on upstreeam tributaaries, as well as abou
ut 5,800 km
m of leveees, had little success at
holding th
he flood waters
w
in ch
heck. No doubt
d
debatte will now
w focus on the utility of
levees in flood conttrol. Leveees are effecctive in pro
otecting maany areas during
d
flood
ds,
me cases they
t
actually exacerbate the pro
oblem by restricting
r
t flow th
the
hat
yet in som
would ottherwise haave spread
d over a floodplain. They
T
are expensive
e
to build and
a
maintain,, and their overall
o
effeectiveness has
h been an
nd will con
ntinue to bee questioned
d.
Most criticism was directed at the U.S. Army Corps of Engineers, which has spent
about $25 billion in this century to build 500 dams and more than 16,000 km of
levees. No one doubts that some of these projects have been successful, at least
within the limits of their design. But critics charge that such flood control projects
make the problem of flooding worse, particularly because f1oodprone areas tend to
be developed once the projects are completed even though nothing can be done to
prevent some floods. As a consequence, the most destructive and most costly type of
natural disaster continues to be flooding.
_ INTRODUCTION
Among the terrestrial planets, the Earth is unique in having abundant liquid water.
Fully 71% of the Earth's surface is covered by water. and a small but important
quantity of water vapor is present in its atmosphere.
The volume of water on Earth is estimated at 1.36 billion km', most of which
(97.2%) is in the oceans. About 2% is frozen in glaciers, and the remaining 0.8%
constitutes all the water in streams, lakes, swamps, groundwater, and the atmosphere
(@ Table 12-1). Only a tiny portion of the total water on Earth is in streams, but
running water is nevertheless the most important erosional agent modifying the
Earth's surface, Even in most desert regions the effects of running water are manifest,
although channels are dry most of the time.
In addition to its significance as a geologic agent, running water is important for
many other reasons. It is a source of fresh (nonsaline) water for industry, domestic
use, and agriculture, and about 8% of the electricity used in North America is
generated by falling water at hydroelectric stations. Streams have been, and continue
to be, important avenues of commerce. Much of the interior of North America was
first explored by following such large streams as the St. Lawrence. Mississippi, and
Missouri rivers. Much of this discussion of running water is necessarily descriptive,
bur one should always be aware that streams are dynamic systems that must
continually respond to change. For example, paving in urban areas increases surface
runoff to streams, while other human actions such as building dams and impounding
reservoirs also alter the dynamics of a stream system. Natural changes, too, affect
stream dynamics. When more rain falls in a stream's drainage area due to long-term
climatic change, more water flows in the stream's channel, and greater energy is
available for erosion and transport of sediments. In short, streams continually adjust to
change.
_ THE HYDROLO
H
OGIC CYC
CLE
Water is continuallly recycleed from th
he oceans, through the atmosp
phere, to the
continentts, and bacck to the o
oceans. Thiis continuaal recycling
g of waterr is called the
hydrologiic cycle (~
~ Figure 12
2-2). The hydrologic
h
cycle, wh
hich is pow
wered by so
olar
radiation,, is possib
ble becausse water changes
c
ph
hases easilly under Earth
E
surfface
condition
ns. Huge q
quantities of
o water ev
vaporate from'
fr
the oceans
o
each
h year as the
surface waters
w
are heated
h
by so
olar energy
y. Approxim
mately 85%
% of all waater that entters
the atmo
osphere is derived from the oceans; the
t
remain
ning 15% comes frrom
evaporation of water on land.
w
evap
porates, thee vapor risses into th
he atmosph
here where the comp
plex
When water
processess of conden
nsation and
d cloud form
mation occcur. About 80",1, of thee precipitattion
falls direcctly into th
he oceans, iin which caase the hyd
drologic cy
ycle is limitted to a thrreestep proceess of evap
poration, co
ondensation
n, and preciipitation.
