSWIRMWMFOOR
BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE
INTERVALS FOR NORTH CAROLINA'S COASTAL PLAINT
William V Sweet and Jens W Geratz2
ABSTRACT Bankfull hydraulic geometry relationships relate
stream channel geometry to watershed size for specific physio
graphic regions This paper presents bankfull hydraulic geometry
relationships and recurrence intervals for the Southeastern Plain
ecoregion and the flatwoods subtype of the Middle Atlantic Coastal
Plain ecoregion found within North Carolinas Coastal Plain phys
iographic province Cross sectional and longitudinal survey data
from gaged and ungaged streams were used to compute channel
dimension and profile information Power functions were devel
oped relating drainage area to bankfull discharge cross sectional
area width and mean depth Recurrence intervals of bankfull
events were estimated from gaged streams using both a Log Pear
son Type III distribution of peak annual discharge and a partial
duration series of average daily discharge Results from both
methods indicate that average bankfull recurrence intervals for the
study area are below one year Determinations of recurrence inter
vals by the Log Pearson Type III distribution were for the most
part inconclusive (less than one year) while a partial duration
series estimated a 0 19 year average ranging from 0 11 to 0 31
years
(KEY TERMS bankfull environmental engineering flood frequen
cy partial duration series bankfull hydraulic geometry relation
ships surface water hydrology)
Sweet William V and Jens W Geratz 2003 Bankfull Hydraulic Geometry Rela
tionships and Recurrence Intervals for North Carolinas Coastal Plain J of the
American Water Resources Association (JAWRA) 39(4) 861 871
INTRODUCTION
Bankfull hydraulic geometry relationships allow
comparisons of bankfull flows (channel forging
events) and associated dimensional parameters to
various watershed sizes (Lindley 1919 Leopold and
Maddock 1953 Wolman 1955 Nixon 1959 Myrick
and Leopold 1963 Emmett 1972 1975 Dunne and
Leopold 1978) Hydraulic geometry relationships are
region specific describing areas of similar physiogra
phy and climate Bankfull hydraulic geometry rela
tionships are valuable tools that aid in the prediction
of bankfull channel characterization on ungaged
reaches both natural and modified and for guiding
natural channel design
Bankfull is associated with flows that fill a stream
channel to the elevation of the active floodplain
These events are most effective at moving sediment
loads over time (Wolman and Miller 1960 Andrews
1980 Carling 1988) and are essentially responsible
for sizing the channel dimensions of a stable system
( Rosgen 1994) The recurrence interval for bankfull
events is traditionally determined by a Log Pearson
Type III distribution of annual peak discharge and
has been estimated to occur between a 14 and 16
year period throughout much of the United States
and the Mountain and Piedmont physiographic
provinces of North Carolina (Dunne and Leopold
1978 Leopold 1994 Rosgen 1996 Harman et al
1999 2000 Castro and Jackson 2001)
With stream restoration currently receiving much
attention it is imperative that bankfull hydraulic
geometry relationships be developed for individual
physiographic provinces for channel design purposes
In addition accurate recurrence interval and flood
duration estimates of bankfull events are important
to agricultural interests developments situated on
flood prone lands and for stream and floodplain
restoration activities This paper presents regional
bankfull hydraulic geometry relationships for the
Coastal Plain of North Carolina and results from a
flood frequency analysis of both a Log Pearson Type
'Paper No 02013 of the Journal of the American Water Resources Association Discussions are open until February 1 2004
211espectively Project Scientist and Senior Scientist EcoScience Corporation 1101 Haynes Street Suite 101 Raleigh North Carolina
27604 (E MaiUGeratz geratz @ecosciencenc com)
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 861 JAWRA

SWEET AND GERATZ
III distribution and a partial duration series to deter
mine an average recurrence interval for bankfull
events
STUDY AREA
North Carolina is commonly divided into three
major physiographic provinces the Mountains Pied
mont and Coastal Plain These provinces are separat
ed by dominant geographic delineations that have
unique topography soils climate and vegetation
Recent efforts (Griffith and Omernik 2000) have sub
divided the physiographic provinces into ecoregtons
which depict ecological systems on regional scales
that have distinct geology hydrology climate and
biology Within a given ecoregion additional subdivi
sions further classify specific subtypes that are depen
dent upon locally dominant characteristics The
Coastal Plain physiographic province (Coastal Plain)
of North Carolina is separated into the Southeastern
Plains (SP) and Middle Atlantic Coastal Plain
(MRCP) ecoregions (Figure 1) For the current study
the MACP ecoregion was further segmented following
delineations depicted by Griffith and Omernik (2000)
into a flatwoods subtype or a lowlands swamps and
floodplains subtype
The Coastal Plain province is characterized as a
predominately sandy and flat low elevation plain
Stream gradients are quite low and become increas
ingly small from the SP to the MACP ecoregion with
slopes ranging from 0 5 percent to less than 0 01 per
cent Unconsolidated sediment formations thicken
toward the coast with surficial deposits progressively
changing from sand in the SP ecoregion and flatwoods
MACP subtype to poorly drained loamy soils within
the lowlands swamps and floodplains MACP sub
type Within the lowlands swamps and floodplains
MACP subtype bottomlands border alluvial systems
whose floodplains contain clay and organic deposits
that form restrictive lenses with very low permeabili
ty (Griffith and Omernik 2000) Mean annual precipi
tation throughout the entire Coastal Plain ranges
from 48 to 58 inches with the highest amounts occur
ring near the coast (Hirth 1998)
The SP ecoregion is characterized by gently rolling
topography that supports a mosaic of pasture wood
land cropland and forested areas Natural vegetation
is mostly oak/hickory /pine forest on upland slopes
with bottomland hardwood species in isolated flood
plains and low terraces The flatwoods MACP subtype
is dominated by loblolly shortleaf pine forests in well
drained soils and oak gum and cypress stands along
streams and terraces The lowlands swamps and
floodplains MACP subtype is characterized by low
Figure 1 Locations of Gaged and Ungaged Stream Reaches in the Southeastern Plain (SP) and Meddle Atlantic Coastal Plain (MRCP)
Ecoregions With MACP Subtypes (modified from Griffith and Omernik 2000) for the Coastal Plain of North Carolina
JAWRA 862 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
230 24
•16
• g 91 b
At
• 9 qq
� 18
#
th�r °���
04
2 2
0 10 20 30
Miles
F
0 loom [as
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Number Location
os ffir eek
arora Branch-
3
Bear Creek
Beaver Dam Branch
ac nc
Bullard anc
e River
a m'jt11i
a ree
LEGEND
Ste Locations
A Gage
• Reference
Major Ecoregions of
the Coastal Plain Province
0 Soufheastem Plains
Noddle Atlantk
Coastal Plain
Flatwoods Subtype
©Lowlands, Swamps
and Floodplarns Subtype
Herrings ars n
Herrings Marsh Run
12
Hood Creek
Island nee
14
Kellys
Is
mil Ru n
16
1 a unta Swamp
7
New River
Rockfish Greek
o ton ree
T24
ree
to owRiver reek
UTt o n ee
wam
Figure 1 Locations of Gaged and Ungaged Stream Reaches in the Southeastern Plain (SP) and Meddle Atlantic Coastal Plain (MRCP)
Ecoregions With MACP Subtypes (modified from Griffith and Omernik 2000) for the Coastal Plain of North Carolina
JAWRA 862 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINAS COASTAL PLAIN
relief poorly drained flats interspersed with broad
shallow valleys and tidally influenced streams near
ing the estuaries Naturally vegetated communities
include high and low pocosins pond/pine woodlands
cypress tupelo forests coastal marshes and barrier
dune systems (Griffith and Omermk 2000)
METHODS
Regional bankfull hydraulic geometry relationships
were developed for the SP ecoregi<on and the flatwoods
subtype of the MACP ecoregion of North Carolinas
Coastal Plain (Figure 1) The hydraulic geometry
relationships were composed of data from U S Geo
logical Survey (USGS) gaged streams and ungaged
streams located within remote portions of the above
mentioned regions Since the majority of USGS gage
sites within the Coastal Plain tend to monitor larger
watersheds the ungaged streams represented smaller
watersheds to supplement the data set
Site Selection and Criteria
Since most USGS gages are located immediately
downstream of roadway crossings careful attention
was given to avoid sites with significant historical
disturbances usually in the form of straightened and
incised channel banks Gaged watersheds included
current and recently discontinued stations that fit the
following criteria a minimum eight year record of
hydrological data (flood frequency analysis only on
sites with a >10 year record) minimal land use
changes in the last 10 years no immediate upstream
impoundments and less than 10 percent impervious
area in the watershed
Ungaged streams identified for the study included
reaches with small drainage areas that had natural
channel conditions — largely undeveloped and unal
tered by agricultural logging or other anthropogemc
disturbances Site selection criteria for the ungaged
streams were the same as the gage sites minus the
flow record requirements In addition a survey was
made to assess the channel bed banks and vegeta
tion for signs of either channel or stream bank insta
bilities The stream bed was observed for scouring or
deposition including the occurrence of bars or islands
in the channel cut faces on bars cutoffs and flood
chutes and unusually steep riffles Channel incision
evaluations using low bank height ratios (i e low
bank height /maximum bankfull depth) were per
formed along each stream profile to assess stability
Low bank height ratios those approaching 10 were
considered indicative of a stable channel particularly
when observed over a significant length of stream
( Rosgen 1997)
Field Protocol and Analysis
Prior to each gage survey the following data were
obtained from the USGS for the site stage discharge
rating tables established reference marks gage sum
maries (Form 9 207) and peak annual discharge
series Peak annual discharge was graphed using the
USGS accepted (Dalrymple 1960) Weibull plotting
position (Gordon et al 1992) to obtain flood recur
rence interval information for the site In addition
aerial photography (1998 three foot resolution) for a
one mile stretch of stream was centered on the gage
site allowing stream channel sinuosity calculations
for the local reach Watershed sizes for gage sites
were provided by the USGS
During each gage survey the protocol for calibrat
ing bankfull discharge at USGS stream gage stations
was followed (Leopold 1994 Rosgen 2001) Gage sur
vey procedures included the centering of the longitu
dinal survey which spanned greater than 20 bankfull
widths about the gage plate location A least square
fit of the data from the bankfull survey determined
bankfull stage at the gage plate location which
identified bankfull discharge from the rating table
Bankfull hydraulic geometry was obtained by analy
sis of cross sectional surveys performed along stream
riffles found within the boundary of the longitudinal
profile The bankfull hydraulic geometry determnna
tion was based on elevations of consistent bankfull
indicators which included the top of banks back of
point bars significant breaks in bank slopes and
highest scour lines (Leopold 1994) The bankfull
hydraulic geometry relationships were developed
without regard to recurrence intervals associated
with bankfull discharge
Prior to each ungaged stream survey watershed
sizes were estimated using USGS 124 000 digitized
topographic maps and USGS 14 digit hydrologic unit
mapping provided by the North Carolina Center for
Geographical Information Analysis (CGIA) For each
ungaged stream a longitudinal profile survey includ
ing measurements of bankfull low bank water level
and thalweg were performed over distances greater
than 20 bankfull widths Cross sectional surveys were
performed at representative riffle locations to deter
mine bankfull hydraulic geometry Bankfull identifi
cation was based upon indicators that included the
back of point bars and the top of banks Plan form
surveys were used to calculate stream sinuosity
Stream roughness coefficients (n) were estimated for
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 863 JAWRA

SWEET AND GERATZ
each of the ungaged streams using a version of Arce
ment and Schneiders (1989) weighted method for
Cowan s (1956) roughness component values and
applied to the following equation (Manning 1891) to
obtain bankfull discharge (Qbkf) estimates
Qbkf = R 486 / n] * [A * R2/3 * S1/2]
where A equals bankfull area R equals bankfull
hydraulic radius and S equals average water surface
slope measured over for the reach during surveying
efforts
Post survey analysis for the gaged and ungaged
streams compared water surface and bankfull eleva
tions over the surveyed reach to ensure that there
was consistency in profiles which would indicate
proper bankfull identification (Leopold 1994) Also
samples of surficial bed deposits that were collected
at all sites were dried sieved and weighed (Gordon
et al 1992) allowing for preliminary bed material
assessments for stream classification purposes only
Each stream was summarily categorized under the
Rosgen method (1994) for stream classification pur
poses
Recurrence Interval Analysis
Bankfull recurrence intervals were estimated by
two methods for nine gaged streams One method
computed recurrence intervals (Pope et al 2001) for
various floods (Q2 Q5 Q10 Q500) by traditional
procedures (USGS 1982) utilizing Log Pearson Type
III distributions of peak annual discharge (conven
tional method) To determine bankfull recurrence
intervals for the current study the Pope et al (2001)
results were graphed onto a semi log plot (recurrence
interval [logged x axis] versus discharge [y axis]) and
fit with a least square regression line (R2 > 0 95 in all
cases) that was extrapolated to bankfull discharge
values The equation of this line was empirically
solved yielding site specific recurrence intervals of
bankfull discharge If the extrapolated values were
below the one year period the results were deemed
inconclusive (Langbein 1949) and reported as less
than one year
Recurrence intervals were also computed by a par
teal duration series of historical average daily dis
charge (USGS 2001) The partial duration method
determines flood frequency by counting the number of
independent maximum daily flows above a specified
level (bankfull) and dividing by the total number
of years of record for that station Only discrete bank
full events separated by at least a one day period
between successive floods were included in the series
The resultant series was graphed using the Weibull
plotting position to allow graphical representation
and numeric interpolation of the bankfull recurrence
interval The average duration of events at or above
bankfull levels was also calculated for each gage site
and equaled the total number of days a site s flow was
above bankfull discharge divided by the total number
of events It should be noted that the vast majority of
bankfull events at any given gage site were separated
by more than a week
RESULTS
Bankfull hydraulic geometry data for 24 gaged and
ungaged streams within the Coastal Plain of North
Carolina are shown in Table 1 Results represent
watershed sizes ranging from 0 6 to 182 square miles
and include bankfull measurements recurrence inter
vals calculated by both the conventional and partial
duration methodologies and stream classification
(Rosgen 1994) In general the streams surveyed for
the current study were located in well developed allu
vial floodplains with dense mature forests and
streamside communities Most of the surveyed
streams had sandy bed material with a few main
taining elevated levels of organic muck within the
channel and floodplain The streams exhibited low
gradient riffle pool structures with low width/depth
ratios k 12) indicative of E stream types (Rosgen
1994) Several streams exhibited slightly higher
width/depth ratios (> 12) as well as point bars which
are traits characteristic of C stream types (Rosgen
1994)
The bankfull hydraulic geometry relationships
shown in Figures 2(a through d) are computed from a
least square power function regression of bankfull
characteristics versus watershed area for 22 of the 24
surveyed sites The hydraulic geometry relationships
do distinguish between data points from the SP ecore
gion and flatwoods subtype of MACP ecoregion but
no additional stratification was performed The
regression equations for the bankfull hydraulic geom
etry relationships along with R squared values based
on a natural log distribution are shown below
(a) Abkf = 9 43 (A,)O 74 R2 = 0 96
(b) Wbkf = 9 64 (AW)O 38 R2 = 0 95
(c) Dbkf = 0 98 (AW)O 36 R2 = 0 92
(d) Qbkf = 8 79 (AW)O 76 R2 = 0 92
where Abk£ equals bankfull cross sectional area (sq
ft) AW equals watershed drainage area (sq mi) Wbk£
JAWRA 864 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

