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22.A Hydrologic and Hydraulic Processes
2.B Geomorphic Processes
2.C Physical and Chemical Characteristics
2.D Biological Community Characteristics
2.E Functions and Dynamic Equilibrium
hapter 1 provided an overview of
stream corridors and the many
per-
spectives from which they should be
viewed in terms of scale, equilibrium,
and space. Each of these views can beseen as a “snapshot” of different aspects
of a stream corridor.
Chapter 2 presents the stream corridor in
motion, providin a basic understandin
of the different processes that ma!e the
stream corridor loo! and function the way
it does. "hile Chapter 1 presented still
imaes, this chapter provides “film
footae” to describe the processes, char-
acteristics, and functions of stream corri-
dors throuh time.
Section 2.A: Hydrologic and Hydraulic
Processes
#nderstandin how water flows into and
throuh stream corridors is critical to
restorations. $ow fast, how much, how
deep, how often, and when
water flows are
important basic questions that
must be answered to
Figure 2.1: A stream corridor
in motion. %rocesses, characteris-
tics, and functions shape stream
corridors and ma!e them loo!
the way they do.
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Hydrologic and Hydraulic Pr ocesses 22
ma!e appropriate decisions about
stream corridor restoration.
Section 2.B: Geomorphic Processes
&his section combines basic hydro-
loic processes with physical or
eomorphic functions and charac-teristics. "ater flows throuh
streams but is affected by the
!inds of soils and alluvial features
within the channel, in the
floodplain, and in the uplands. &he
amount and !ind of sediments
carried by a stream larely
determines its equi- librium
characteristics, includin si'e,shape, and profile. (uccessful
stream corridor restoration,
whether active )requirin direct
chanes* or passive )manaement
and removal of disturbance fac-
tors*, depends on an
understandin of how water and
sediment are re- lated to channel
form and function and on what processes are involved with
channel evolution.
Section 2.C: Physical and Chemical
Characteristics
&he quality of water in the stream
corridor is normally a primary ob-
+ective of restoration, either to im-
prove it to a desired condition, or
to sustain it. estoration should
consider the physical and chemical
characteristics that may not be
readily apparent but that are
nonetheless critical to the functions
and processes of stream corridors.
Chanes in soil or water chemistry
to achieve restoration oals usually
involve manain or alterin ele-
ments in the landscape or corridor.
Section 2.D: Biological Community
Characteristics
&he fish, wildlife, plants, and hu-
mans that use, live in, or +ust visit
the stream corridor are !ey ele-
ments to consider in restoration.
&ypical oals are to restore, create,
enhance, or protect habitat to
ben- efit life. t is important tounder- stand how water flows,
how sediment is transported, and
how eomorphic features and
processes evolve however, a
prerequisite to successful
restoration is an under- standin
of the livin parts of the system
and how the physical and chemical
processes affect the streamcorridor.
Section 2.: Functions and
Dynamic !uili"rium
&he si/ ma+or functions of
stream corridors are0 habitat,
conduit, barrier, filter, source,
and sin!.
&he interity of a stream
corridor ecosystem depends on
how well these functions
operate. &his section discusses
these functions and how they
relate to dynamic equilibrium.
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cycle.
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!egetati"e #ype $ Precipitation %nter cepted
Forests
Deciduous 13
Coniferous 28
Crops
Alfalfa 36
Corn 16
Oats
Grasses 1!"2!
Precipitation can do one of three things
once it reaches the earth. $t can return
to the atmosphere% move into the soil%
or run off the earth’s surface into a
stream, la&e, wetland, or other water
body. All three pathways play a role in
determining how water moves into,
across, and down the streamcorridor .
This section is divided into two subsec-
tions. The first subsection focuses on
hydrologic and hydraulic processes in
the lateral dimension, namely, the
movement of water from the land into
the channel. The second subsection
concentrates on water as it moves in the
longitudinal dimension, specifically as
streamflow in the channel.
Hydrologic and HydraulicProcesses Across t#e $tr eamCorridor
'ey points in the hydrologic cycle serve
as organiational headings in this sub-
section
■ $nterception, transpiration, and
evapotranspiration.
■ $nfiltration, soil moisture, and
ground water .
■ " unoff.