About 20%
2
of all precipitatiion falls on
n land as rain
r
and sn
now, and th
he hydrolo
ogic
cycle invo
olves moree steps: evaaporation, condensatio
c
on, movem
ment of water vapor frrom
the ocean
ns to the continents, precipitatio
on, and ru
unoff and in
nfiltration. Some of the
precipitattion evaporrates as it ffalls and reeenters thee hydrologiic cycle ass vapor; waater
evaporateed from lak
kes and streams also reenters th
he cycle as vapor as does
d
moistture
evaporateed from plaants by transspiration (Figure 12-2).
Each yeear about 36
6,000 km3 of the preccipitation falling
fa
on laand returns to the oceans
by runofff, the surfaace flow off streams. The water returning to the oceans by run
noff
enters
the Earth's ultimate reservoir
r
whhere it begiins the hydrologic cyccle again. Some of the
precipitatiion falling on
o land is ttemporarily stored in laakes, snow fields, and glaciers or
seeps beloow the surfface where it is tempo
orarily storeed as grounndwater. Th
his water is
effectivelyy removed from the ssystem for up to thou
usands of yyears, but eventually,
glaciers melt,
m
lakes feeed streamss, and groun
ndwater Bow
ws into streaams or direcctly into the
oceans (F
Figure 12-2
2). Our conncern here is with th
he comparaatively smaall quantity
returning to the oceaans as runofff, for the energy
e
of ru
unning wateer is respon
nsible for a
great manny surface feeatures.
RUNN
NING WA
ATER
The amouunt of runofff in any arrea during a rainstorm depends onn infiltratio
on capacity,
the maxim
mum rate thaat soil or othher surface materials can absorb w
water. Infiltrration
capacity depends
d
on
n several faactors, inclu
uding the in
ntensity andd duration of rainfall.
Loosely packed, dry soils absorbb water fasteer than tightly packed, wet soils.
If rain is
i absorbed as fast as iit falls, no surface
s
runo
off occurs. Should the infiltrationn
capacity be
b exceedeed, or shouuld surface materials become saaturated, ex
xcess waterr
collects onn the surfacce and, if a sslope existss, moves downhill. Eveen on steep slopes Bow
w
is initiallyy slow, and
d hence littlee or no ero
osion occurss, but as waater continu
ues movingg
downslope, it accelerrates and maay move by
y sheet flout, a more-or-lless continu
uous film off
water Bow
wing over the surfac e. Sheet Bow
B
is not confined tto depressio
ons, and itt
accounts for
f sheet eroosion, a partticular problem on som
me agricultuural lands (ssee Chapterr
6).
In channnel flow, surfface runofff is confined
d to long, trrough like ddepressions. Channels
vary in sizze from rillls containinng a tricklin
ng stream of
o water to the Amazo
on River of
South Am
merica, which is 6,4500 km long and up to
o 2.4 km w
wide and 90
0 m deep.
Channelizzed flow is described
d
byy various teerms includiing rill, broook, creek. stream,
s
and
river, mosst of which are distinguuished by size and volume. The te
term stream
m carries no
connotatioon of size and is useed here to refer to all
a runoff cconfined to
o channels
regardlesss of size.
Streams receive water
w
from sseveral sou
urces, including sheet flow and rain
r
fallingg
c
Faar more imp
portant, though. is the water supplied by soill
directly innto stream channels.
moisture and
a ground
dwater, bothh of which flow downsslope and ddischarge in
nto streams.
In humid areas wherre groundw
water is plen
ntiful, streaams may m
maintain a fairly
f
stablee
flow year round, eveen during ddry seasons,, because th
hey are conntinuously supplied
s
byy
mount of water
w
in streaams of aridd and semiaarid regionss
groundwaater. In conttrast, the am
fluctuates widely beecause thesee streams depend
d
more on infreequent rainstorms andd
surface ruunoff for theeir water suppply.
Stream Gradient,
G
Velocity,
V
and
d Discharg
ge
fl
downhill from a ssource area to a lower elevation w
where they empty into
Streams flow
another sttream, a lak
ke, or the seea. The slop
pe Over whiich a stream
m flows is itts gradient.