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BANKFULL HYDRAULIC GEOMETRY RtLATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINAS COASTAL PLAIN
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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 865 JAWRA

1000
z
loo
a;
X
a 10
9
as
100
;
a 10
A
� 1
A
(a)
SWEET AND GERATZ
!f�
1 i /•
E
i
t if ti
i
01 1 10 100 1000
Drainage Area (sq mi )
(c)
0 1 4-
01
1 10 100 1000
Drainage Area (sq mi )
b
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100
b
x
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1+
01
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W 1000
x
v
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�w
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(d)
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/ /d
t / �
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t I ;
1t t+
}
t I i, 1 ),,
If
11
I i ii r 1 I If
1 10 100 1000
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Figure 2 Bankfull Hydraulic Geometry Relationships (a through d) With 95 Percent Prediction Limits (dash) Composed of Gage
(triangles) and Ungaged Sites (circles) for the Southeastern Plain Ecoregion (hollow symbols) and Flatwoods Subtypes
of the Middle Atlantic Coastal Plain Ecoregion (filled symbols) Within the Coastal Plain of North Carolina
equals bankfull width (ft) Dbk£ equals bankfull mean
depth (ft) and Qbkf equals bankfull discharge (cfs)
It should be noted that the original effort attempt
ed to create bankfull hydraulic geometry relation
ships valid for the entire Coastal Plain of North
Carolina However two gage sites (Cashie River and
Van Swamp) had bankfull hydraulic geometry signifi
cantly distinct from the rest of the surveyed Coastal
Plain sites A subsequent GIS mapping investigation
found that the above mentioned sites were uniquely
located within the lowlands swamps and floodplains
MACP subtype Due to temporal constraints and a
lack of additional long term gaging stations further
surveys within the lowlands swamps and floodplains
MACP subtype were prohibitive Therefore the
hydraulic geometry relationships developed within
the current study exclude these two data sets as well
as all subsequent flood frequency analyses Even
though the presented bankfull hydraulic geometry
relationships may not be applicable within the low
lands swamps and floodplains MACP subtype the
results from the two surveys are considered impor
tant to current and future stream restoration efforts
and are therefore included and briefly discussed
JAWRA 866 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
/
1 I I
1 10 100 1000
Drainage Area (sq mi )
b
1000
100
b
x
10
1+
01
10000
W 1000
x
v
A 100
�w
M 10
(d)
1 �-
01
f� /Ar
/ /d
t / �
T
1 10 100 1000
Drainage Area (sq mi )
t I ;
1t t+
}
t I i, 1 ),,
If
11
I i ii r 1 I If
1 10 100 1000
Drainage Area (sq rni )
Figure 2 Bankfull Hydraulic Geometry Relationships (a through d) With 95 Percent Prediction Limits (dash) Composed of Gage
(triangles) and Ungaged Sites (circles) for the Southeastern Plain Ecoregion (hollow symbols) and Flatwoods Subtypes
of the Middle Atlantic Coastal Plain Ecoregion (filled symbols) Within the Coastal Plain of North Carolina
equals bankfull width (ft) Dbk£ equals bankfull mean
depth (ft) and Qbkf equals bankfull discharge (cfs)
It should be noted that the original effort attempt
ed to create bankfull hydraulic geometry relation
ships valid for the entire Coastal Plain of North
Carolina However two gage sites (Cashie River and
Van Swamp) had bankfull hydraulic geometry signifi
cantly distinct from the rest of the surveyed Coastal
Plain sites A subsequent GIS mapping investigation
found that the above mentioned sites were uniquely
located within the lowlands swamps and floodplains
MACP subtype Due to temporal constraints and a
lack of additional long term gaging stations further
surveys within the lowlands swamps and floodplains
MACP subtype were prohibitive Therefore the
hydraulic geometry relationships developed within
the current study exclude these two data sets as well
as all subsequent flood frequency analyses Even
though the presented bankfull hydraulic geometry
relationships may not be applicable within the low
lands swamps and floodplains MACP subtype the
results from the two surveys are considered impor
tant to current and future stream restoration efforts
and are therefore included and briefly discussed
JAWRA 866 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINAS COASTAL PLAIN
The average recurrence interval of bankfull events
for the gaged streams is estimated to be 0 19 year
with a range of 0 11 to 0 31 year as estimated by the
partial duration series method (Table 1) A determi
nation of an average bankfull recurrence interval by a
Log Pearson Type III distribution of peak annual dis
charge was not possible since all but one site had a
recurrence interval below one year The average dura
tion of bankfull events for the gaged streams ranges
from 14 to 8 8 days (Table 1)
DISCUSSION
The bankfull hydraulic geometry relationships for
the Coastal Plain of North Carolina exhibit strong
relationships (R2 > 0 9) that are considered valid for
the majority of the physiographic province Variability
within the bankfull hydraulic geometry relationships
may be attributed to differences of regional land uses
(Leopold 1994 Rosgen 1994) and presumably water
surface gradients quantity of instream debris and
bank vegetation and underlying geology that affects
water storage capacity within a given watershed
Bank full Recurrence Interval
Flood frequency analysis reveals that the recur
rence interval of bankfull events in the Coastal Plain
is much shorter than 15 year interval estimated by
previous state and nationwide investigations (Dunne
and Leopold 1978 Leopold 1994 Harman et al
1999 2000 Castro and Jackson 2001) The Log
Pearson Type III distribution of peak annual dis
charge for the Coastal Plain indicates subannual
bankfull recurrence intervals The partial duration
series further refines the recurrence interval estimat
ing a 0 19 year average which implies that bankfull
occurs more than five times a year The shortened
recurrence interval of bankfull in the Coastal Plain is
attributed to high precipitation inputs onto a wide
expanse of near level topography with large surface
storativity an elevated water table and slow flushing
rates
The substantially shorter recurrence interval of
bankfull events in the Coastal Plain has a couple of
important implications Besides providing reliable
flood frequency information recurrence intervals can
influence the development of bankfull hydraulic
geometry relations Presently standard practice by
Rosgen (2001) utilizes recurrence intervals from
either a Log Pearson Type III distribution or the
Weibull plotting position of peak annual discharge
data to verify correct field identification of bankfull
indicators along often modified channels at USGS
gaging sites Rosgen (2001) states that appropriately
chosen bankfull elevations along a given reach will
have a stage derived (USGS rating table) discharge
with an associated recurrence interval between the
one to two year span Unfortunately without an
enhanced range of accurate bankfull event recurrence
intervals relationship developers within low gradient
regions similar to the Coastal Plain may overestimate
actual bankfull elevations in the field to be in line
with the currently accepted one to two year range
A shortened recurrence interval for bankfull events
also indicates that stream systems in the Coastal
Plain are spilling their banks more frequently than
other regions This fact has important repercussions
for agricultural and urban planning interests and
might also affect the sizing of land acquisitions during
stream restoration efforts that attempt to create nat
urally shaped stream channels with active flood
plains To more precisely determine the frequency of
bankfull events the current study chose to analyze a
partial duration series of average daily discharge for
each gage station This analysis was actually an
afterthought decided upon after a number of field vis
its to remote gage sites revealed that discharges asso
crated with the tops of bank (bankfull) had a
recurrence interval of less than a one year period as
computed by methods utilizing the peak annual dis
charge series
General acceptance of the conventional method by
most governmental and private agencies was intend
ed to permit a consistent and proven technique to
estimate flood frequencies at USGS gaging stations
However the method was intended to tackle the
issues of peak flood/flow frequency estimations for the
national flood damage abatement program (USGS
1982) not for determining accurate magnitude and
frequency estimates of relatively small scale phe
nomenon such as bankfull events The conventional
method is preferred because of its effective treatment
of positive skew which provided a reasonably good fit
to empirical flood distributions from a wide range of
watersheds throughout the United States As with
most probability distributions the fit provides most
accurate estimates near the middle values for the
range of the historical data (Gordon et al 1992)
On the other hand a partial duration series allows
assessments of recurrence intervals for bankfull flows
with discharges less than the minimum annual peak
flow for a given data set (NERC 1975 USGS 1982
IAE 1987) Unlike the conventional method which
attempts to determine future probabilities of exceed
ing a particular flood level based only on the maxi
mum flood peak from each year of a given data set
the partial duration series method analyzes the entire
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 867 JAWRA

SWEET AND GERATZ
historical daily data set to more precisely isolate the
frequency of particular flood magnitudes Compar
isons between the two methods for a 41 year flow
record from the Swift Creek gage site highlight the
enhanced resolution of the partial duration series for
high frequency events (Figure 3) Figure 3 also illus
trates the intended purpose and strength of the con
ventional method — to smoothly fit the peak annual
series graphed using the Weibull plotting position
which is another method that cannot resolve recur
rence intervals below a one year period
There are advantages associated with each of the
above strategies used to generate flood frequency
information Inherent properties of Log Pearson Type
III distribution lends itself well to flood data that tend
to have a lower limit but no upper limit (Gordon et
al 1992) The distribution permits fairly straightfor
ward extrapolation beyond the temporal boundaries
of the data set (Chow 1964) allowing frequency pre
dictions of large scale floods previously unobserved at
or around a gage location However smaller floods
will occur more frequently than indicated by distribu
tions or plotting positions based on peak annual
7000
6000
5000
N
4000
(D
rn
cu
La 3000
2000
1000
0
01
{ I
t t_ —
series because several peaks in one year may be
higher than the highest flood in others The National
Research Council (NRC 1999) recognizes this report
ing bias of the conventional method and cautions
against the use of Log Pearson Type III distributions
to describe high frequency floods including annual
and multidecadal events In any case the longer the
available data set the more accurate results from
either method become provided that no significant
climatic events or management actions occur (USGS
1982 NRC 1999)
Bankfull Hydraulic Relationship Comparisons
As noted in the result section surveyed results
from two gaged streams the Cashie River and Van
Swamp sites substantially deviate from bankfull
trends and were excluded during relationship devel
opment (Table 1) These two sites are located adjacent
to drowned river valleys and swampy wetlands of the
Croatan Albemarle and Pamlico Estuarine System
(CAPS) and uniquely positioned within the lowlands
_4
T-T
1
i_..._».�._ T
10
Average Recurrence Interval (years)
100
Figure 3 Recurrence Interval Method Comparison for Swift Creek Gage Site With Bankfull Discharge (dashed line) Weibull Plotting
Position for Both the Partial Duration Series of Average Daily Flow (circle) and for the Peak Annual Series (triangle) Which is
Overlaid by the Log Pearson Type III Distribution (square) of the Peak Annual Series With a Regression Fit (solid line)
JAWRA 868 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
-
T":.r..,,,.........m-
10
Average Recurrence Interval (years)
100
Figure 3 Recurrence Interval Method Comparison for Swift Creek Gage Site With Bankfull Discharge (dashed line) Weibull Plotting
Position for Both the Partial Duration Series of Average Daily Flow (circle) and for the Peak Annual Series (triangle) Which is
Overlaid by the Log Pearson Type III Distribution (square) of the Peak Annual Series With a Regression Fit (solid line)
JAWRA 868 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINAS COASTAL PLAIN
swamps and floodplains subtype of the MACP ecore
gion (Figure 1)
Undisturbed sections of the lowlands swamps and
floodplains MACP subtype are characterized by bot
tomlands that are perched on ancient marine terraces
underlain by mucky histosols with extremely low per
meability and high surface storativity (Griffith and
Omermk 2000) These topographic flats have mini
mal regional stream gradients as well as numerous
debris obstructions beaver dams and relatively close
spatial connectivity to their receiving waters As
bankfull flows flood into components of the CAPS
system downstream water level may rise slightly
and reduce barotropic (i e gradient driven) forces
upstream from the barrier or confluence This combs
nation of factors further diminishes regional water
surface gradients and allows for extensive flooding
with heightened instream and floodplain deposition
Accordingly the Cashie River and Van Swamp gage
sites have prolonged average bankfull event durations
(18 1 and 13 0 days respectively) that are typical of
alluvial swamp systems (Brinson 1993) which flood
during the early months of each year when evapo
transpiration rates are low Physiographic considera
tions with emphasis upon minimal barotropic forcing
and reduced channel flushing as well as channel and
floodplain composition may constructively differenti
ate bankfull hydraulic geometry for the lowlands
swamps and floodplains MACP subtype from the rest
1 0000
w 1000
U
bbo
100
0
pq 10
(a)
/
/
/
/
10
Drainage Area (sq nv )
100
of the Coastal Plain Distinct from the rest of the sur
veyed sites and indicative of streams of the region
these two streams have developed low flow channels
within their expansive floodplain which is flooded for
extended parts of the year
Likewise physiographic properties unique to the
Coastal Plain such as low water surface gradients
and sandy soils with shallow aquacludes render
bankfull hydraulic geometry relationships distinct
from the other North Carolina provinces Figure 4
shows the Coastal Plain relationships (from the cur
rent study) with those produced by Harman et al
(1999 2000) for North Carolinas Mountain and Pied
mont provinces For comparative purposes the rela
tionships have been truncated and averaged over
watershed sizes ranging between 1 and 100 square
miles Comparisons reveal that average bankfull dis
charge of streams within the Coastal Plain is approxi
mately 10 percent of that of the Mountain and
Piedmont provinces (Figure 4a) Also the average
bankfull cross sectional area of Coastal Plain streams
is approximately 50 percent of the other two provinces
(Figure 4b) while its width is approximately 50 and
75 percent and its depth is 100 and 70 percent of the
Mountain and Piedmont provinces respectively
l000 (b)
100
� 10
va
/
/
/r
/
/
/
/
r t i
10 100
Drainage Area (sq mi )
Figure 4 17runcated Bankfull Hydraulic Geometry Relationships of (a) Bankfull Discharge and (b) Bankfull
Area for the Coastal Plain (circle) and the Overlapping Mountain (square) and Piedmont
(triangle) Provinces of North Carolina (Harman et al 1999 2000)
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 869 JAWRA