-nterception, & ranspiration, and
Evapotranspiration
*ore than two-thirds of the precipita-
tion falling over the +nited #tates evap-
orates to the atmosphere rather than
being discharged as streamflow to the
oceans. This short-circuiting of the
hydrologic cycle occurs because of thetwo processes, interception and transpi-
ration.
%nterception
A portion of precipitation never reaches
the ground because it is intercepted by
vegetation and other natural and con-
structed surfaces. The amount of water
intercepted in this manner is determined
by the amount of interception storage
available on the above-ground surfaces.
$n vegetated areas, storage is a function
of plant type and the form and density
of leaves, branches, and stems (Table
2.1). actors that affect storage in
forested areas include
■ /eaf shape. 0onifer needles hold
water more efficiently than leaves.
1n leaf surfaces droplets run togeth-
er and roll off. 2eedles, however ,
&eep droplets separated.
■ /eaf teture. "ough leaves store more
water than smooth leaves.
■ Time of year. /eafless periods provide
less interception potential in the
canopy than growing periods% howev-
er, more storage sites are created by
leaf litter during this time.
■ 3ertical and horiontal density. The
more layers of vegetation that precip-
itation must penetrate, the less li&ely
it is to reach the soil.
■ Age of the plant community. #ome
vegetative stands become more dense
with age% others become less dense.
The intensity, duration, and fre!uency
of precipitation also affect levels of in-
terception.
Figure 2.3 shows some of the pathways
rainfall can ta&e in a forest. "ainfall at
#a"le 2.1: Percentage o$ precipitation inter%
cepted $or &arious &egetation types.
$ource% Dunne and &eopold 1'8.
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the beginning of a storm initially fills
interception storage sites in the canopy.
As the storm continues, water held in
these storage sites is displaced. The dis-
placed water drops to the net lower
layer of branches and limbs and fills
storage sites there. This process is re-
peated until displaced water reaches thelowest layer, the leaf litter. At this point,
water displaced off the leaf litter either
infiltrates the soil or moves downslope
as surface runoff.
Antecedent conditions, such as mois-
precipitation
canopyinterceptionand evaporation
ture still held in place from previous
storms, affect the ability to intercept
and store additional water. 4vaporation
will eventually remove water residing
in interception sites. 5ow fast this t#roug#fall
t#roug#fall
stemflo(
litterinterceptionandevaporation
process occurs depends on climaticconditions that affect the evaporation
rate.
$nterception is usually insignificant in
areas with little or no vegetation. 6are
soil or roc& has some small imperme-
able depressions that function as inter-
ception storage sites, but typically most
mineral soil
t#roug#fall
net rainfall enteringt#e soil
of the precipitation either infiltrates the
soil or moves downslope as surface
runoff. $n areas of froen soil, intercep-
tion storage sites are typically filled
with froen water. 0onse!uently, addi-
tional rainfall is rapidly transformed
into surface runoff.
$nterception can be significant in large
urban areas. Although urban drainage
systems are designed to !uic&ly move
storm water off impervious surfaces, the
urban landscape is rich with storage
sites. These include flat rooftops, par&-
ing lots, potholes, crac&s, and otherrough surfaces that can intercept and
hold water for eventual evaporation.
#ranspiration and E"apotranspiration
Transpiration is the diffusion of water
vapor from plant leaves to the atmos-
phere. +nli&e intercepted water, which
originates from precipitation, transpired
Figure 2.': #ypical path(ays $or $orest rain$all.
portion of precipitation never reaches the
round because it is intercepted by veetation
and other surfaces.
water originates from water ta&en in by
roots.
Transpiration from vegetation and evap-
oration from interception sites and
open water surfaces, such as ponds and
la&es, are not the only sources of water
returned to the atmosphere. #oil mois-
ture also is sub7ect to evaporation.
4vaporation of soil moisture is, how-
ever, a much slower process due to cap-
illary and osmotic forces that &eep the
moisture in the soil and the fact that
vapor must diffuse upward through soil
pores to reach surface air at a lower
vapor pressure.