If the sourrce (headwaaters) of a sstream is 1,000 m abov
ve sea levell and the strream flows
50t) km too the sea, it drops 1,00 0 m verticaally over a horizontal
h
ddistance of 500
5 km (Ii>
Figure 12-3). Its grad
dient is calcculated by dividing th
he vertical ddrop by thee horizontal
distance; in this example, it is 1,000 m/500 km = 2 m/km.
Gradients vary considerably, even along the course of a single stream. Generally,
streams are steeper in their upper reaches where their gradients may be tens of meters
per kilometer, but in their lower reaches the gradient may be as little as a few
centimeters per kilometer.
Stream velocity and discharge are closely related variables.
Velocity is simply a measure of the downstream distance traveled per unit of time, and
is usually expressed in feet per second (ft/sec) or meters per second (m/sec). Variations
in flow velocity occur not only with distance along a stream channel but also across a
channel's width. Flow velocity is slower and more turbulent near a stream's banks or
bed because of friction than it is farther from these boundaries (Ii> Figure 12-4). Other
controls on velocity include channel shape and roughness. Broad, shallow channels and
narrow, deep channels have proportionally more water in contact with their perimeters
than do channels with semicircular cross sections (Ii> Figure 12-5). Consequently the
water in semicircular channels flows more rapidly because it encounters less frictional
resistance.
Channel roughness is a measure of the frictional resistance within a channel. Frictional
resistance to flow is greater in a channel containing large boulders than in one with
banks and a bed composed of sand or clay. In channels with abundant vegetation, flow
is slower than in barren channels of comparable size.
The most obvious control on velocity is gradient, and one might think that the steeper
the gradient, the greater the flow velocity. In fact, the average velocity generally
increases in a downstream direction, even though the gradient decreases in the same
direction. Three factors contribute to this: First, velocity increases continuously, even as
gradient decreases, in response to the acceleration of gravity unless other factors retard
flow. Secondly, in their upstream reaches, streams commonly have boulder-strewn,
broad, shallow channels, so flow resistance is high and velocity is correspondingly
slower. Downstream, channels generally become more semicircular, and the bed and
banks are usually composed of finer-grained materials, reducing the effects of friction.
Thirdly, the number of tributary streams joining a larger stream increases in a
downstream direction, so the total volume of water (discharge) increases, and increasing
discharge results in increased velocity.
Discharge is the total volume of water in a stream moving past a particular point in a
given period of time. To determine discharge. one must know the dimensions of a
channel-that is, its cross-sectional area (A)-and its flow velocity (V). Discharge (Q) can
then be calculated by the formula Q = VA; it is generally expressed in cubic feet
per second (ft3/sec) or cubic meters per second (m3/sec).
STREAM
M EROSION
Erosion innvolves thee physical rremoval of dissolved substances
s
and loose particles
p
off
soil and rock
r
from a source arrea. Accord
dingly, the sediment
s
trransported in
i a stream
m
consists of
o both disssolved mateerials and so
olid particlees. Some oof the dissolvved load of a
stream iss acquired from the sstream bed
d and bank
ks where sooluble rock
ks such as
limestonee and dolosttone are exxposed. But much of itt is carried into stream
ms by sheett
flow and by
b groundw
water.
The sollid sedimen
nt carried iin streams ranges from
m clay sizzed particlees to large
boulders. Much of th
his sedimentt finds its way
w into streams by maass wasting
g (~ Figure
12-6), butt some is derived
d
direectly from the stream
m bed and bbanks. The power of
running water,
w
called
d hydraulic aaction, is su
ufficient to set
s particless in motion
Anotherr process off erosion in streams is abrasion, in
n which expposed rock is
i worn andd
scraped by
b the impact of solidd panicles. If running
g water is ttransporting
g sand andd
gravel. thhe impact of these p articles abrrades exposed rock ssurfaces. On
ne obviouss
manifestaation of abrrasion is thhe occurrencce of potholles in the beeds of streaams. Thesee
circular to oval holees occur whhere eddyin
ng currents containing sand and gravel
g
swirll
around annd erode dep
pressions innto solid rocck.