SWEET AND GERATZ
CONCLUSIONS
The bankfull hydraulic geometry relationships for
the Coastal Plain physiographic province of North
Carolina provide valuable templates that can guide
stream channel classification and restoration efforts
within the SP ecoregion and the flatwoods subtype of
the MRCP ecoregion However caution is advised for
restoration efforts near pocosins bottomlands and
wetlands within or adjacent to the lowlands swamps
and floodplains subtype of the MACP ecoregion For
these cases stream channel parameters may vary sig
nificantly and reference reaches located within the
surrounding vicinity that share similar physiography
should be mimicked
Bankfull events occur much more frequently within
the Coastal Plain of North Carolina than reported in
other state and nationwide surveys Bankfull recur
rence intervals were computed by a Log Pearson Type
III distribution of peak annual discharge and by a
partial duration series of average daily flow Results
from the former method indicated a subannual recur
rence interval for bankfull but was inconclusive in
estimating values with any more precision The lad
der method estimated a 0 19 year average recurrence
interval for the Coastal Plain with a 0 11 to 0 31 year
range Future development of regional bankfull
hydraulic relationships within other low gradient
regions along the Southeast Atlantic and Gulf Coast
should consider this lower range of naturally occur
ring recurrence intervals In particular the step wise
procedures guiding the development of regional
hydraulic geometry relationships (Leopold 1994 Ros
gen 2001) should be refined to include an adjusted
range of bankfull recurrence intervals Findings from
the current study suggest that when the Log Pearson
Type III distribution of peak annual discharge is
unable to discern recurrence intervals below one year
a 0 1 to 0 3 year interval should be chosen using a
partial duration series of daily discharge
Development of additional bankfull hydraulic
geometry relationships may be possible for the low
lands swamps and floodplains subtype of the MACP
ecoregion within the Coastal Plain of North Carolina
Appropriate delineation for regional relationship
development should give close consideration to water
surface slopes localized geology and water storativity
within the watershed With bankfull hydraulic geome
try relationships developed for all of North Carolinas
physiographic provinces a more spatially continuous
statewide relationship might be possible through a
deterministic modeling effort that integrates all data
sets
LITERATURE CITED
Andrew E D 1980 Effective and Bankfull Discharges of Streams
in the Yampa River Basin Colorado and Wyoming Journal of
Hydrology 46 311 330
Arcement Jr G J and V R Schneider 1989 Guide for Selecting
Manning s Roughness Coefficients for Natural Channels and
Floodplains U S Geological Survey Water Supply Paper 2339
38 pp
Brinson M M 1993 A Hydrogeomorphic Classification for Wet
lands Wetlands Research Program Technical Report WRP DE
4
Carling P 1988 The Concept of Dominant Discharge Applied to
Two Gravel Bed Streams in Relations to Channel Stability
Thresholds Earth Surface Processes and Landforms 13 355
367
Castro J M and P L Jackson 2001 Bankfull Discharge Recur
rence Intervals and Regional Hydraulic Geometry Relation
ships Journal of the American Water Resources Association
37(5) 1249 1262
Chow V T 1964 Handbook of Applied Hydrology McGraw Hill
New York New York
Cowan W L 1956 Estimating Hydraulic Roughness Coefficients
Agricultural Engineering 37 473 475
Dalrymple T 1960 Flood Frequency Analyses Manual of Hydrol
ogy Part 3 Flood Flow Techniques U S Geological Survey
Water Supply Paper 1543 A
Dunne T and L B Leopold 1978 Water in Environmental Plan
rang W H Freeman Co San Francisco California
Emmett W W 1972 The Hydraulic Geometry of Some Alaskan
Streams South of the Yukon River U S Geological Survey
Open File Report 102 pp
Emmett W W 1975 The Channels and Waters of the Upper
Salmon River Area Idaho U S Geological Survey Professional
Paper 870 A 116 pp
Griffith G E and J Omermk 2000 Draft Level III and IV of
North Carolina U S Geological Survey Map and Description
Reston Virginia Scale 1 250 000
Gordon N D T A McMahon and B L Finlayson 1992 Stream
Hydrology An Introduction for Ecologists John Wiley and Sons
New York New York
Harman W A G D Jennings J M Patterson D R Clinton
L 0 Slate A G Jessup J R Everhart and R E Smith 1999
Bankfull Hydraulic Geometry Relationships for North Carolina
Streams In Wildland Hydrology Proceedings Darren S Olsen
and John P Potyondy (Editors) AWRA TPS 99 3 pp 401 408
Harman W A D E Wise M A Walker R Morris M A Cantrell
M Clemmons G D Jennings D Clinton and J Patterson
2000 Bankfull Regional Curves for North Carolina Mountain
Streams In Water Resources in Extreme Environments Pro
ceedmgs Douglas L Kane (Editor) AWRA TPS 00 1 pp 185
190
Hirth Daniel K 1998 North Carolina Rainfall Map North Car
olina Department of Environment and Natural Resources Divi
Sion of Water Quality Groundwater Section Raleigh (map)
IAE (Institution of Australian Engineers) 1987 Australian Rain
fall and Runoff A Guide to Flood Estimation (1) The Institution
of Engineers Australia Barton ACT
Langbein W B 1949 Annual Floods and the Partial Duration
Flood Series EOS Transactions American Geophysical Union
30 879 881
Leopold L B and T Maddock 1953 The Hydraulic Geometry of
Stream Channels and Some Physiographic Implications U S
Geological Survey Professional Paper 252 57 pp
Leopold L B 1994 A View of the River Harvard University
Press Cambridge Massachusetts
JAWRA 870 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

BANKFULL HYDRAULIC GEOMETRY RELATIONSHIPS AND RECURRENCE INTERVALS FOR NORTH CAROLINAS COASTAL PLAIN
Lindley E S 1919 Regime Channels In Proceedings of the Pun
,lab Engineering Congress pp 63 74
Manning R 1891 On the Flow of Water in Open Channels and
Pipes Transactions of the Institution of Civil Engineers of Ire
land 20 161 20
Myrick R M and L B Leopold 1963 Hydraulic Geometry of a
Small tidal Estuary U S Geological Survey Professional Paper
422 B 18 pp
NERC (National Environment Research Council) 1975 Flood
Studies Report London England
NRC (National Research Council) 1999 Improving American River
Flood Frequency Analyses National Academy Press Washing
ton D C
Nixon M A 1959 A Study of Bankfull Discharges of the Rivers of
England and Wales Proceedings of the Institution of Civil Engi
neers 12 157 174
Pope B F G D Tasker and J C Robbins 2001 Estimating the
Magnitude and Frequency of Floods in Rural Basins of North
Carolina (Revised) U S Geological Survey Water Resources
Investigations Report 014207 Raleigh North Carolina
Rosgen D L 1994 A Classification of Natural Rivers Catena
22 169 199
Rosgen D L 1996 Applied River Morphology Wildland Hydrolo
gy Inc Pagosa Springs Colorado
Rosgen D L 1997 A Geomorphological Approach to Restoration
of Incised Rivers In Proceedings of the Conference on Manage
ment of Landscapes Disturbed by Channel Incision S S Y
Wang E J Lanendoen and F D Shields Jr (Editors) Univer
sity of Mississippi Oxford Mississippi ISBN 0 937099 05 8 pp
1222
Rosgen D L 2001 Step Wise Procedure for Calibrating Bankfull
Discharge at USGS Stream Gaging Stations In River Morphol
ogy and Applications Wildland Hydrology Inc Pagosa
Springs Colorado
USGS (U S Geological Survey) 1982 Guidelines for Determining
Flood Flow Frequency Interagency Advisory Committee on
Water Data Bulletin No 17B of the Hydrology Subcommittee
Reston Virginia
USGS (U S Geological Survey) 2001 USGS Water Resources of
North Carolina Daily Streamflow for North Carolina Avail
able at http / /waterdata usgs gov /nc/nwis /discharge Accessed in
June 2001
Wolman M G 1955 The Natural Channel of Brandywine Creek
Pennsylvania U S Geological Survey Professional Paper 271
56 pp
Wolman M G and J P Miller 1960 Magnitude and Frequency of
Forces in Geomorphic Processes Journal of Geology 68 54 74
JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 871 JAWRA

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6

EARTH SURFACE PROCESSES AND LANDFORMS VOL 14 11 -26 (1989)
A MODEL OF CHANNEL RESPONSE IN DISTURBED ALLUVIAL
CHANNELS
ANDREW SIMON
U S Geological Survey Nashville Tennessee U S A
Received 1 November 1986
Revised 21 March 1988
ABSTRACT
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic
changes along modified reaches and tnbutary streams Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance and lowered bed levels by as much as 61 in Following degradation, reaches upstream of
the area of maximum disturbance expenenced a secondary aggradation phase in response to excessive incision and
gradient reduction Aggradation downstream of the area of maximum disturbance reached 012 in per year with the
greatest rates occurring near the stream mouths
The adjustment of channel geometry and phases of channel evolution are charactenzed by six process- onented stages of
morphologic development — premodified constructed degradation threshold aggradation and restabilization Down
cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and
angle of the bank material is exceeded (threshold stage) Channel widening continues through the aggradation stage as the
slough line develops as an initial site of lower bank stability The bank profile develops three dynamic elements (1)
vertical face (70 to 90 ), (2) upper bank (25 to 50 ) and (3) slough line (20 to 25 ) Alternate channel bars form during the
restabilization stage and represent incipient meandenng of the channel
KEY WORDS Unstable channels Degradation Aggradation Channelization Empincal model
INTRODUCTION
Changes imposed on a fluvial system be they natural or man induced tend to be absorbed by the system
through a senes of channel adjustments (Gilbert, 1880 Mackin 1948 Lane, 1955 Hack 1960• Schumm 1973
Bull 1979) Due to the drastic changes in energy conditions imposed by channel dredging, enlarging and
straightening, a rejuvenated condition is established within the fluvial system much like that imposed by uplift
or a natural lowenng of base level The predominant difference is one of time scale Channel adjustments
caused by climatic changes or uplift may be exceedingly slow and progressive practically imperceptible by
man s standards Conversely large scale channel modifications by man result in a sudden and significant
shock to the fluvial system that causes migrating knickpoints and observable morphologic changes This
shortened time scale presents an opportunity to document successive process - response mechanisms through
the course of fluvial adjustment over time and space Channel modifications from 1959 to 1978 through much
of three mayor nver systems in West Tennessee create a natural laboratory for the study of channel
adjustments in rejuvenated fluvial networks (Table I)
PURPOSE AND SCOPE
The U S Geological Survey in cooperation with the Tennessee Department of Transportation (TDOT) began
this study in 1981 as a result of bridge failures on modified channels in West Tennessee The purpose was to
0197 - 9337/89/010011- 16$08 00
© 1989 by John Wiley & Sons Ltd
k

F
12 A SIMON
document and empirically model stream channel changes and process- response mechanisms following large
scale channelization projects in West Tennessee TDOT was interested in obtaining estimates of channel
changes in alluvial channel morphology throughout West Tennessee for the purpose of designing adequate
river crossing structures
BACKGROUND
West Tennessee is an area of 27500 km' bounded by the Mississippi River on the west and the Tennessee
River divide on the east (Figure 1) The region is characterized by unconsolidated highly erosive formations
predominantly of Quaternary age (U S Department of Agriculture 1980) The major rivers of the region
(Obion Forked Deer and Hatchie) flow in channels comprising medium sand beds and silt -clay banks In
contrast, many small tributary streams flow through extensive silt deposits of reworked Wisconsin loess and
have strikingly similar bed and bank material properties (Simon, in press) Those tributaries (such as Cub
and Porters Creeks) that head in the eastern sandier Tertiary and Cretaceous formations flow on medium
sand beds A lack of bedrock control of base level assures unrestricted bed level adjustment
Accounts of river conditions following land clearing throughout the region (mid to late 1800s) describe
extremely sinuous sluggish sediment choked streams that frequently overflowed their banks for 3 to 10 days
at a fame (Hidinger and Morgan 1912) Tlus channel infilling, attributed to mayor deforestation and severe
upland erosion in the late 1800s prompted channel dredging and straightening in West Tennessee near the
EXPLANATION ^Y
Basin boundary ,o
Clearing and snagging
Channelization
Year work was completed
0
0
35
Base from U S Gedrdcal Siavey
State base map 1967
revised 1973
Figure 1 Extent of channel modifications in West Tennessee