6ecause it is virtually impossible to sep-
arate water loss due to transpiration
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"ater is sub+ect to evaporation whenever it is
e/posed to the atmosphere. 2asically this process
involves0
■ &he chane of state of water from liquid to
vapor
■ &he net transfer of this vapor to theatmosphere
&he process beins when some molecules in the
liquid state attain sufficient !inetic enery
)primari- ly from solar enery* to overcome the
forces of surface tension and move into the
atmosphere. &his movement creates a vapor
pressure in the atmosphere.
&he net rate of movement is proportional to the
difference in vapor pressure between the water
surface and the atmosphere above that surface.
3nce the pressure is equali'ed, no more
evapora- tion can occur until new air, capable ofholdin more water vapor, displaces the old
saturated air. Evaporation rates therefore vary
accordin to lati- tude, season, time of day,
cloudiness, and wind enery. 4ean annual la!e
evaporation in the #nited (tates, for e/ample,
varies from 25 inches in 4aine and "ashinton
to about 67 inches in the desert (outhwest
) Figure 2.) *.
)2! inc#es
2!"3! inc#es
3!"*! inc#es
*!"+! inc#es
+!"6! inc#es
6!"! inc#es
!"8! inc#es,8! inc#es
Figure 2.): *ean annual la+e e&aporation $or the period 1,)-1,//.
$ource% Dunne and &eopold -1'8 modified from /o#ler et al. -1'+'.
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gravitationa
l
force
cap
illary
fo
rce
capilla
ry
force
from water loss due to evaporation, the
two processes are commonly combined
and labeled evapotranspiration. 4vapo-
transpiration can dominate the water
balance and can control soil moisture
content, ground water recharge, and
streamflow.
The following concepts are importantwhen describing evapotranspiration
■ $f soil moisture conditions are limit-
ing, the actual rate of evapotranspira-
tion is below its potential rate.
■ 8hen vegetation loses water to the
atmosphere at a rate unlimited by
the supply of water replenishing the
roots, its actual rate of evapotranspi-
ration is e!ual to its potential rate of
evapotranspiration.
The amount of precipitation in a region
drives both processes, however. #oil
types and rooting characteristics also
play important roles in determining the
actual rate of evapotranspiration.
-nfiltration, (oil 4oisture, and
8round " ater
Precipitation that is not intercepted or
flows as surface runoff moves into thesoil. 1nce there, it can be stored in the
upper layer or move downward through
the soil profile until it reaches an area
completely saturated by water called the
phreatic zone.
%n&iltration
0lose eamination of the soil surface re-
veals millions of particles of sand, silt,
and clay separated by channels of differ -
ent sies (Figure 2.5). These macropores
include crac&s, pipes left by decayed
roots and wormholes, and pore spaces
between lumps and particles of soil.
8ater is drawn into the pores by gravity
and capillary action. 9ravity is the
dominant force for water moving into
rain
rain
(ettedgrains
drygrains
(ettedgrains
drygrains
drygrains
(ettedgrains
the largest openings, such as worm or
root holes. 0apillary action is the domi-
Figure 2./: Soil pro$ile. "ater is drawn into
the pores in soil by ravity and capillary action.
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rainfall.+ inc#es0#r
infiltration.+ inc#es0#r
A. %n&iltration 'ate (rain&all rate) *hich is less thanin&iltration capacity
B. 'uno&& 'ate (rain&all rate minusin&iltration capacity
rainfall1.+ inc#es0#r
infiltration1 inc#0#r
Figure 2.-: 0n$iltration and runo$$. (urface runoff occurs when rainfall intensity e/ceeds infiltration
capacity .
nant force for water moving into soilswith very fine pores.
The sie and density of these pore
openings determine the water ’s rate of
entry into the soil. Porosity is the term
used to describe the percentage of the
total soil volume ta&en up by spaces be-
tween soil particles. 8hen all those
spaces are filled with water, the soil is
said to be saturated.
#oil characteristics such as teture andtilth (looseness) are &ey factors in deter -
mining porosity. 0oarse-tetured, sandy
soils and soils with loose aggregates
held together by organic matter or small
amounts of clay have large pores and,
thus, high porosity. #oils that are tightly
pac&ed or clayey have low porosity.
Infiltration is the term used to describe
the movement of water into soil pores.