TRANSPORT OF SEDIMENT LOAD
Streams transport a solid load of sedimentary particles and a dissolved load consisting
of ions taken into solution by chemical weathering. Sedimentary particles are
transported either as suspended load or as bed load. Suspended load consists of the
smallest particles, such as silt and clay, which are kept suspended by fluid turbulence.
Bed load consists of the coarser particles such as sand and gravel. Fluid turbulence is
insufficient to keep large particles suspended, so they move along the stream bed.
However, part of the bed load can be suspended temporarily as when an eddying current
swirls across a stream bed and lifts sand grains into the water. These particles move
forward at approximately the flow velocity, but at the same time they settle toward the
stream bed where they come to rest, to be moved again later by the same process. This
process of intermittent bouncing and skipping is saltation
Particles too large to be suspended even temporarily are transported by rolling or
sliding. Obviously, greater flow velocity is required to move particles of these sizes.
The maximum-sized particles that a stream can carry define its competence, a factor
related to flow velocity. Capacity is a measure of the total load a stream can carry. I t
varies as a function of discharge; with greater discharge, more sediment can be carried.
A small, swiftly flowing stream may have the competence to move gravel-sized
particles but not to transport a large volume of sediment, so it has a low capacity. A
large', slow-flowing stream. on the other hand, has a low competence, but may have a
very large suspended load, and hence a 1arge capacity.
STREAM DEPOSITION
Streams can transport sediment a considerable distance from the source area. Some of
the sediments deposited in the Gulf of Mexico by the Mississippi River came from
such distant sources as Pennsylvania, Minnesota, and Alberta, Canada. Along t he way,
deposition may occur in a variety of environments, such as stream channels, the
floodplains adjacent to channels, and the points where streams flow into lakes or the
seas or flow from mountain valleys onto adjacent lowlands.
Streams do most of their erosion, sediment transport, and deposition when they flood.
Consequently, stream deposits, collectively called alluvium, do not represent the
continuous day-to-day activity of streams, but rather those periodic, large-scale events
of sedimentation associated with flooding.
Braided Streams and Their Deposits
Braided streams possess an intricate network of dividing and rejoining channels
(~Figure 12-7). Braiding develops when a stream is supplied with excessive sediment,
which over time is deposited as sand and gravel bars within its channel. During highwater stages, these bars are submerged, but during low-water stages, they are exposed
and divide a single channel into multiple channels (Figure 12-7). Braided streams have
broad, shallow channels. They are generally characterized as bed load-transport
streams, and t heir deposits are composed mostly of sheets of sand and gravel.
Meandering Streams and Their Deposits
Meandering streams possess a single, sinuous channel with broadly looping curves
called meanders (~ Figure 12-8). Such stream channels are semicircular in cross section
along straaight reachees, but at m
meanders they are mark
kedly asym
mmetric, beiing deepestt
near the outer
o
bank, which com
mmonly descends vertically into th
the channel. The outerr
bank is caalled the cut bank becauuse flow velocity and tu
urbulence arre greatest on
o that sidee
of the chaannel wheree it is erodeed. In contrrast, flow velocity is aat a minimu
um near thee
inner bankk, which slo
opes gently into the chaannel (~ Fig
gure 12-9a)..
As a coonsequence of the uneqqual distrib
bution of flo
ow velocityy across meeanders, thee
cut bank is eroded and
a depositiion occurs along the opposite
o
sidde of the ch
hannel. Thee
his manner is a point bar; it consists of crooss-bedded sand or, inn
deposit foormed in th
some casees, gravels (Figure
(
12-99b).
It is nott uncommo
on for meannders to become so sinuous that the thin neeck of landd
separatingg adjacent meanders
m
iss eventually
y cut off durring a floodd. The valleey floors off
meanderinng streams are commoonly marked
d by crescen
nt shaped oxxbow lakes, which aree
actually cuutoff meand
ders (Figurees 12-8 and
d ~ 12-10). These
T
oxbow
w lakes maay persist ass
lakes for some timee, but are eeventually filled
f
with organic m
matter and fine-grained
f
d
sediment carried
c
by floods.
f
Oncee filled, oxb
bow lakes are called meeander scars.