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 13
turn of the century The most recent channel modification programme in West Tennessee occurred between
1959 and 1978 and is the subject of this study (Table I) The modified channels listed in Table I by no means
include all of the modified streams in the region but were selected to represent a spectrum of West Tennessee
alluvial channels based on size location and bed and bank properties and to represent the magnitude and
extent of modifications Channel lengths were shortened as much as 44 per cent gradients were increased as
much as 600 percent and beds were lowered as much as 5 2 m (Simon in press)
DATA COLLECTION
Channel morphology data collected from 1982 to 1984 were compared to previous surveys to determine
channel changes with time Sources of these earlier surveys include the U S Geological Survey Army Corps of
Engineers Soil Conservation Service and the Obion- Forked Deer Basin Authority These data consist of bed
elevations and gradients channel top widths and channel lengths before during, and after modification
Where available gauging station records were used to record annual changes in the water surface elevation at
a given discharge Changes in the stage - discharge relation imply similar changes on the channel bed and can
be used to document bed level trends (Blench 1973 Wilson 1979 Robbins and Simon 1983 Williams and
Wolmdn 1984)
The identification and dating of various geomorphic surfaces were diagnostic in determining relative
stability of a reach and the status of bank slope development (Simon and Hupp 1986 Hupp and Simon
1986) Data collection involved dating of riparian vegetation (1) on newly stabilized surfaces to determine the
timing of initial stability for that surface and (2) on unstable bank surfaces to estimate rates of bank retreat
BED LEVEL ADJUSTMENT
Adjustment of the channel bed at a site follows channel modifications immediately and can be described
through time Channel bed adjustments through time (irrespective of the temporal and spatial scales applied)
Table I Type extent and dates of recent channel modifications on studied streams (1959 -1978)
Basin
Stream
Type of modification
Length
(km)
Dates
Obion
Obion River
Enlarging and straightening
751
1959 -66
Clearing and snagging
68
1976
Enlarging and straightening
—
1974 -77
North Fork
Enlarging and straightening
175
1967
Obion River
Clearing and snagging
174
1974 -76
Hoosier Creek
Enlarging and straightening
119
1967
Rutherford Fork
Enlarging
119
1967
Obion River
Clearing and snagging
288
1973 -78
South Fork
Enlarging
96
1967 1969
Obion River
Clearing and snagging
275
1976 -78
Forked Deer
North Fork Forked
Enlarging and straightening
69
315
1973
1974 -77
Deer River
Clearing and snagging
21 1
1976 -78
Pond Creek
Clearing and snagging
South Fork Forked
Enlarging and straightening
71
1969
Deer River
Clearing and snagging
365
1973 -77
Meridian Creek
Enlarging and straightening
26
1959?
Enlarging and straightening
84
1969
Enlarging
26
1969
Cane
Cane Creek
Enlarging and straightening
520
1970
(lower Hatchie)
Enlarging and straightening
20-9
1978
Hyde Creek
Enlarging and straightening
13
1970
Upper Hatchie
Cub Creek
Enlarging and straightening
156
1970
Porters Creek
Enlarging and straightening
344
1972

14 A SIMON
are best described mathematically by nonlinear functions which asymptotically approach a condition of
mimmum variance The description of channel adjustment and evolution by nonlinear decay functions is well
documented (Schumm and Lichty 1965 Graf 1977 Bull 1979 Hey 1979 Robbins and Simon 1983)
However there seems to be considerable disagreement as to the mathematical form of the function Graf
(1977) used exponential functions to describe the relaxation time necessary to achieve equilibrium following
a disturbance Simon and Robbins (1987) used similar equations to model gradient adjustment through time
Williams and Wolman (1984) found that hyperbolic functions were appropriate for describing degradation in
alluvial streams downstream from dams In this study both exponential and power equations were initially
fitted to the observed data. The power function gave consistently superior matches of the empirical data and
was therefore used to describe bed level adjustment through time The general form is
E =a(tr (1)
where E= elevation of the bed for a given year in metres above sea le,,el,
a= coeflRcient determined by regression, representing the premodified elevation of the bed in metres
above sea level
t= time since beginning of adjustment process in years where to= 10 and
b = dimensionless exponent determined by regression and indicative of the nonlinear rate of change
on the bed
Trends of bed level change through time at gauging stations (where bed elevation is measured every 4 to 6
weeks) having penods of record of up to 20 years (Table I1) served to support the well known concept of
nonlinear adjustment (Figure 2) Once it was established that power functions provided the best fit to this
gauged data, data from periodically surveyed sites were similarly fitted (Figure 2) Sites with just two recorded
82
at
1n
no
2
Z
_? 79
n
Ye
Note: Denotes specific gage data sod
e n South Fork Forked Dee River
River kilometer = 12 71
e
0
0
r =094
e
6
y 1964 1968 1972 1976 1980 1984
� 89
O
m
Q
O �
Q
87
W
Be
85
84
A Rutherford Fork Obion River
River kllometa = 7 88
r = 0 93
e
r =096
1958 1962 1966 1970 1974 1978 1982 1986
143
0
IQ
e
141
0
140
139 r =100
138
Porters Crook
River kilameter = 27 51
137 ` '
1970 1974 1978 1982 1986
90
89
Be
87
86
85
84
83 F
82 L
1862
e South F rk Oblon RI or
e Rl or Idiom for = 9 33
0
X, e
ee
r =088
r =093
1965 1970 1974 1978 1982 1986
Figure 2 Examples of fitting power equations to degradation and aggradation trends through time
te1

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
Table II Sites with calculated aggradation ( +b) and degradation ( —b)
15
Stream
b
n
r2
RKM
To
Stream
b
n
r2
RKM
To
Cane Creek
— 001620
7
0.99
2389
1969
Obion River
— 002220
10
095
110.22
1965*
00168
2
—
2389
1980
00463
10
74
110.22
1974*
—02022
4
1-00
20.24
1969
00235
16
76
10008
1968*
01052
2
—
2024
1980
00908
19
93
8640
1965*
—03300
3
100
1446
1869
00518
15
84
5503
1963*
00770
2
—
1446
1980
00585
16
74
3347
1960*
—03131
2
—
1009
1969
00352
2
—
1009
1980
Pond Creek
—00828
5
81
1829
1977
—04126
3
91
653
1969
—00799
4
84
1580
1977
—02011
4
92
406
1969
—01233
4
97
1178
1977
00835
2
—
406
1980
—00900
5
79
171
1977
Cub Creek
—00243
3
69
1113
1969
—00342
3
87
9.22
1969
—00565
4
88
348
1969
Porters Creek
—01069
6
100
2751
1971
—00905
5
91
248
1969
—01320
7
99
1802
1971
00272
2
—
248
1976
--00578
6
100
1430
1971
Hoosier Creek
—00843
3
100
829
1967
Rutherford Fork
Obion River
00149
19
60
4811
1965*
—01130
4
94
481
1966
—00317
4
91
2880
1977
—02081
3
67
88
1965
—00493
3
1-00
2446
1977
00274
2
—
88
1968
—00991
4
79
1673
1972
—02630
3
99
02
1965
00356
4
99
1673
1977
—01728
9
93
788
1965*
00433
10
88
788
1974*
Hyde Creek
00281
2
—
381
1975
South Fork
Forked Deer
River
— 00895
6
59
4441
1976
— 00737
2
—
381
1969
— 00950
10
92
2623
1974*
—01070
4
92
222
1969
— 00978
5
76
2140
1969
— 01380
3
99
119
1969
— 01264
5
96
1915
1969
—02050
4
00
-02
1969
—01630
15
94
12 71
1969*
01180
13
92
5 31
1969*
Meridian Creek
—00326
3
99
594
1965
—00580
4
98
473
1964
South Fork
Obion River
00133
13
90
5535
1969*
— 00341
3
99
241
1969
— 00054
4
26
4570
1972
— 00190
3
99
154
1967
— 00238
6
50
3733
1972
North Fork
Forked Deer
River
— 00740
4
95
3846
1977
— 00661
7
90
3089
1977*
—01076
5
52
3247
1974
— 00573
5
87
27 -03
1972
—00839
4
96
3028
1978
— 00932
4
94
1834
1972
—01720
10
95
8 53
1973*
— 02430
12
87
933
1965*
— 02297
3
87
616
1972
00544
8
88
933
1975*
North Fork
Obion River
00111
15
69
5937
1969*
— 00206
2
—
4248
1979
— 00490
2
—
3395
1975
— 00372
13
80
2896
1972*
— 01240
6
93
1583
1965*
— 02470
4
85
949
1965*
00303
5
89
949
1967*
Note b nonlinear gradation rate n, number of observations r2 coefficient of determination RKM river kilometre; To, start of observed
gradation process, * specific gauge data used, where n is low statistical significance may be limited)

16 A SiMON
bed elevations were Included because of confidence in the general trend of adjustment at a site and to
increase areal coverage of the data network
The initial response of a channel reach (degradation or aggradatlon) depends on the location of the reach
relative to the Imposed channel modifications Reaches located upstream from the disturbance will eventually
go through an initial phase of general degradation, followed by a period of general aggradation in these cases
the data set was split, and Equation 1 was fitted to the degradation and aggradation periods separately The
last data point of the degradation curve (lowest elevation) was also used as to for the aggradation curve
The exponent b (Equation 1) is a measure of the nonlinear rate of degradation (negative b) or aggradation
(positive b) and was used as the primary variable in identifying longitudinal adjustment trends Values of b
used in this study are listed by stream in Table II The data described by Equation 1 imply that bed level
response at a site is initially rapid and then diminishes as bed elevation asymptotically approaches a condition
of no net change (Begin et al 1981)
Because adjustments of channel width are functionally related to changes that occur on the channel bed
(Table III) a bed level model was developed and is summarized using the Obion River system as an example
(Figure 3) The area of maximum disturbance (AMD) is defined as the upstream terminus of channel work
However if a stream is channelized throughout its length stream power is greatest at the mouth and the
AMD is located there (Simon in press)
Stream reaches are divided into the following four groups based on the location of the reach relative to the
Imposed AMD in the fluvial network and the dominant process occurring on the channel bed (1)
downstream aggradation (2) migrating degradation (3) secondary aggradation and (4) premodified
aggradation These reach types in the Obion River drainage system are represented in Figure 3 and denote not
only temporal data at sites in the network but also longitudinal bed response through time with distance from
the AMD
Maximum amounts of degradation (largest negative b values) occurred just upstream of the AMD (as a
response to the significant increase in stream power imposed by the channel work here in 1967 A in Figure 3)
and decreased nonlinearly with distance upstream (curve C in Figure 3) Note that curve C also represents the
headward migration of the degradation process and therefore has a temporal as well as spatial component (To
in Table xI) Rates of migration on the Obion River Forks are 16 km year-' Degradation occurred for 10 to
15 years at sites just upstream of the AMD and has lowered bed levels as much as 61 in Effects of the imposed
disturbance decreased with distance upstream resulting in minimal degradation rates at about river kilometre
150 (b = 0 0) The absence of net erosion or deposition on the channel bed (b = 0 0) is analagous to Bull s (1979)
threshold of critical stream power Further upstream (E in Figure 3� channel beds of the Obion River system
(including upstream reaches of the North South and Rutherford Forks) remain unaffected by the
downstream channel work and aggrade at low background rates
Sites downstream of the AMD (line B in Figure 3) aggraded immediately following channel modification
with material delivered from eroding reaches upstream Maximum recorded rates of aggradation are
0 12 m year -' close to the mouth of the Obion River Aggradation near D in Figure 3 occurs at previously
degraded sites where gradient has been significantly reduced by incision and knickpoint migration Flows
become incapable of transporting the greater bed material loads being generated from upstream channel
Table III Coefficient of detemunation (r2) for
relations between changes in top width (D W) and
the absolute value of nonlinear degradation rates
(b)
Basin
r2
n
Cane Creek
0.49
9
Forked Deer River
0.60
13
Obion River
080
18
Upper Hatclue River
0.64
7

0 010
V) 0 005
W
U
0 0 000
d
Z
O -0005
Q
-0010
O
tL
O
-0015
Q
W -0020
o_
Q
Z -0025
Z
Z —0 030
L-
0
cr- —0035
O
a
Ei
0 -0 040
Z
-0045
25
CHANNEL RESPONSE iN DISTURBED ALLUVIAL CHANNELS
B
■
THRESHOLD OF CRITICAL STREAM POWER ■
D■ _
— -' —
E
aw
■
■
C
A Area of maximum
disturbance (1967)
B Line representing downstream
aggradetion
C Curve representing migrating
degradation
A
D Location of secondary
aggradation
E Location of premodified
aggrodation
■ Observed data (b)
45 65 as 105 125 145 165
DISTANCE ABOVE MOUTH OF OBION RIVER IN KILOMETERS
Figure 3 Model or bed level response for the Obion River system
17
beds The channel aggrades and thereby increases gradient and transporting capacity Schumm and Parker
(1973) describe similar results in an experimental watershed This secondary aggradation migrates headward
with time and is apparently a response to excessive lowering (overadlustment) by the degradation phase
(Simon m press) Hey (1979) and Alexander (1981) similarly argue for alternating phases of degradation and
aggradation following rejuvenation of a channel
With the short time scales involved in the application of the disturbance and the channels consequent
response the evolution of channel form over time and space can be monitored Numerical results such as
those plotted in Figure 3 can be derived for most alluvial systems where even limited data are available
Results could be applied over longer time scales with the inclusion of dendrochronologic data (as in Graf
1977) or to experimental and conceptual models of channel evolution such as Schumm and Parker (1973) and
Hey (197% respectively it has been shown that bed level adjustment does follow nonlinear trends both over
time at a site (Figure 2) and over time with distance upstream (Figure 3)
WIDTH ADJUSTMENT AND BANK SLOPE DEVELOPMENT
Channel widening by mass wasting processes is common in adjusting channels in West Tennessee Bank
failure in the loess derived alluvium is induced by the overheightening and oversteepening of the bank by
degradation and by undercutting and seepage forces at the toe of the bank (Table ii1) Piping and tension
cracks in the bank materials enhance bank failures by internally destabilizing the bank (Simon in press)
Channel bank material of West Tennessee streams is completely alluvial comprised of loess- denved
Quaternary sediments which are predominantly nonplastic dispersive silts of low cohesion (Simon in press)
In situ shear strength tests confirm the existence of low cohesive strengths generally less than 14 kilopascals
(k Pa) These highly erodible materials tend to maintain high moisture contents due to the relatively slow
downward adjustment of the water table The mean degree of saturation for studied channel banks is

18 A. SIMON
91 per cent (182 tests) even though most samples were taken during periods of low flow (Simon and Hupp
1987) Moderate nses in river stage are sufficient to complete saturation of the channel banks and result in
mass failures upon recession of river stage Figure 4 shows the frequency distributions of the primary soil
mechanics variables cohesion friction angle and field density Note that the histograms do not display a
normal distribution, but are markedly skewed towards lower strength characteristics Friction angle and
cohesion values vary within accepted limits for the types of materials tested (Lohnes and Handy 1968)
Maximum values of cohesion represent tests taken at depth in localized clay rich deposits Statistical
properties of the soil mechanics data are given in Table IV
Changes in channel width following the most recent mayor modifications of West Tennessee stream
channels are summarized in Table V Mean widening data serve only to provide a point of reference as to
magnitude and variability of width changes Minimum and maximum values of channel widening (Table V)
do reflect a realistic range of width changes throughout the stream lengths studied
�o
w
eo
40
U
IN)
m
i3
40
� w
U
u
ss
o
to
a!i "
is
e
L
s C s A
COHESION DI WIDPASCALS
a
�a
io
w
W
w
so
10
m
MCnON AMML IN DEGREES
is a O a M
nEW Dowy Di ioWNEMNS
Figure 4 Frequency distributions of soil mechanics data for studied streams
Table IV Statistical properties of selected soil mechanics data
[kPa, kdopascals, kN kilonewtons]
Standard
n
Mean
error
Minimum
Maximum
Cohesion (kPa)
168
859
059
014
543
Friction angle
168
301
062
830
424
Field Density (kN)
182
186
022
113
481
[kPa, kdopascals, kN kilonewtons]