The infiltration rate is the amount of
water that soa&s into soil over a givenlength of time. The maimum rate that
water infiltrates a soil is &nown as the
soil’s infiltration capacity.
$f rainfall intensity is less than infiltra-
tion capacity, water infiltrates the soil at
a rate e!ual to the rate of rainfall. $f the
rainfall rate eceeds the infiltration ca-
pacity, the ecess water either is de-tained in small depressions on the soil
surface or travels downslope as surface
runoff (Figure 2.6).
The following factors are important in
determining a soil’s infiltration rate
■ 4ase of entry through the soil surface.
■ #torage capacity within the soil.
■ Transmission rate through the soil.
Areas with natural vegetative cover andleaf litter usually have high infiltration
rates. These features protect the surface
soil pore spaces from being plugged by
fine soil particles created by raindrop
splash. They also provide habitat for
worms and other burrowing organisms
and provide organic matter that helps
bind fine soil particles together. 6oth of
these processes increase porosity and
the infiltration rate.
The rate of infiltration is not constant
throughout the duration of a storm.
The rate is usually high at the begin-
ning of a storm but declines rapidly as
gravity-fed storage capacity is filled.
A slower, but stabilied, rate of infiltra-
tion is reached typically : or ; hours
into a storm. #everal factors are in-
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P r o p o r t i o n b y ! o l u m e
volved in this stabiliation process,
including the following
■ "aindrops brea&ing up soil aggregates
and producing finer material, which
then bloc&s pore openings on the sur -
face and reduces the ease of entry.
■ 8ater filling fine pore spaces and
reducing storage capacity.
■ 8etted clay particles swelling and
effectively reducing the diameter of
pore spaces, which, in turn, reduces
transmission rates.
#oils gradually drain or dry following a
storm. 5owever, if another storm occurs
before the drying process is completed,
there is less storage space for new water .
!.6!
!.+!
!.*!
!.3!
!.2!
!.1!
!
unfilledpore space
&ine
loam
porosity
field
capacity
(iltingpoint
clay
#eavyclay loam
clay loam
Therefore, antecedent moisture condi-
tions are important when analying
available storage.
+oil ,oistur e
sandy loam
sandy loam
fine sand
sand
lig#t clay loam
silt loam
After a storm passes, water drains out of
upper soils due to gravity. The soil re-
mains moist, however, because some
amount of water remains tightly held in
fine pores and around particles by sur-
face tension. This condition, called field
capacity, varies with soil teture. /i&e
porosity, it is epressed as a proportion
by volume.
The difference between porosity and
field capacity is a measure of unfilled
pore space (Figure 2.7). ield capacity
is an approimate number, however, be-
cause gravitation drainage continues in
moist soil at a slow rate.
#oil moisture is most important in the
contet of evapotranspiration. Terrestrial
plants depend on water stored in soil.
As their roots etract water from pro-
gressively finer pores, the moisture con-
tent in the soil may fall below the field
capacity. $f soil moisture is not replen-
ished, the roots eventually reach a point
where they cannot create enough suc-
tion to etract the tightly held interstitial
Figure 2.: ater%holding properties o$ &arious
soils. "ater-holdin properties vary by te/ture.
9or a fine sandy loam the appro/imate dif fer-
ence between porosity, 5.:;, and field
capacity ,
5.25, is 5.2;, meanin that the unfilled pore
space is 5.2; times the soil volume. &he dif fer-
ence between field capacity and wiltin point is
a measure ofunfilled
pore space.$ource% Dunne and &eopold 1'8.
pore water. The moisture content of the
soil at this point, which varies depend-
ing on soil characteristics, is called the
permanent wilting point because plants
can no longer withdraw water from the
soil at a rate high enough to &eep up
with the demands of transpiration, caus-
ing the plants to wilt.
Deep percolation is the amount of water that passes below the root one of
crops, less any upward movement of
water from below the root one ().
Ground -ater
The sie and !uantity of pore openings
also determines the movement of water
within the soil profile. 9ravity causes
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water to move vertically downward.
This movement occurs easily through
larger pores. As pores reduce in sie due
to swelling of clay particles or filling of
pores, there is a greater resistance to
flow. 0apillary forces eventually ta&e
over and cause water to move in any
direction.