Floods an
nd Floodpla
ain Depositts
Most streams period
dically receiive more water
w
than their
t
channnel can carrry, so they
spread accross low-lying, relatiively flat areas calleed floodplaains adjacen
nt to theirr
channels (Figure
(
12-8
8; see Persppective 12-1
1). Some flo
oodplains arre composed
d mostly off
sand and gravel
g
that were
w deposiited as poin
nt bars. Wheen a meandeering stream
m erodes its
cut bank and
a depositts on the oppposite bank, it migrattes laterallyy across its floodplain.
As lateral migration occurs,
o
a suuccession off point bars develops (~~ Figure 12--11a).
Many floodplains
fl
are dominaated by fin
ne-grained sediments,
s
mostly mu
ud. When a
stream ovverflows itss banks andd floods, the
t velocity
y of the w
water spillin
ng onto thee
floodplainn diminishess rapidly beecause the water
w
encou
unters greatter frictionaal resistancee
to flow as it spreads out as a broad, shallow sheet. In response to the diminished
velocity, ridges of sandy alluvium called natural levees are deposited along the margins
of the stream channel (Figure 12-11 b).
The flood waters spilling from a main channel carry large quantities of mud beyond
the natural! Levees and onto the floodplain. During the waning stages of a flood, the
flood waters may flow very slowly or not at all, and the suspended silt and clay
eventually settle as layers of mud.
Annual property damage from flooding in the United States exceeds $100 million.
And in spite of the completion of more and more flood control projects, the amount of
property damage is not decreasing. In fact, floodplains are attractive sites for settlement
due to the combination of fertile soils, level surfaces for construction, and proximity to
water for industry, agriculture, and domestic uses. However, these human activities
generally increase the potential for flooding. Urbanization greatly increases surface
runoff because concrete and asphalt compact and cover surface materials, thereby
reducing their infiltration capacity. Storm drains in urban areas quickly carry water to
nearby streams, many of which flood much more commonly than they did in the past.
Deltas
When a stream flows into another body of water, its flow velocity decreases rapidly and
deposition occurs. As a result, a delta forms, causing the local shoreline to build out, or
prograde (~ Figure 12-12). The simplest prograding deltas exhibit a characteristic vertical
sequence in which bottomset
beds are successively overlain by forest beds and topset beds (Figure 12-12a). This
sequence develops when a stream enters another body of water, and the finest sediments
are carried some distance beyond the stream's mouth, where they settle from suspension
and form bottomset beds. Nearer the stream's mouth, foresee beds are formed as sand
and silt are deposited in gently inclined layers. The topset beds consist of coarse-grained
sediments deposited in a network of distributary channels traversing the top of the delta.
In effect, streams lengthen their channels as they extend across prograding deltas
(Figure 12-12).
Many small deltas in lakes have the three-part division described above, but large
marine deltas are usually much more complex. The Mississippi River delta consists of
long fingerlike sand bodies. each deposited in a distributary channel that progrades far
seaward (Figure 12-12c). Such deltas are commonly called bird~-foot deltas because the
projections resemble the toes of a bird.
Progradation of marine deltas is one way that potential reservoirs for oil and gas are
formed. Much of the oil and gas production of the Gulf Coast of Texas comes from
buried delta deposits, and the present-day deltas of the Niger River in Africa and the
Mississippi River are also known to contain reserves of oil and gas. The marshes
between. distributary channels of deltas are dominated by nonwoody vegetation and are
potential areas of coal formation.