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 19
Table V Range of changes in channel top width (DW) by stream
Years since most
DW recent major
(m) channelization
Stream Mean Min Max n (from 1983)
Cane Creek
182
14
25
6
13
Cub Creek
21
0.0
67
4
13
Hoosier Creek
142
70
2
3
16
Hyde Creek
31
0.3
54
3
18
Meridian Creek
34
24
43
2
16
North Fork Forked Deer River
10.3
30
25
4
10
North Fork Obion River
10.4
0.6
25
5
16
Obion River
378
20
59
4
17
Pond Creek
29
21
43
4
5
Porters Creek
77
37
12
3
11
Rutherford Fork Obion River
72
0.0
18
5
14
South Fork Forked Deer River
136
49
27
6
14
South Fork Obion River
119
0.0
36
7
14
Processes and stages of bank retreat and slope development
The processes and successive forms of bank retreat and bank slope development reflect the Interaction of
hillslope and fluvial processes Interpretations of these processes are based largely on bed level adjustment
trends and botanical evidence A complete cycle of slope development from the premodified condition
through stages of adjustment to the eventual reestablishment of stable bank conditions assumes that
degradation is of sufficient magnitude to instigate unstable bank conditions and a 20- to 40 -year period of
mass wasting However in grossly unstable channels comprised of silt -clay alluvium much longer amounts of
time may be required for the complete restabilization of the channel The Idealized representation of six stages
of bank retreat and bank slope development (Figure 5) represent distinguishable bank morphologies
characteristic of the various reach types descnbing bed level adjustment (Table VI) By applying a space for
time substitution longitudinal variation in bank slope development can then be used to denote morphologic
change through time Sand bed channels will require 20 to 40 years of channel adjustment to pass through the
first five stages of the model In these types of channels a total of 50 to 100 years is assumed necessary for the
restabilization of the channel banks (stage VI) and for the development of incipient meanders
Premodified Stage (1) Premodified bank conditions are assumed to be the result of natural fluvial
processes and land use practices Bank failure by mass wasting processes generally does not occur and banks
are considered stable Data referring to stable bank conditions along West Tennessee channels are available
from the literature agency work plans the upstream most reaches of present day (1987) adjusting networks
and the nonchannelized Hatchie River
Premodified bank conditions (stage I in Figure 5) are characterized generally by low angle slopes (20 to
25 ) convex upper bank and concave lower bank shapes and established woody vegetation along the top
bank and downslope towards the low flow channel Channel reaches with these characteristic bank
conditions generally have width -depth ratios (as measured from the flood plain surface) between 6 and 10
and narrow slowly with time due to limited aggradation and bank accretion
Premodified channel widths (prior to 1915) along the downstream ends of the forks of the Obion and
Forked Deer Rivers ranged from 15 to 28 m depending on drainage area (Morgan and McCrory 1910
Hidinger and Morgan, 1912) The present premodified sites (upstream most) on the forks of the Obion River
are approximately 23 m wide
Constructed Stage (11) Construction of a new channel involves reshaping the existing channel banks or
repositioning the entire channel In either case the banks are generally steepened heightened, and made linear
(stage iI in Figure 5) West Tennessee channels are generally constructed as trapezoids with bank slopes

20 A SIMON
1
4y� II III Ilia
PREMODIFIED STAGE CONSTRUCTED STAGE Undercutung �\
DEGRADATION STAGE
1V t IVa
-- Vertical face —4
7090 r �— Previous profile
Slab and rotational \
failures
Previous Upper bank
Pop out failures ~ profile 25 50 ��
Degraded channel
THRESHOLD STAGE bottom
V
Vertical face --\
70 90 t — Previous profile
Upper bank \
25 40°
Slough line Fluvial deposition
20 25
AGGRADATION STAGE
VI Via
— — — Substantial bed _ Substantial bed level
Vertical face level recovery recovery
70 90 Convex Moderately dispersive
Upper bank \ Non dispersive shape materials
25 35 materials \
Fluvial deposition Fluvial deposition
Slough line 20
Note Scale Is relative
RESTABILIZATION STAGE
Figure 5 The six stages of bank slope development
ranging from 18 to 34 Channel widths are Increased and vegetation is removed in order to increase channel
conveyance It is appropriate to consider the constructed condition (stage iI) as a transition from the stable,
premodified state to the more unstable degradation stage when further heightening and steepening of the
banks occur by fluvial processes
Degradation Stage (III) The degradation stage (stage III in Figure 5) is characterized by rapid erosion of
the channel bed and the consequent increase in bank heights Downcutting generally does not steepen bank
slopes directly but results in bank angles that approximate the angle of internal friction (Skempton 1951
Carson and Kirkby 1972 Simon and Hupp 1986) Steepening occurs when moderate flows attack basal
surfaces and remove toe material (stage I1Ia in Figure 5) Pop out failures due to excess pore pressure at the
bank toe similarly cause steepening Ideally widening by mass wasting does not occur because the critical
bank height and angle have not yet been exceeded Bed degradation with constant channel width (Pickup,
1975) is thus possible in these materials if the amount of incision is limited
The degradation stage is probably the most important in determining the magnitude of channel widening
that will occur because the amount of incision partly controls the bank failure threshold Sites close to the
AMD (where degradation is greatest) are therefore expected to widen appreciably
Threshold Stage (IV) Stage IV (Figure 5) is the result of continued degradation and basal erosion that
further heightens and steepens the channel banks The critical bank height is exceeded and bank slopes and
shapes become the product of mass wasting processes

C
E
a
0
u
{v
CL
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ao
m
in
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F
CHANNEL
RESPONSE IN DISTURBED ALLUVIAL CHANNELS
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22 A SIMON
Degraded reaches of West Tennessee streams have mean bank angles ranging from 55 to 65 degrees
However toe removal often creates almost vertical banks Given the low cohesive strengths and the ambient
high moisture contents of the channel banks critical bank heights at saturation rarely exceed 5 in (Simon and
Hupp 1987) Degraded reaches of the studied streams having bank heights commonly in excess of 8 in,
therefore fail readily
Slab failures occur due to excessive undercutting and the loss of support for the upper part of the bank
Pop -out failures at the base of the bank due to ground water saturation and excess pore water pressure
further oversteepen the bank and lead to slab failure
Deep seated rotational failures shear along an arc and often become detached from the top bank surface by
piping Water drains into the bank mass along the pipes thereby reducing shear strength and increasing pore -
water pressure and lubrication along the potential failure surfaces Failures can occur as a saturated mass of
bank material that leave 18 to 3 0 in long slickensides along the vertical section of the failure surface (often a
pipe) and come to rest in a lobate form at the base of the bank. The failed material loses shear strength and, in
its saturated state can be removed easily by moderate flows Rotational failures and debris flows under
saturated conditions along overheightened reaches of the Rutherford and South Forks Obion River and Cane
Creek tend not to reduce bank angles as much as rotational failures under drier conditions These failure
mechanisms are considered worst case conditions in dispersive loess derived materials as occurs in the Cane
Creek basin
Rotational failures also occur in less dispersive materials where piping is not as prevalent Banks fall as a
single mass of material along an arc leaving elliptical scars 15 to 60 m long The failed material does not come
to rest at the base of the bank but forms a definable surface higher on the bank profile at slopes ranging from
25° to 50 (stage IV in Figure 5) This surface the upper bank (Simon and Hupp 1986) is often identified by
tilted and fallen vegetation A second definable surface, the vertical face representing the top section of the
mayor failure plane also is developed during the threshold stage (stage IV in Figure 5 and Table VI)
Bank retreat by slab and rotational failures continues through the threshold stage developing the vertical
face and upper bank as two distinguishable bank forms (stage IV in Figure 5) The threshold stage is the first of
two that are dominated by active channel widening (Table VI) Failed material is generally removed by
moderate to high flows, thereby retaining the overheightened and oversteepened bank profile and giving the
banks a scalloped appearance
Aggradation Stage (V) Stage V is marked by the onset of aggradation on the channel bed and often can be
identified by sand deposited on bank surfaces Bank retreat dominates the vertical face and upper bank
sections because bank configurations still exceed the critical conditions of the material The failed material on
the upper bank is subject to secondary low angle slides resulting from frequent wetting by nses in stage and
from the added weight of fluvially deposited materials The low angle failures reduce the angle of the upper
bank and extend it downslope (stage V in Figure 5)
Previously failed material on the upper bank also moves downslope by low angle slides and shows viclence
of fluvial reworking and deposition This combination of mass wasting and fluvial processes creates a low
angle surface (20 to 25 ) termed the slough line extending downslope from the upper bank (Simon and
Hupp 1986) Woody vegetation reestabh§hang on the slough line shows only mild tilting from secondary slips
and can be used to date the timing of renewed bank stability (stage V in Figure 5 Hupp and Simon 1986)
Rates of deposition on the slough line can be obtained by dividing the depths of burial above the root collar of
trees by the age of the trees
Bank stability as indicated by establishing vegetation on the slough line extends upslope away from the
channel with time Bank angles on the upper bank and slough line surfaces continue to flatten through stage V
due to secondary low angle slides and fluvial reworking The range of bank angles given for the vertical face
and the upper bank (Table VI) are in general agreement with values reported in previous studies of unstable
slopes comprised of loess ( Lohnes and Handy 1968) and loess - denved alluvium (Throne et al 1981)
Three classes of slope angles were identified by Lohnes and Handy (1968) in their classic study of Iowa and
West Tennessee loess 10 to 41 degrees 50 to 52 degrees and 68 to 85 degrees The second and third classes
relate to the upper bank and vertical face as described in this report respectively The angles reported by
Lohnes and Handy (1968) for their second class (50 to 52 degrees) are steeper than those observed in West

CHANNEL RESPONSE iN DISTURBED ALLUVIAL CHANNELS 23
Tennessee during this study (25 to 50 degrees) The reason is that with the addition of water from seepage or
from rises in river stage effective strength and therefore slope angles are reduced (Lutton 1974) The
differences reported here suggest that mass wasting and bank slope development along streams of loess
derived alluvium occur by processes similar to those that occur on loess slopes However the addition of
fluvial and fluvial related processes result in slope forms that are consistent but slightly different from those of
upland loess slopes The genesis and range of angles of the slough line further support this argument These
low angle surfaces (20 to 25 degrees) are not explicable in the Lohnes and Handy (1968 p 254) model because
their study did not deal with channels and the processes of fluvial reworking and low angle sliding in
saturated materials of residual strength In fact, the 10 to 41 degree class identified by Lohnes and Handy
(1968) from topographic maps may indeed represent old slough lines emanating downslope from what are
now terraces
Assuming that the critical height of the entire bank is still exceeded and (or) dispersion and piping continue
to weaken the vertical face parallel retreat along the vertical face and flattening of the upper bank and slough
line may continue as the channel creates a new flood plain at an elevation lower tharLthe previous one Highly
disturbed channels such as Cane Creek which is cut through loess derived sediments and lacks a coarse
sediment load for aggradation tend to aggrade extremely slow following degradation The lack of bed level
recovery leads to an extended period when bank profiles exceed the cntical bank height and angle allowing
for continued bank retreat and flood plain development It is estimated that the development of a new and
distinct flood plain on channel such as Cane Creek will occur over 1000 years or more Stage V would then
represent the final stage of bank slope development in these types of channels
In sand bed channels aggradation may be of sufficient magnitude to stabilize the vertical face and upper
bank through a reduction in bank height below the critical height Bed levels of sand bed channels such as the
forks of the Obion and Forked Deer Rivers and the lower reaches of Cub Porters and Meridian Creeks nse
and become stage VI channels over time spans that can approximate 20 to 40 years
The aggradation stage (stage V) of bank slope development occurs first in downstream reaches and
progresses upstream with trends of secondary aggradation A meandering low water thalweg and alternate
channel bars also begin to develop during late stage V further reducing gradient and stream power
Restabilization Stage (Vi) The restabilization stage is marked (1) by a significant reduction of bank heights
by aggradation on the channel bed and (2) by fluvial deposition on the upper bank and slough line surfaces
Bank retreat along the vertical face by intense mass wasting processes subsides because bank heights no
longer exceed cntical heights Woody vegetation extends upslope towards the base of the vertical face and the
former floodplain surface becomes a terrace (stage VI in Figure 5)
In channels where bank material is only moderately resistant and bed level has sufficiently aggraded to
cause more frequent wetting of vertical face the uppermost section of the bank may take a convex shape due
to fluvial reworking and deposition (stage VI in Figure 5) In heavily aggrading reaches of the Obion and
North and South Forks Forked Deer Rivers the floodplam continues to be a conduit for moderately high
flows Woody vegetation is reestablished at the top of the bank and on the flood plain surface
Bank slope development through stage VI assumes either significant bed aggradation which occurs along
downstream reaches of sand bed channels and (or) limited initial downcutting Flattening of the upper bank
and slough line surfaces by secondary slides continues due to weaker residual strengths increased moisture
contents and the additional weight of fluvially deposited material
Summary of slope development and channel evolution
The six stages of bank slope development represent a conceptual model of width adjustment Stages
(premodified constructed degradation threshold aggradation, and restabilization) are induced by a
succession of interactions between fluvial and hillslope processes (Table VI) By associating the six bank slope
development stages with the dominant hillslope and fluvial processes and with characteristic channel forms a
conceptual model of channel evolution over time and space was developed (Table VIi) The model does not
suggest that each adjusting reach will undergo all six stages but implies that specific trends of bed level
response will result in a series of mass wasting processes and definable bank and channel forms However the
L