8ater will continue to move downward
until it reaches an area completely satu-
rated with water, the phreatic zone or
one of saturation (Figure 2.8). The top
of the phreatic one defines the ground
water table or phreatic surface.
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underground scenarios. or eample,
perched ground water occurs when a shal-
low a!uitard of limited sie prevents
water from moving down to the
phreatic one. 8ater collects above the
a!uitard and forms a mini-phreatic
one. $n many cases, perched ground
water appears only during a storm orduring the wet season. 8ells tapping
perched ground water may eperience a
shortage of water during the dry season.
Perched a!uifers can, however, be im-
portant local sources of ground water .
Artesian wells are developed in con-
fined a!uifers. 6ecause the hydrostatic
pressure in confined a!uifers is greater
than atmospheric pressure, water levels
in artesian wells rise to a level where at-
mospheric pressure e!uals hydrostatic pressure. $f this elevation is above the
ground surface, water can flow freely
out of the well.
8ater also will flow freely where the
ground surface intersects a confined
a!uifer. The piezometric surface is the
level to which water would rise in wells
tapped into confined a!uifers if the
wells etended indefinitely above the
ground surface. Phreatic wells draw
water from below the phreatic one in
unconfined a!uifers. The water level in
a phreatic well is the same as the
ground water table.
Practitioners of stream corridor restora-
tion should be concerned with locations
where ground water and surface water
are echanged. Areas that freely allow
movement of water to the phreatic one
are called recharge areas. Areas where the
water table meets the soil surface orwhere stream and ground water emerge
are called springs or seeps.
The volume of ground water and the
elevation of the water table fluctuate
according to ground water recharge
and discharge. 6ecause of the fluctua-
tion of water table elevation, a stream
channel can function either as a
recharge area (influent or losing
stream) or a discharge area (effluent
or gaining stream).
unof f
8hen the rate of rainfall or snowmelt
eceeds infiltration capacity, ecesswater collects on the soil surface and
travels downslope as runoff. actors
that affect runoff processes include cli-
mate, geology, topography, soil charac-
teristics, and vegetation. Average annual
runoff in the contiguous +nited #tates
ranges from less than : inch to more
than ;> inches (Figure 2.9).
Three basic types of runoff are intro-
duced in this subsection (Figure2.10
)■ 1verland flow
■ #ubsurface flow
■ #aturated overland flow
4ach of these runoff types can occur in-
dividually or in some combination in
the same locale.
"erland Flo*
8hen the rate of precipitation eceeds
the rate of infiltration, water collects onthe soil surface in small depressions
(Figure 2.11). The water stored in these
spaces is called depression storage. $t
eventually is returned to the atmos-
phere through evaporation or infiltrates
the soil surface.
After depression storage spaces are filled,
ecess water begins to move downslope
as overland flow, either as a shallow
sheet of water or as a series of small
rivulets or rills. 5orton (:=??) was the
first to describe this process in the liter-
ature. The term Horton overland flow or
5ortonian flow is commonly used.
The sheet of water increases in depth
and velocity as it moves downhill. As it
travels, some of the overland flow is
trapped on the hillside and is called sur-
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precipitation
precipitation
r oun
(ater tale
saturatedoverlandflo(
Figure 2.15: Flo(
paths o$ (ater o&
a sur$ace. &he potion of precipitati
that runs off or
infiltrates to the
round water tab
depends on the s
permeability rate
surface rouhness
and the amount,
duration, and inte
ty of precipitation
$n some situations, infiltrated storm
water does not reach the phreatic one
because of the presence of an a!uitard.
$n this case, subsurface flow does not
mi with baseflow, but also discharges
water into the channel. The net result,
whether mied or not, is increased
channel flow.
+aturated "erland Flo*
$f the storm described above continues,
the slope of the water table surface can
continue to steepen near the stream.4ventually, it can steepen to the point
that the water table rises above the
channel elevation. Additionally, ground
water can brea& out of the soil and
travel to the stream as overland flow.
This type of runoff is termed quic
return flow.
The soil below the ground water brea&-
out is, of course, saturated. 0onse-
!uently, the maimum infiltration rate
is reached, and all of the rain falling
on it flows downslope as overland
runoff. The combination of this direct
pands further up the hillside. 6ecause
!uic& return flow and subsurface flow
are so closely lin&ed to overland flow,
they are normally considered part of
the overall runoff of surface water .