Alluvial Fans
F
Alluvial fans
f
are lobaate depositss of alluvium on land (~ Figure 1 2-13). They
y form bestt
on lowlannds adjacen
nt to highlaands in ariid and sem
miarid regioons where little
l
or noo
vegetationn exists to stabilize surrface materials. When periodic
p
rainnstorms occcur, surfacee
materials are quickly
y saturated aand runoff begins. During a partiicularly heaavy rain, alll
of the surfface flow in
n a drainagee area is fun
nneled into a mountainn canyon leeading to ann
adjacent lowland. Ass long as thhe stream is confined in the mounntain canyon
n, it cannott
spread latterally. Butt when it ddischarges from the canyon ontoo the lowlaand area, itt
quickly sppreads out, its
i velocity diminishes,, and deposiition ensuess.
The alluuvial fans th
hat develop by the proccess just described are mostly acccumulationss
of sand annd gravel, a large propportion of which
w
is dep
posited by sstreams. In some casess
the water flowing th
hrough a m
mountain canyon pick
ks up so m
much sedim
ment that itt
becomes a viscous mudflow.
m
Coonsequently
y, mudflow deposits m
make up a laarge part off
many alluvial fans.
ο DRAIINAGE BA
ASINS AND
D DRAINA
AGE PATIE
ERNS
A stream such as the Mississipppi River con
nsists of a main
m stream
m and all of the smallerr
tributary strreams that supply
s
wateer to it. Thee Mississipp
pi and all off its tributarries, or anyy
other drainnage system
m for that m
matter, carry surface ru
unoff from an area kn
nown as thee
drainage basin. Indiividual draainage basiins are sep
parated froom adjacen
nt ones byy
topographhically higheer areas callled divides (~ Figure 12-14).
Various drrainage pattterns are reccognized baased on the regional arrrangement of
o channels
in a drainnage system
m. The moost common
n is dendrittic drainage, which con
nsists of a
network of
o channels resemblingg tree brancching (~ Figure 12-15aa). Dendritic drainage
develops on
o gently slloping surfaaces where the materiaals respond more or lesss homogeneously too erosion. Areas
A
of Ratt-lying sediimentary rocks and som
me terrains of igneous
or metamoorphic rockss usually di splay a den
ndritic drainaage pattern..
Rectangularr drainage iss characteriized by chaannels with right anglee bends and
d tributariess
that join larger
l
stream
ms at right angles (Fig
gure 12-15b
b). The possitions of th
he channelss
are stronggly controlleed by geoloogic structu
ures, particu
ularly regioonal joint sy
ystems thatt
intersect at
a right angles.
In some parts
p
of the eastern Unnited States, such as Viirginia and Pennsylvan
nia, erosionn
of folded sedimentarry rocks deevelops a landscape
l
of
o alternatinng parallel ridges andd
T ridges consist of more resisttant rocks, such as saandstone, whereas
w
thee
valleys. The
valleys ovverlie less reesistant roc ks such as shale. Main
n streams foollow the trrends of thee
valleys. Shhort tributarries rowingg from the adjacent
a
ridg
ges join the main stream at nearlyy
right anglees, hence th
he name trelllis drainage (Figure
(
12-1
15c).
from a ceentral high area (Figurre 12-15d). Radial draainage dewllaps all larg
ge, isolatedd
volcanic mountains
m
and in areaas where th
he Earth's crust
c
has b een arched
d up by thee
intrusion at'
a plutons such
s
as lacc oliths.
οIn some areas streams flow iill and out of swamp
ps and lakees with irreegular flow
w
directions. Drainage patterns ccharacterizeed by such irregularitty are calleed derangedd
(Figure 122-15e). The presence oof deranged drainage in
ndicates that
at it develop
ped recentlyy
and has not yet fo
ormed an organized drainage system.
s
In areas of Minnesota,,
Wisconsinn, and Micchigan thatt were glaaciated untiil about 100,000 yearrs ago, thee
previouslyy established drainage ssystems weere obliterateed by glaciaal ice.
οFollowinng the final retreat of tthe glacierss, drainage systems beecame estab
blished, but
have not yet
y become fully organiized.