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CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 25
conceptual framework of the simultaneous retreat of the vertical face and flattening along surfaces below is
supported by the observations of other investigators (Carson and Kirkby 1972, p 184) ¢t
The model described in this report makes the following assumptions (1) there is no local bedrock control of
bed level (2) overadjustment and secondary response are active processes (3) the bed and banks are free to
adjust to imposed changes and (4) successive stages of evolution are not interrupted by other disturbances
Extrapolation of the six stage conceptual model of bank slope development and channel evolution should be
particularly appropriate for areas of the Mississippi embayment and the central United States Application
over a broader geographical area is conceptually justified on the basis that similar processes can create similar �t
forms Variations in time scales and forms from the idealized model will occur due to local bedrock control
and variations in relief soil properties and climatic conditions Still the model reflects the overadlustments
inherent to fluvial response and may be useful in determining expected changes in alluvial channel
morphology over the course of a major adjustment cycle
SUMMARY
Channel modifications between 1959 and 1978 in West Tennessee caused a series of morphologic changes
along modified reaches and tributary streams Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance (AMD) and lowered bed levels by as much as 61 metres Aggradation occurs
downstream of the AMD with the greatest rates near the stream mouths Initially degraded sites experience a
secondary aggradation phase in response to excessive incision and gradient reduction
Adjustments of channel geometry and phases of channel evolution are characterized by six process
oriented stages of morphologic development— premodified constructed degradation threshold aggra
dation and rest ibilization Downcutting and toe removal occur immediately following construction along
stream reaches upstream of the AMD (degradation stage) Bank failure by mass wasting begins during the
threshold stage when the critical height and angle of the bank material is exceeded Top -bank widening
continues concurrently with deposition on the channel bed during the aggradation stage as the slough line
develops and becomes an initial site of lower bank stability The development of the bank profile is defined in
terms of three observable and dynamic surfaces (1) vertical face (70 to 90 ) (2) upper bank (25 to 50 ) and (3)
slough line (20 to 25 ) Alternate channel bars form during the restabilization stage and represent incipient
meandering of the channel The vertical face may concurrently take a convex shape from continual reworking
of materials by stream flow
REFERENCES
Alexander D 1981 Threshold of critical power in streams Discussions and reply Bulletin of the Geological Soc teti of Ant rn a Bulletin,
92,310-312
Begin Z B Meyer D F and Schumm S A 1981 Development of longitudinal profiles of alluvial channels in response to base level
lowering Earth Surface Pricesses 6, 49-68
Bull W B 1979 Threshold of critical power in streams Bulletin of the Geological Society of America part 1 90 453 -464
Carson M A and Kirkby M 1 1972 Hillclope Form and Process Cambridge University Press London 475 pp
Gilbert G K 1880 Report on the geology of the Henry Mountains (2) Geographical and Geological Survey of the Roc k1 M,untann
Reg, n United States Government Printing Office Washington, 170 pp
Graf W L 1977 'The rate law in fluvial geomorphology American Journal of Science 277 178 191
Hack J T 1960 Interpretation of erosional topography in humid temperate regions American Journal of Science 2S&A 80 97
Hey R D 1979 Dynamic process- response model of river channel development Earth Surface Proemes 4, 59 -72
Hidinger L L and Morgan A E 1912 Drainage problems of Wolf Hatchte, and South Fork of Forked Deer Rivers, in west Tenn%=
in The Resources of Tennes%ee Tennessee Geological Survey 2 No 6 231 249
Hupp C R and Simon A 1986 Vegetation and bank slope development Proceedings of the 4th Federal Interagene) Sedimentation
Conference Las Vegas, Nevada, March 1986 2, 5-83 to 5-92
Lane, E W 1955 The importance of fluvial morphology in hydraulic engineering Proceedings of the American Society of C►ul
Enguitering 81 No 745 17
Lohnes R A and Handy R L 1968 Slope angles in friable loess Journal of geology 76, No 3 247 -258
Lutton R 1 1974 Use of loess soil for modeling rock mechanics Miscellaneous Report S 74 28 U S Army Engineer, Waterways
Experiment Station Vicksburg, Mississippi
Mackin 1 H 1948 Concept of a graded river Bulletin of the Geological Society of America Bulletin, 59 463 511
Morgan A E and McCrory S H 1910 Drainage of lands overflowed by the North and Middle Forks of the Forked Doer River and the

26 A SIMON
Rutherford Fork of the Obion River in Gibson County Tennessee Bulletin of the Tennessee State Geological Survey 3-B, 17-43
Pickup G 1975 Downstream variations in morphology flow conditions and sediment transport in an eroding channel Zeitschrift fur
Geomorphologie 19 443-459
Robbins, C H and Simon A 1983 Man induced channel adjustment in Tennessee streams United States Geological Survey Water
Resources Investigations Report 914098 129
Schumm, S A. 1973 Geomorphic thresholds and the complex response of drainage systems in Monsawa, M (Ed ), Fluvial
Geomorphology Binghamton State University of New York 229 -310
Schumm, S A. and Lichty R W 1965 `Time, space and causality in geomorphology American Journal of Science 263 110 -119
Sehumm, S A and Parker R S 1973 Implications of complex response of drainage systems for Quaternary alluvial stratigraphy
Nature 243 99-100
Simon, A In press Gradation processes and channel evolution in modified West Tennessee streams process, response and form United
States Geological Survey Professional Paper 1470
Simon, A and Hupp C. R 1986 Channel widening characteristics and bank slope development along a reach of Cane Creek, West
Tennessee in Subitzsky Seymour (Ed.) Selected Papers in the Hydrologic Sciences, United States Geological Survey Water Supply
Paper 2290 113 -126
Simon, A and Hupp C. R 1987 Geomorphc and vegetative recovery processes along modified Tennessee streams an interdisciplinary
approach to disturbed fluvial systems Forest Hydrology and Watershed Management International Association of Hydrologic
Sciences, Publication, 167 251 -26Z
Simon, A and Robbins, C H 1987 Man induced gradient adjustment of the South Fork Forked Deer River West Tennessee
Environmental Geology and Water Sciences 9 No 2 109 -118
Skemtpton, A. W 1953 `Soil mechanics in relation to geology Proceedings of Yorkshire Geological Society 29 33-62
Thorne:, C R Murphey J B and Little, W C 1981 Bank stability and bank material properties in the bluflline streams of nortfiwest
Mississippi Stream Channel Stability United States Department of Agriculture Sedimentation Laboratory Oxford Mississippi,
Appendm D 257 pp
United States Department of Agriculture 1980 'Summary report final Obion- Forked Deer River Basin Tennessee Sod Conservation
Service 43 pp
Williams, G P and Wolman, M G 1984 Downstream effects of dams on alluvial rivers U.S Geological Survey Professional Paper 1286
83 pp
Wilson, K V 1979 Changes in channel characteristics 1938 -1974 of the Homodutto River and tributaries Mississippi U S Geological
Survey Open File Report 79 -534, 18 pp

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EARTH SURFACE PROCESSES AND LANDFORMS, VOL 14 11 -26 (1989)
A MODEL OF CHANNEL RESPONSE IN DISTURBED ALLUVIAL
CHANNELS
ANDREW SIMON
U S Geological Survey Nashville Tennessee U S A
Received 1 November 1986
Revised 21 March 1988
ABSTRACT
Dredging and straightening of alluvial channels between 1959 and 1978 in West Tennessee caused a series of morphologic
changes along modified reaches and tributary streams Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance and lowered bed levels by as much as 61 in Following degradation reaches upstream of
the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and
gradient reduction Aggradation downstream of the area of maximum disturbance reached 0.12 in per year with the
greatest rates occurring near the stream mouths
The adjustment of channel geometry and phases of channel evolution are characterized by six process - oriented stages of
morphologic development — premodified constructed degradation, threshold aggradation and restabilization Down
cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and
angle of the bank material is exceeded (threshold stage) Channel widening continues through the aggradation stage as the
slough line develops as an initial site of lower bank stability The bank profile develops three dynamic elements (1)
vertical face (70 to 90 ) (2) upper bank (25 to 50 ) and (3) slough line (20 to 25 ) Alternate channel bars form during the
restabilization stage and represent incipient meandering of the channel
KEY WORDS Unstable channels Degradation Aggradation Channelization Empirical model
INTRODUCTION
Changes imposed on a fluvial system be they natural or man induced tend to be absorbed by the system
through a senes of channel adjustments (Gilbert 1880 Mackin 1948 Lane 1955 Hack 1960• Schumm 1973
Bull 1979) Due to the drastic changes in energy conditions imposed by channel dredging, enlarging and
straightening a rejuvenated condition is established within the fluvial system much like that imposed by uplift
or a natural lowering of base level The predominant difference is one of time scale Channel adjustments
caused by climatic changes or uplift may be exceedingly slow and progressive practically imperceptible by
man s standards Conversely large -scale channel modifications by man result in a sudden and significant
shock to the fluvial system that causes migrating knickpoints and observable morphologic changes This
shortened time scale presents an opportunity to document successive process - response mechanisms through
the course of fluvial adjustment over time and space Channel modifications from 1959 to 1978 through much
of three major river systems in West Tennessee create a natural laboratory for the study of channel
adjustments in rejuvenated fluvial networks (Table I)
PURPOSE AND SCOPE
The U S Geological Survey in cooperation with the Tennessee Department of Transportation (TDOT) began
this study in 1981 as a result of bridge failures on modified channels in West Tennessee The purpose was to
0197 - 9337/89/010011 - 16$08 00
© 1989 by John Wiley & Sons Ltd

12 A SIMON
document and empirically model stream channel changes and process- response mechanisms following large
scale channehzation projects in West Tennessee TDOT was interested in obtaining estimates of channel
changes in alluvial channel morphology throughout West Tennessee for the purpose of designing adequate
river crossing structures
BACKGROUND
West Tennessee is an area of 27500 km' bounded by the Mississippi River on the west and the Tennessee
River divide on the east (Figure 1) The region is characterized by unconsolidated highly erosive formations
predominantly of Quaternary age (U S Department of Agriculture 1980) The mayor rivers of the region
(Obion Forked Deer and Hatchie) flow in channels comprising medium sand beds and silt -clay banks In
contrast, many small tributary streams flow through extensive silt deposits of reworked Wisconsin loess and
have strikingly similar bed and bank material properties (Simon in press) Those tributaries (such as Cub
and Porters Creeks) that head in the eastern sandier Tertiary and Cretaceous formations flow on medium
sand beds A lack of bedrock control of base level assures unrestricted bed level adjustment
Accounts of river conditions following land clearing throughout the region (mid to late 1800s) describe
extremely sinuous sluggish sediment choked streams that frequently overflowed their banks for 3 to 10 days
at a time (Hidmger and Morgan, 1912) This channel infilling, attributed to major deforestation and severe
upland erosion in the late 1800s prompted channel dredging and straightening in West Tennessee near the
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Basin boundary ,a
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Figure 1 Extent of channel modifications in West Tennessee

CHANNEL RESPONSE iN DISTURBED ALLUVIAL CHANNELS 13
turn of the century The most recent channel modification programme in West Tennessee occurred between
1959 and 1978 and is the subject of this study (Table 1) The modified channels listed in Table I by no means
include all of the modified streams in the region but were selected to represent a spectrum of West Tennessee
alluvial channels based on size location and bed and bank properties and to represent the magnitude and
extent of modifications Channel lengths were shortened as much as 44 per cent gradients were increased as
much as 600 per cent and beds were lowered as much as 5 2 m (Simon, in press)
DATA COLLECTION
Channel morphology data collected from 1982 to 1984 were compared to previous surveys to determine
channel changes with time Sources of these earlier surveys include the U S Geological Survey Army Corps of
Engineers Soil Conservation Service and the Obion- Forked Deer Basin Authority These data consist of bed
elevations and gradients channel top widths and channel lengths before during, and after modification
Where available gauging station records were used to record annual changes in the water surface elevation at
a given discharge Changes in the stage - discharge relation imply similar changes on the channel bed and can
be used to document bed level trends (Blench 1973 Wilson 1979 Robbins and Simon 1983 Williams and
Wolman 1984)
The identification and dating of various geomorphic surfaces were diagnostic in determining relative
stability of a reach and the status of bank slope development (Simon and Hupp 1986 Hupp and Simon
1986) Data collection involved dating of riparian vegetation (1) on newly stabilized surfaces to determine the
timing of initial stability for that surface and (2) on unstable bank surfaces to estimate rates of bank retreat
BED LEVEL ADJUSTMENT
Adjustment of the channel bed at a site follows channel modifications immediately and can be described
through time Channel bed adjustments through time (irrespective of the temporal and spatial scales applied)
Table I Type extent and dates of recent channel modifications on studied streams (1959 -1978)
Basin
Stream
Type of modification
Length
(km)
Dates
Obion
Obion River
Enlarging and straightening
751
195946
Clearing and snagging
68
1976
Enlarging and straightening
—
1974 -77
North Fork
Enlarging and straightening
175
1967
Obion River
Clearing and snagging
174
1974 -76
Hoosier Creek
Enlarging and straightening
119
1967
Rutherford Fork
Enlarging
119
1967
Obion River
Clearing and snagging
288
1973 -78
South Fork
Enlarging
96
1967 1969
Obion River
Clearing and snagging
275
1976 -78
Forked Deer
North Fork Forked
Enlarging and straightening
69
1973
Deer River
Clearing and snagging
315
1974 -77
Pond Creek
Clearing and snagging
21 1
1976 -78
South Fork Forked
Enlarging and straightening
71
1969
Deer River
Clearing and snagging
365
1973 -77
Meridian Creek
Enlarging and straightening
26
1959'
Enlarging and straightening
84
1969
Enlarging
26
1969
Cane
Cane Creek
Enlarging and straightening
520
1970
(lower Hatchie)
Enlarging and straightening
20.9
1978
Hyde Creek
Enlarging and straightening
13
1970
Upper Hatchie
Cub Creek
Enlarging and straightening
156
1970
Porters Creek
Enlarging and straightening
344
1972