Hydrologic and HydraulicProcesses Along t#e $tr eamCorridor
8ater flowing in streams is the
collection of direct precipitation and
water that has moved laterally from theland into the channel. The amount and
timing of
this lateral movement directly influencesFigure 2.11: 6&erland $lo( and depression
storage. 3verland moves downslope as an
irreular sheet.
$ource% Dunne and &eopold 1'8.
surface
detention dept# andvelocity ofoverland flo(
increasedo(nslope
precipitation and !uic& return flow is
called saturated overland flow. As the
storm progresses, the saturated area e-
depression storage-dept# of depressionsgreatly e9aggerated
streamc#annel
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, e a n
, o n t h l y
D i s c h a r g e / c & s 0
: A$;:Os. +nfortunately, the length of
record regarding wet and dry years is
short (in geologic time), ma&ing it is
difficult to predict broad-scale persis-
tence of wet or dry years.
#easonal variations of streamflow are
more predictable, though somewhat
complicated by persistence factors. 6e-
cause design wor& re!uires using histor-
ical information (period of record) as a
basis for designing for the future, flow
information is usually presented in a
probability format. Two formats are es-
pecially useful for planning and design-
ing stream corridor restoration
■ !low duration" the probability a given
streamflow was e!ualed or eceeded
over a period of time.
■ !low frequency" the probability a
given streamflow will be eceeded
(or not eceeded) in a year .
(#ometimes this concept is modified
and epressed as the average number
of years between eceeding Cor not
eceedingD a given flow.)
Figure 2.12 presents an eample of a
flow fre!uency epressed as a series of
probability curves. The graph displays
months on the -ais and a range ofmean monthly discharges on the y-ais.
The curves indicate the probability that
the mean monthly discharge will be
less than the value indicated by the
curve. or eample, on about percent chance that the
1+!!!
1!!!!
+!!!
!Oct. >ov. Dec. ?an. :e. @ar. April @ay ?une ?uly Aug. $ept.
,onth
Figure 2.12: An e7ample o$ monthly pro"a"ility cur&es. 4onthly probability that the mean
monthly dischare will be less than the values indicated. =a!ima iver near %ar!er, "ashinton.
)>ata from #.(. rmy Corps of Enineers.*
$ource% Dunne and &eopold 1'8.
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21 Chapter 23 +tream Corridor Processes) Characteristics) and
discharge will be less than =,>>> cfs
and a B> percent chance it will be less
than ;,>>> cfs.
Ecoloical -mpacts of 9low
The variability of streamflow is a pri-
mary influence on the biotic and abiotic
processes that determine the structureand dynamics of stream ecosystems
(0ovich :==?). 5igh flows are impor-
tant not only in terms of sediment
transport, but also in terms of recon-
necting floodplain wetlands to the
channel.
This relationship is important because
floodplain wetlands provide spawning
and nursery habitat for fish and, later in
the year, foraging habitat for waterfowl.
/ow flows, especially in large rivers,
create conditions that allow tributary
fauna to disperse, thus maintaining
populations of a single species in sev-
eral locations.
$n general, completion of the life cycle
of many riverine species re!uires an
array of different habitat types whose
temporal availability is determined
by the flow regime. Adaptation to this
environmental dynamism allows river-ine species to persist during periods
of droughts and floods that destroy
and recreate habitat elements (Poff
et al. :==E).
2. 4eomorp#ic Pr ocesses
#eomorphology is the study of surface
forms of the earth and the processes
that developed those forms. The hydro-
logic processes discussed in the previ-
ous section drive the geomorphic
processes described in this section. $n
turn, the geomorphic processes are the
primary mechanisms for forming the
drainage patterns, channel, floodplain,
terraces, and other watershed and
stream corridor features discussed in
0hapter :.
Three primary geomorphic processes
are involved with flowing water, as fol-
lows)
■ $rosion, the detachment of soil parti-
cles.
■ %ediment transport , the movement of
eroded soil particles in flowing water .