14 A. SIMON
are best described mathematically by nonlinear functions which asymptotically approach a condition of
minimum variance The description of channel adjustment and evolution by nonlinear decay functions is well
documented (Schumm and Lichty 1965 Graf 1977 Bull 1979 Hey 1979 Robbins and Simon 1983)
However there seems to be considerable disagreement as to the mathematical form of the function Graf
(1977) used exponential functions to describe the relaxation time necessary to achieve equilibrium following
a disturbance Simon and Robbins (1987) used similar equations to model gradient adjustment through time
Williams and Wolman (1984) found that hyperbolic functions were appropriate for describing degradation in
alluvial streams downstream from dams In this study both exponential and power equations were initially
fitted to the observed data. The power function gave consistently superior matches of the empirical data, and
was therefore used to describe bed level adjustment through time The general form is
E =a(tr (1)
where E= elevation of the bed for a given year in metres above sea le-,el,
a= coefficient determined by regression, representing the premodified elevation of the bed in metres
above sea level
t =time since beginning of adjustment process in years where to= 1-0 and
b= dimensionless exponent determined by regression and indicative of the nonlinear rate of change
on the bed.
Trends of bed level change through time at gauging stations (where bed elevation is measured every 4 to 6
weeks) having periods of record of up to 20 years (Table In, served to support the well known concept of
nonlinear adjustment (Figure 2) Once it was established that power functions provided the best fit to this
gauged data, data from periodically surveyed sites were similarly fitted (Figure 2) Sites with just two recorded
Lim
at
no
Z
Z
1 79
Noty Den to op Ilk: page data sod
e e South fork Forked Deer River
River kilometer = 1271
e
e
0
r =094
A
0
V) 1964 1968 1972 1976 1980 1984
> as
O
m
a
z as
O
a
97
W
86
85
84
e e R thertord Fork Oblon RI or
-
River kilometer = 7 88
r =093
e
r =095
1958 1962 1966 1970 1974 1978 1962 1986
143
e
142
e
141
0
140
138 r = too
138
137 L--
1970
90
89
88
87
88
85
84
83
82
1962 1966 1970 1974 1978 1982 1986
Porters Creek
River kilometer = 27 51
1974 1978 1982 1986
A e So th r rk Ohio Ri
e RI a kilomot r = 9 33
e
0 0
°e o
r =088
r =093
Figure I Examples of fitting power equations to degradation and aggradatlon trends through time
J

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS
Table 11 Sites with calculated aegradation ( +b) and degradation ( —b)
15
Stream
b
n
rZ
RKM
To
Stream
b
n
r2
RKM
To
Cane Creek
— 0.01620
7
0,99
2389
1969
Obion River
— 002220
10
0.95
11a22
1965*
1978
00168
2
—
2389
1980
—01720
00463
10
74
11022
1974*
12
—02022
4
1-00
20,24
1969
87
00235
16
76
100-08
1968*
933
01052
2
—
20.24
1980
00908
19
93
8640
1965*
Obion River 00111
—03300
3
1-00
1446
1869
00518
15
84
5503
1963*
—
00770
2
—
1446
1980
00585
16
74
3347
1960*
1975
—03131
2
—
10.09
1969
—00372
13
80
2896
1972*
00352
2
—
10-09
1980
Pond Creek
—00828
5
81
1829
1977
—04126
3
91
653
1969
1965*
—00799
4
84
1580
1977
00303
—02011
4
92
406
1969
—01233
4
97
1178
1977
00835
2
—
406
1980
—00900
5
79
171
1977
Cub Creek
—00243
3
69
1113
1969
—00342
3
87
922
1969
—00565
4
88
348
1969
Porters Creek
—01069
6
100
2751
1971
—00905
5
91
248
1969
—01320
7
99
1802
1971
00272
2
—
248
1976
--00578
6
1-00
1430
1971
Hoosier Creek
—00843
3
100
829
1967
Rutherford Fork
Obion River
100149
19
60
4811
1965*
—01130
4
94
481
1966
—00317
4
91
2880
1977
—02081
3
67
88
1965
— -00493
3
1-00
2446
1977
00274
2
—
88
1968
—00991
4
79
1673
1972
—02630
3
99
02
1965
00356
4
99
1673
1977
—01728
9
93
788
1965*
00433
10
88
788
1974*
Hyde Creek
00281
2
—
381
1975
South Fork
Forked Deer
River
— 00895
6
59
4441
1976
— 00737
2
—
381
1969
— 00950
10
92
2623
1974*
— 01070
4
92
222
1969
—00978
5
76
2140
1969
— 01380
3
99
1 19
1969
— 01264
5
96
1915
1969
— 02050
4
00
02
1969
—01630
15
94
1271
1969*
01180
13
92
5 31
1969*
Meridian Creek
—00326
3
99
5 94
1965
--00580
4
98
473
1964
South Fork
Obion River
00133
13
90
5535
1969*
— 00341
3
99
241
1969
— 00054
4
26
4570
1972
—00190
3
99
154
1967
—00238
6
50
37 33
1972
North Fork
Forked Deer
River —00740
4
95
3846
1977
—00661
7
90
3089
1977
—01076
5
52
3247
1974
—00573
5
87
2703
1972
—00839
4
96
3028
1978
—00932
4
94
1834
1972
—01720
10
95
853
1973*
—02430
12
87
933
1965*
—02297
3
87
616
1972
00544
8
88
933
1975
North Fork
Obion River 00111
15
69
5937
1969*
—00206
2
—
4248
1979
—00490
2
—
33.95
1975
—00372
13
80
2896
1972*
—01240
6
93
1583
1965*
—02470
4
85
949
1965*
00303
5
89
949
1967*
Note b nonlinear gradation rate n, number of observations, r2 coefficient of determination RKM river kilometre; Tee start of observed
gradation process, * specific gauge data used, where n is low statistical significance may be limited)

16 A SIMON
bed elevations were included because of confidence in the general trend of adjustment at a site and to
increase areal coverage of the data network
The initial response of a channel reach (degradation or aggradation) depends on the location of the reach
relative to the imposed channel modifications Reaches located upstream from the disturbance will eventually
go through an initial phase of general degradation followed by a period of general aggradation in these cases
the data set was split and Equation 1 was fitted to the degradation and aggradation periods separately The
last data point of the degradation curve (lowest elevation) was also used as to for the aggradation curve
The exponent b (Equation 1) is a measure of the nonlinear rate of degradation (negative b) or aggradation
(positive b) and was used as the primary variable in identifying longitudinal adjustment trends Values of b
used in this study are listed by stream in Table II The data described by Equation 1 imply that bed level
response at a site is initially rapid and then diminishes as bed elevation asymptotically approaches a condition
of no net change (Begin et al 1981)
Because adjustments of channel width are functionally related to changes that occur on the channel bed
(Table III) a bed level model was developed and is summarized using the Obion River system as an example
(Figure 3) The area of maximum disturbance (AMD) is defined as the upstream terminus of channel work
However if a stream is channelized throughout its length stream power is greatest at the mouth and the
AMD is located there (Simon in press)
Stream reaches are divided into the following four groups based on the location of the reach relative to the
unposed AMD in the fluvial network and the dominant process occurring on the channel bed (1)
downstream aggradation (2) migrating degradation (3) secondary aggradation and (4) premodified
aggradation These reach types in the Obion River drainage system are represented in Figure 3 and denote not
only temporal data at sites in the network but also longitudinal bed response through time with distance from
the AMD
Maximum amounts of degradation (largest negative b values) occurred just upstream of the AMD (as a
response to the significant increase in stream power imposed by the channel work here in 1967 A in Figure 3)
and decreased nonlinearly with distance upstream (curve C in Figure 3) Note that curve C also represents the
headward migration of the degradation process and therefore has a temporal as well as spatial component (To
in Table 11) Rates of migration on the Obion River Forks are 16 km year - i Degradation occurred for 10 to
15 years at sites just upstream of the AMD and has lowered bed levels as much as 61 in Effects of the imposed
disturbance decreased with distance upstream resulting in minimal degradation rates at about river kilometre
150 (b = 0 0) The absence of net erosion or deposition on the channel bed (b = 0 0) is analagous to Bull s (1979)
threshold of critical stream power Further upstream (E in Figure 3), channel beds of the Obion River system
(including upstream reaches of the North South and Rutherford Forks) remain unaffected by the
downstream channel work and aggrade at low background rates.
Sites downstream of the AMD (line B in Figure 3) aggraded immediately following channel modification
with material delivered from eroding reaches upstream Maximum recorded rates of aggradation are
0.12 in year-' close to the mouth of the Obion River Aggradation near D in Figure 3 occurs at previously
degraded sites where gradient has been significantly reduced by incision and knickpoint migration Flows
become incapable of transporting the greater bed material loads being generated from upstream channel
Table III Coefficient of determination (0) for
relations between changes in top width (D W) and
the absolute value of nonlinear degradation rates
(b)
Basin
r,
n
Cane Creek
0.49
9
Forked Deer River
0.60
13
Obion River
0.80
18
Upper Hatchie River
0.64
7

0 010
a
in 0 005
in
U
90 0 000
0_
Z
O -0005
Q
-0010
C7
t`
O -0015
Q
o✓ —0020
o:
a
Z —0025
J
Z
Z -0 030
L.
O
W -0035
O
Q
U
0 -0040
Z
—0043
25
CHANNEL RESPONSE iN DISTURBED ALLUVIAL CHANNELS
B
■
THRESHOLD OF CRITICAL STREAM POWER ■
p■ _
_ —
E
■
■
C
A Area of maximum
disturbance (1967)
B Una representing downstream
aggradation
C Curve representing migrating
degradation
A
D Location of secondary
aggradation
E Location of premodified
aggrodation
■ Observed data (b)
45 65 as 105 125 U5 165
DISTANCE ABOVE MOUTH OF OBION RIVER IN KILOMETERS
Figure 3 Model of bed level response for the Obion River system
17
beds The channel aggrades and thereby increases gradient and transporting capacity Schumm and Parker
(1973) describe similar results in an experimental watershed This secondary aggradation migrates headward
with time and is apparently a response to excessive lowering (overadjustment) by the degradation phase
(Simon in press) Hey (1979) and Alexander (198 1) similarly argue for alternating phases of degradation and
aggradation following rejuvenation of a channel
With the short time scales involved in the application of the disturbance and the channels consequent
response the evolution of channel form over time and space can be monitored Numerical results such as
those plotted in Figure 3 can be derived for most alluvial systems where even limited data are available
Results could be applied over longer time scales with the inclusion of dendrochronologic data (as in Graf
1977) or to experimental and conceptual models of channel evolution such as Schumm and Parker (1973) and
Hey (1979L respectively It has been shown that bed level adjustment does follow nonlinear trends both over
time at d site (Figure 2) and over time with distance upstream (Figure 3)
WIDTH ADJUSTMENT AND BANK SLOPE DEVELOPMENT
Channel widening by mass wasting processes is common in adjusting channels in West Tennessee Bank
failure in the loess derived alluvium is induced by the overheightening and oversteepening of the bank by
degradation and by undercutting and seepage forces at the toe of the bank (Table IiI) Piping and tension
cracks in the bank materials enhance bank failures by internally destabilizing the bank (Simon in press)
Channel bank material of West Tennessee streams is completely alluvial comprised of loess derived
Quaternary sediments which are predominantly nonplastic dispersive silts of low cohesion (Simon in press)
In situ shear strength tests confirm the existence of low cohesive strengths generally less than 14 kilopascals
(kPa) These highly erodible materials tend to maintain high moisture contents due to the relatively slow
downward adjustment of the water table The mean degree of saturation for studied channel banks is

18 A. SIMON
91 per cent (182 tests) even though most samples were taken during penods of low flow (Simon and Hupp
1987) Moderate rises in river stage are sufficient to complete saturation of the channel banks and result in
mass failures upon recession of river stage Figure 4 shows the frequency distributions of the pnmary soil
mechanics variables cohesion, friction angle and field density Note that the histograms do not display a
normal distribution, but are markedly skewed towards lower strength characteristics Friction angle and
cohesion values vary within accepted limits for the types of materials tested (Lohnes and Handy 1968)
Maximum values of cohesion represent tests taken at depth in localized clay rich deposits Statistical
properties of the soil mechanics data are given in Table IV
Changes in channel width following the most recent mayor modifications of West Tennessee stream
channels are summarized in Table V Mean widening data serve only to provide a point of reference as to
magnitude and variability of width changes Minimum and maximum values of channel widening (Table V)
do reflect a realistic range of width changes throughout the stream lengths studied
70
so
so
w
70
:o
10
ii
40
a
30
3 m
C
20
�i a
10
e
s • m s� n
CANESION IN KIIAP/SCAtS
so
so
7o
so
so
w
so
20
to
FRICTION AN" IN OMRMS
m m v m r
F= OEM" IN KItANEWTONS
Figure 4 Frequency distributions of sod mechanics data for studied streams
Table IV Statistical properties of selected soil mechanics data
Standard
n Mean error Minimum Maximum
Cohesion (kPa)
Friction angle
Field Density (kN)
168 859 0159 014 543
168 30.1 %2 830 424
182 186 0,22 113 481
[kPa, kdopascals; kN kilonewtons]

CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 19
Table V Range of changes in channel top width (DW) by stream
Years since most
DW recent mayor
Stream Mean Min m) Max n c (from 1983)n
(from 1983)
Cane Creek
182
14
25
6
13
Cub Creek
21
0.0
67
4
13
Hoosier Creek
142
70
2
3
16
Hyde Creek
31
03
54
3
18
Meridian Creek
34
24
43
2
16
North Fork Forked Deer River
10.3
10
25
4
10
North Fork Obion River
104
0.6
25
5
16
Obion River
378
20
59
4
17
Pond Creek
29
21
43
4
5
Porters Creek
77
37
12
3
11
Rutherford Fork Obion River
72
0.0
18
5
14
South Fork Forked Dar River
136
49
27
6
14
South Fork Obion River
119
0.0
36
7
14
Processes and stages of bank retreat and slope development
The processes and successive forms of bank retreat and bank slope development reflect the interaction of
hillslope and fluvial processes Interpretations of these processes are based largely on bed level adjustment
trends and botanical evidence A complete cycle of slope development from the premodified condition
through stages of adjustment to the eventual reestablishment of stable bank conditions assumes that
degradation is of sufficient magnitude to instigate unstable bank conditions and a 20 to 40 -year period of
mass wasting However in grossly unstable channels comprised of silt -clay alluvium much longer amounts of
time may be required for the complete restabilization of the channel The idealized representation of six stages
of bank retreat and bank slope development (Figure 5) represent distinguishable bank morphologies
characteristic of the various reach types descnbing bed level adjustment (Table VI) By applying a space for
time substitution longitudinal variation in bank slope development can then be used to denote morphologic
change through time Sand bed channels will require 20 to 40 years of channel adjustment to pass through the
first five stages of the model In these types of channels, a total of 50 to 100 years is assumed necessary for the
restabilization of the channel banks (stage VI) and for the development of incipient meanders
Premodified Stage (I) Premodified bank conditions are assumed to be the result of natural fluvial
processes and land use practices Bank failure by mass wasting processes generally does not occur and banks
are considered stable Data referring to stable bank conditions along West Tennessee channels are available
from the literature agency work plans, the upstream most reaches of present day (1987) adjusting networks
and the nonchannelized Hatchie River
Premodified bank conditions (stage I in Figure 5) are characterized generally by low angle slopes (20 to
25 ) convex upper bank and concave lower bank shapes, and established woody vegetation along the top
bank and downslope towards the low flow channel Channel reaches with these characteristic bank
conditions generally have width -depth ratios (as measured from the flood plain surface) between 6 and 10
and narrow slowly with time due to limited aggradation and bank accretion
Premodified channel widths (pnor to 1915) along the downstream ends of the forks of the Obion and
Forked Deer Rivers ranged from 15 to 28 m depending on drainage area (Morgan and McCrory 1910
Hidinger and Morgan, 1912) The present premodified sites (upstream most) on the forks of the Obion River
are approximately 23 m wide
Constructed Stage (11) Construction of a new channel involves reshaping the existing channel banks or
repositioning the entire channel In either case the banks are generally steepened heightened and made linear
(stage i1 in Figure 5) West Tennessee channels are generally constructed as trapezoids with bank slopes

20
I
PREMODIFIED STAGE
A SIMON
II
CONSTRUCTED STAGE
III Ilia
Undercuthng ,
DEGRADATION STAGE
IV Iva
-- Vertical face
Slab and rotational
7090 \— Previous profile
\
failures
Previous Upper bank
Pop out failures Y profile 25 50 ��
� Degraded channel
THRESHOLD STAGE bottom
V
Vertical face --\
70 90 t-- Previous profile
Upper bank \
25 40
Slough line fluvial deposition
20 25
AGGRADATION STAGE
VI
Vertical face –' —\ Substantial bed
7090 \ level recovery
\ Non - dispersive
Upper bank \ materials
25 35
Fluvial di
Slough line 20
Note Scale is relative
Via
Substantial bed level
\ recovery
Convex \ Moderately dispersive
shape \ materials
iposition Fluvial deposition
RESTABILIZATION STAGE
Figure 5 The six stages of bank slope development
ranging from 18 to 34 Channel widths are Increased and vegetation is removed in order to increase channel
conveyance It is appropriate to consider the constructed condition (stage II) as a transition from the stable,
premodifit:d state to the more unstable degradation stage when further heightening and steepening of the
banks occur by fluvial processes
Degradation Stage (III) The degradation stage (stage III in Figure 5) is characterized by rapid erosion of
the channel bed and the consequent increase in bank heights Downcutting generally does not steepen bank
slopes directly but results in bank angles that approximate the angle of internal friction (Skempton 1953
Carson and Kirkby 1972 Simon and Hupp 1986) Steepening occurs when moderate flows attack basal
surfaces and remove toe material (stage 11Ia in Figure 5) Pop -out failures, due to excess pore pressure at the
bank toe similarly cause steepening Ideally widening by mass wasting does not occur because the critical
bank height and angle have not yet been exceeded Bed degradation with constant channel width (Pickup,
1975) is thus possible in these materials if the amount of incision is limited
The degradation stage is probably the most important in determining the magnitude of channel widening
that will occur because the amount of incision partly controls the bank failure threshold Sites close to the
AMD (where degradation is greatest) are therefore expected to widen appreciably
Threshold Stage (IV) Stage IV (Figure 5) is the result of continued degradation and basal erosion that
further heightens and steepens the channel banks The critical bank height is exceeded and bank slopes and
shapes become the product of mass wasting processes

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22 A SIMON
Degraded reaches of West Tennessee streams have mean bank angles ranging from 55 to 65 degrees
However toe removal often creates almost vertical banks Craven the low cohesive strengths and the ambient
lugh moisture contents of the channel banks critical bank heights at saturation rarely exceed 5 in (Simon and
Hupp 1987) Degraded reaches of the studied streams having bank heights commonly in excess of 8 m,
therefore fail readily
Slab failures occur due to excessive undercutting and the loss of support for the upper part of the bank
Pop out failures at the base of the bank due to ground water saturation and excess pore water pressure
further oversteepen the bank and lead to slab failure
Deep seated rotational failures shear along an arc and often become detached from the top bank surface by
piping Water drains Into the bank mass along the pipes thereby reducing shear strength and increasing pore -
water pressure and lubrication along the potential failure surfaces Failures can occur as a saturated mass of
bank material that leave 18 to 3 0-m long slickensides along the vertical section of the failure surface (often a
pipe) and come to rest in a lobate form at the base of the bank The failed material loses shear strength and, in
its saturated state can be removed easily by moderate flows Rotational failures and debris flows under
saturated conditions along overheightened reaches of the Rutherford and South Forks Obion River and Cane
Creek tend not to reduce bank angles as much as rotational failures under drier conditions These failure
mechanisms are considered worst -case conditions in dispersive loess derived materials as occurs in the Cane
Creek basin
Rotational failures also occur in less dispersive materials where piping is not as prevalent Banks fail as a
single mass of material along an arc, leaving elliptical scars 15 to 60 in long The failed material does not come
to rest at the base of the bank but forms a definable surface higher on the bank profile at slopes ranging from
25° to 50° (stage IV in Figure 5) This surface the upper bank (Simon and Hupp 1986) is often identified by
tilted and fallen vegetation A second definable surface the vertical face representing the top section of the
mayor failure -plane also is developed during the threshold stage (stage IV in Figure 5 and Table VI)
Bank retreat by slab and rotational failures continues through the threshold stage developing the vertical
face and upper bank as two distinguishable bank forms (stage IV in Figure 5) The threshold stage is the first of
two that are dominated by active channel widening (Table VI) Failed material is generally removed by
moderate to high flows, thereby retaining the overheightened and oversteepened bank profile and giving the
banks a scalloped appearance
Aggradation Stage (in Stage V is marked by the onset of aggradation on the channel bed and often can be
identified by sand deposited on bank surfaces Bank retreat dominates the vertical face and upper bank
sections because bank configurations still exceed the critical conditions of the material The failed material on
the upper bank is subject to secondary low angle slides resulting from frequent wetting by nses in stage and
from the added weight of fluvially deposited materials The low angle failures reduce the angle of the upper
bank and extend it downslope (stage V in Figure 5)
Previously failed material on the upper bank also moves downslope by low angle slides and shows . vidence
of fluvial reworking and deposition This combination of mass wasting and fluvial processes creates a low
angle surface (20 to 25 ) termed the slough line extending downslope from the upper bank (Simon and
Hupp 1986) Woody vegetation reestabligh rig on the slough line shows only mild tilting from secondary slips
and can be used to date the timing of renewed bank stability (stage V in Figure 5 Hupp and Simon, 1986)
Rates of deposition on the slough line can be obtained by dividing the depths of burial above the root collar of
trees by the age of the trees
Bank stability as indicated by establishing vegetation on the slough line extends upslope away from the
channel with time Bank angles on the upper bank and slough line surfaces continue to flatten through stage V
due to secondary low angle slides and fluvial reworking The range of bank angles given for the vertical face
and the upper bank (Table VI) are in general agreement with values reported in previous studies of unstable
slopes comprised of loess (Lohnes and Handy 1968) and loess derived alluvium (Throne et al 1981)
Three classes of slope angles were identified by Lohnes and Handy (1968) in their classic study of Iowa and
West Tennessee loess 10 to 41 degrees 50 to 52 degrees and 68 to 85 degrees The second and third classes
relate to the upper bank and vertical face as described in this report respectively The angles reported by
Lohnes and Handy (1968) for their second class (50 to 52 degrees) are steeper than those observed in West

CHANNEL RESPONSE iN DISTURBED ALLUVIAL CHANNELS 23
Tennessee during this study (25 to 50 degrees) The reason is that with the addition of water from seepage or
from uses in river stage effective strength and therefore slope angles are reduced (Lutton 1974) The
differences reported here suggest that mass wasting and bank slope development along streams of loess
derived alluvium occur by processes similar to those that occur on loess slopes However the addition of
fluvial and fluvial related processes result in slope forms that are consistent but slightly different from those of
upland loess slopes The genesis and range of angles of the slough line further support this argument These
low angle surfaces (20 to 25 degrees) are not explicable in the Lohnes and Handy (1968 p 254) model because
their study did not deal with channels and the processes of fluvial reworking and low angle sliding in
saturated materials of residual strength In fact, the 10 to 41 degree class identified by Lohnes and Handy
(1968) from topographic maps may indeed represent old slough lines emanating downslope from what are
now terraces
Assuming that the critical height of the entire bank is still exceeded and (or) dispersion and piping continue
to weaken the vertical face parallel retreat along the vertical face and flattening of the upper bank and slough
line may continue as the channel creates a new flood plain at an elevation lower thanthe previous one Highly
disturbed channels such as Cane Creek which is cut through loess - derived sediments and lacks a coarse
sediment load for aggradation tend to aggrade extremely slow following degradation The lack of bed level
recovery leads to an extended period when bank profiles exceed the critical bank height and angle allowing
for continued bank retreat and flood plain development It is estimated that the development of a new and
distinct flood plain on channel such as Cane Creek will occur over 1000 years or more Stage V would then
represent the final stage of bank slope development in these types of channels
In sand bed channels aggradation may be of sufficient magnitude to stabilize the vertical face and upper
bank through a reduction in bank height below the critical height Bed levels of sand bed channels such as the
forks of the Obion and Forked Deer Rivers and the lower reaches of Cub Porters and Meridian Creeks rise
and become stage VI channels over time spans that can approximate 20 to 40 years
The aggradation stage (stage V) of bank slope development occurs first in downstream reaches and
progresses upstream with trends of secondary aggradation A meandering low water thalweg and alternate
channel bars also begin to develop during late stage V further reducing gradient and stream power
Restabilization Stage (V!) The restabilization stage is marked (1) by a significant reduction of bank heights
by aggradation on the channel bed and (2) by fluvial deposition on the upper bank and slough line surfaces
Bank retreat along the vertical face by intense mass wasting processes subsides because bank heights no
longer exceed critical heights Woody vegetation extends upslope towards the base of the vertical face and the
former floodplain surface becomes a terrace (stage VI in Figure 5)
In channels where bank material is only moderately resistant and bed level has sufficiently aggraded to
cause more frequent wetting of vertical face the uppermost section of the bank may take a convex shape due
to fluvial reworking and deposition (stage VI in Figure 5) In heavily aggrading reaches of the Obion, and
North and South Forks Forked Deer Rivers, the floodplain continues to be a conduit for moderately high
flows Woody vegetation is reestablished at the top of the bank and on the flood plain surface
Bank slope development through stage VI assumes either significant bed aggradation which occurs along
downstream reaches of sand bed channels and (or) limited initial downcutting Flattening of the upper bank
and slough line surfaces by secondary slides continues due to weaker residual strengths increased moisture
contents, and the additional weight of fluvially deposited material
Summary of slope development and channel evolution
The six stages of bank slope development represent a conceptual model of width adjustment Stages
(premodified constructed degradation, threshold aggradation and restabilization) are induced by a
succession of interactions between fluvial and hillslope processes (Table VI) By associating the six bank slope
development stages with the dominant hillslope and fluvial processes and with characteristic channel forms, a
conceptual model of channel evolution over time and space was developed (Table VII) The model does not
suggest that each adjusting reach will undergo all six stages but implies that specific trends of bed level
response will result in a series of mass wasting processes and definable bank and channel forms However the

24
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CHANNEL RESPONSE IN DISTURBED ALLUVIAL CHANNELS 25
conceptual framework of the simultaneous retreat of the vertical face and flattening along surfaces below is
supported by the observations of other investigators (Carson and Kirkby 1972, p 184
The model described in this report makes the following assumptions (1) there is no local bedrock control of
bed level (2) overadjustment and secondary response are active processes (3) the bed and banks are free to
adjust to imposed changes and (4) successive stages of evolution are not interrupted by other disturbances
Extrapolation of the six stage conceptual model of bank slope development and channel evolution should be 1
particularly appropriate for areas of the Mississippi embayment and the central United States Application
over a broader geographical area is conceptually justified on the basis that similar processes can create similar
forms Variations in time scales and forms from the idealized model will occur due to local bedrock control
and variations in relief soil properties and climatic conditions Still the model reflects the overadjustments
inherent to fluvial response and may be useful in determining expected changes in alluvial channel
morphology over the course of a mayor adjustment cycle
SUMMARY
Channel modifications between 1959 and 1978 in West Tennessee caused a series of morphologic changes
along modified reaches and tributary streams Degradation occurred for 10 to 15 years at sites upstream of the
area of maximum disturbance (AMD) and lowered bed levels by as much as 61 metres Aggradation occurs
downstream of the AMD with the greatest rates near the stream mouths Initially degraded sites experience a
secondary aggradation phase in response to excessive incision and gradient reduction
Adjustments of channel geometry and phases of channel evolution are characterized by six process
oriented stages of morphologic development — premodified constructed degradation threshold aggra
dation and restabilization Downcutting and toe removal occur immediately following construction along
stream reaches upstream of the AMD (degradation stage) Bank failure by mass wasting begins during the
threshold stage when the critical height and angle of the bank material is exceeded Top -bank widening
continues concurrently with deposition on the channel bed during the aggradation stage as the slough line
develops and becomes an initial site of lower bank stability The development of the bank profile is defined in
terms of three observable and dynamic surfaces (1) vertical face (70 to 90 ) (2) upper bank (25 to 50 ) and (3)
slough line (20 to 25 ) Alternate channel bars form during the restabilization stage and represent incipient
meandering of the channel The vertical face may concurrently take a convex shape from continual reworking
of materials by stream flow
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Bull W B 1979 'Threshold of critical power in streams Bulletin of the Geological Society of America part 1 90, 453 -464
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26 A SIMON
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Simon A and Robbins, C H 1987 Man induced gradient adjustment of the South Fork Forked Deer River West Tennessee
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