■
%ediment deposition, settling of erod-ed soil particles to the bottom of a
water body or left behind as water
leaves. #ediment deposition can be
transitory, as in a stream channel
from one storm to another, or more
or less permanent, as in a larger
reservoir .
#ince geomorphic processes are so
closely related to the movement of
water, this section is organied into
subsections that mirror the hydrologic processes of surface storm water runoff
and streamflow
■ 9eomorphic Processes Across the
#tream 0orridor
■ 9eomorphic Processes Along the
#tream 0orridor
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4eomorp#ic Processes Acr osst#e $tream Corridor
The occurrence, magnitude, and distrib-
ution of erosion processes in water-
sheds affect the yield of sediment and
associated water !uality contaminants
to the stream corridor .
#oil erosion can occur gradually over a
long period, or it can be cyclic or
episodic, accelerating during certain
seasons or during certain rainstorm
events (Figure 2.13). #oil erosion can
be caused by human actions or by nat-
ural processes. 4rosion is not a simple
process because soil conditions are con-
tinually changing with temperature,
moisture content, growth stage and
amount of vegetation, and the humanmanipulation of the soil for develop-
ment or crop production. Tables 2.2
and 2.3 show the basic processes that
influence soil erosion and the different
types of erosion found within the water-
shed.
4eomorp#ic Processes Alongt#e $tream Corridor
The channel, floodplain, terraces, and
other features in the stream corridor areformed primarily through the erosion,
transport, and deposition of sediment
by streamflow. This subsection de-
scribes the processes involved with
transporting sediment loads down-
stream and how the channel and
floodplain ad7ust and evolve through
time.
(ediment & ransport
#ediment particles found in the streamchannel and floodplain can be catego-
ried according to sie. A boulder is the
largest particle and clay is the smallest
particle. Particle density depends on the
sie and composition of the particle
(i.e., the specific gravity of the mineral
content of the particle).
2o matter the sie, all particles in the
channel are sub7ect to being trans-
ported downslope or downstream.
The sie of the largest particle a stream
can move under a given set of hy-
draulic conditions is referred to as
stream competence. 1ften, only very
high flows are competent to move thelargest particles.
0losely related to stream competence is
the concept of tractive stress, which cre-
ates lift and drag forces at the stream
boundaries along the bed and ban&s.
Tractive stress, also &nown as shear
stress, varies as a function of flow depth
and slope. Assuming constant density,
shape, and surface roughness, the larger
the particle, the greater the amount of
tractive stress needed to dislodge it andmove it downstream.
The energy that sets sediment particles
into motion is derived from the effect
of faster water flowing past slower
water . This velocity gradient happens
because the water in the main body of
flow moves faster than water flowing at
the boundaries. This is because bound-
Figure 2.1': 8aindrop impact. 3ne of
many types of erosion.
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Erosion4Physical Process
Erosion #ype +heet ConcentratedFlo*
,ass-asting
Combination
$#eet and rill 9 9
Bnterill 9
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3ne way to differentiate the sediment load of a
stream is to characteri'e it based on the immediate
source of the sediment in transport. &he total sediment
load in a stream, at any iven time and location, isdivided into
two parts?wash load and bed-material load. &he prima-
ry source of wash load is the watershed, includin sheet
and rill erosion, ully erosion, and upstream streamban!
erosion. &he source of bed material load is primarily the
streambed itself, but includes other sources in the water-
shed.
"ash load is composed of the finest sediment particles
in transport. &urbulence holds the wash load in
suspen- sion. &he concentration of wash load in
suspension is essentially independent of hydraulic
conditions in the stream and therefore cannot be
calculated usin mea- sured or estimated hydraulic
parameters such as velocity or dischare. "ash load
concentration is normally a function of supply i.e., the
stream can carry as much wash load as the watershed
and ban!s can deliver )for sediment concentrations
below appro/imately @555 parts per million*.
2ed-material load is composed of the sediment of si'e
classes found in the streambed. 2ed-material load
moves alon the streambed by rollin, slidin, or +umpin, and may be periodically entrained into the
flow by turbu- lence, where it becomes a portion of the
suspended
load. 2ed-material load is hydraulically controlled
and can be computed usin sediment transport
equations discussed in Chapter 6.
iner-grained particles are more easily
carried into suspension by turbulent ed-
dies. These particles are transported
within the water column and are there-
fore called the suspended load . Although
there may be continuous echange of
sediment between the bed load and
suspended load of the river, as long as
sufficient turbulence is present.
Part of the suspended load may be col-
loidal clays, which can remain in sus-
pension for very long time periods,
depending on the type of clay and
water chemistry.
+ediment #ransport #erminology
#ediment transport terminology can
sometimes be confusing. 6ecause of
this confusion, it is important to define
some of the more fre!uently used
terms.
■ %ediment load , the !uantity of sedi-
ment that is carried past any cross
section of a stream in a specified
period of time, usually a day or a
year. %ediment discharge, the mass
or volume of sediment passing a
stream cross section in a unit of time. Typical units for sediment load
are tons, while sediment discharge
units are tons per day.
■ &ed-material load , part of the total
sediment discharge that is composed
of sediment particles that are the
same sie as streambed sediment.
■ 'ash load , part of the total sediment
load that is comprised of particle
sies finer than those found in thestreambed.
■ &ed load , portion of the total sedi-
ment load that moves on or near the
streambed by saltation, rolling, or
sliding in the bed layer .
■ %uspended bed material load , portion
of the bed material load that is trans-
ported in suspension in the water
column. The suspended bed material
load and the bed load comprise the
total bed material load.
■ %uspended sediment discharge (or sus-
pended load ), portion of the total sed-
iment load that is transported in sus-
pension by turbulent fluctuations
within the body of flowing water .
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Classi&ication +ystem
Based on
,echanismo&
Based on
Particle +i5e
# o t a l s e d i m e n t l o a d-ash load +uspended
load-ash load
+uspendedbed6materialload
Bed6materialload
Bed load Bed load
■ (easured load , portion of the total
sediment load that is obtained by the
sampler in the sampling one.
■ )nmeasured load , portion of the total
sediment load that passes beneath
the sampler, both in suspension and
on the bed. 8ith typical suspended
sediment samplers this is the lower >.? to >.F feet of the vertical.
The above terms can be combined in a
number of ways to give the total
sediment load in a stream (Table 2.4).
5owever, it is important not to com-
bine terms that are not compatible.
or eample, the suspended load and
the bed material load are not compli-
mentary terms because the suspended
load may include a portion of the bed
material load, depending on the energy
available for transport. The total sedi-
ment load is correctly defined by the
combination of the following terms
;otal $ediment &oad F
ed @aterial &oad G 5as# &oad
or
ed &oad G $uspended &oad
or
@easured &oad G nmeasured &oad
#ediment transport rates can be com-
puted using various e!uations or mod-
els. These are discussed in the %tream
*hannel +estoration section of 0hapter G.
#a"le 2.): Sediment load terms.
+tream Po*er
1ne of the principal geomorphic tas&s
of a stream is to transport particles out
of the watershed (Figure 2.15). $n this
manner, the stream functions as a trans-
porting machine% and, as a machine,
its rate of doing wor& can be calculated
as the product of available power multi- plied by efficiency.
%tream power can be calculated as
ϕ H γ I #
8here
ϕ H #tream power (foot-lbsJsecond-
foot)
γ H #pecific weight of water (lbsJft?)
I H @ischarge (ft?Jsecond)
# H #lope (feetJfeet)
#ediment transport rates are directly re-
lated to stream power% i.e., slope and
discharge. 6aseflow that follows the
highly sinuous thalweg (the line that
mar&s the deepest points along the
stream channel) in a meandering
stream generates little stream power%
therefore, the stream’s ability to move
sediment, sediment-transport capacity" islimited. At greater depths, the flow fol-
lows a straighter course, which increases
slope, causing increased sediment trans-
port rates. The stream builds its cross
section to obtain depths of flow and
channel slopes that generate the sedi-
ment-transport capacity needed to
maintain the stream channel.
"unoff can vary from a watershed, ei-
ther due to natural causes or land use
practices. These variations may change
the sie distribution of sediments deliv-
ered to the stream from the watershed
by preferentially moving particular par-
ticle sies into the stream. $t is not un-
common to find a layer of sand on top
of a cobble layer. This often happens
when accelerated erosion of sandy soils
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