BASE
E LEVEL
Streams have
h
a loweer limit to w
which they can erode; this limit iis called baase level (~
Figure 12-16). Theorretically, a stream cou
uld erode itts entire vaalley to verry near sea
level, so sea
s level is commonlyy referred to
o as ultima
ate base levvel. In realitty. though,
streams neever reach ultimate
u
basse level beccause they must
m have ssome gradieent in order
to maintain How, Sttreams flow
wing into deepressions below sea level, such
h as Death
Valley in Californiaa. have a bbase level correspond
ding to the lowest po
oint of the
depressionn and are no
ot limited byy sea level.
In additioon 10 ultim
mate base llevel, stream
ms have loocal or tempporary base'' levels, Forr
example, a lake or another
a
streeam can seerve as a lo
ocal base leevel for the upstream
m
segment of
o a stream (Figure 12--16). Likewise, where a stream floows across particularly
p
y
resistant roock, a waterrfall may deevelop, form
ming a locall base level..
When seaa level rises or tills witth respect to
o the land, or t he landd over whicch a stream
m
flows is upplifted or su
ubsides, chaanges in basse level occcur. During the Pleistoccene Epochh
when exteensive glaciers were prresent on th
he Northern Hemispherre continentts, sea levell
was more than 100m lower thann at present. Accordinglly, streams deepened th
heir valleyss
by adjustiing to a new
w, lower baase level. Rising
R
sea leevel at the eend of the Pleistocenee
caused baase level to
o rise, and the stream
ms respondeed by depoositing sediiments andd
backfillingg previously
y formed vaalleys.
Streams adjust to hum
man interveention, but not
n always in anticipatted or desirrable ways.
Geologistss and engineers are w
well awaree that the process
p
of building a dam and
impoundinng a reservoir creates a local basse level (~ Figure 12- 17a\. Wherre a stream
enters a reservoir, its flow vvelocity diiminishes rapidly
r
andd depositio
on occurs;
consequenntly, reservo
oirs are eveentually fillled with sediment unlless they arre dredged.
Another consequence
c
e of buildinng a dam is that the water disch
charged at the
t dam is
largely sediment freee, but it stilll possessess energy to transport ssediment. Commonly,
C
such streaams simply acquire a nnew sedimeent load by vigorouslyy eroding downstream
from the dam.
d
Draining a lake along
g a stream's course may
y seem like a small chaange that is well worthh
the time and
a expensee to exposee dry land for agricultture or com
mmercial deevelopment.
However, draining a lake lowerrs the base level for that part off the stream
m above thee
lake, and the
t stream will
w very likkely respond
d by rapid down
d
cuttingg (Figure 12
2-17b).
οTHE GRADED
G
STREAM
M
A stream''s longitudinal profile shhows the elevations
e
of
o a channeel along itss length as
viewed inn cross secttion (~Figuure 12-18). The longitudinal proffiles of many streams
show a nuumber of irregularities ssuch as lakees and wateerfalls, whicch are local base levels
(Figure 12- 18a). Over
O
time tthese irregularities tend to be eeliminated by stream
m
processes;; where the gradient iss steep, ero
osion decreaases it, and where the gradient is
100 low too maintain sufficient fflow velocitty for sedim
ment transpoort, deposittion occurs,
steepeningg the grad
dient. In sshort, streaams tend to
t develop a smooth
h, concave
longitudinnal profile of
o equilibriuum, meanin
ng that all parts of the
he system dynamically
d
y
adjust to one
o another (Figure 12--18b).
Streams possessing
p
an equilibrrium profilee are said to be grade
ded streams; that is, a
delicate balance exists
e
betw
ween gradiient, disch
harge, flow
w velocity
y, channell
characteristics, and sediment
s
looad such thaat neither significant eerosion norr depositionn
b
is raarely attaineed, so the concept
c
of a
occurs witthin the chaannel. Such a delicate balance
graded strream is an
n ideal. Nevvertheless, many streams do inddeed appro
oximate thee
graded conndition, alth
hough not aalong their entire
e
coursees and usual
ally only tem
mporarily.
Even thouugh the conccept of a grraded stream
m is an ideaal, we can ggenerally an
nticipate thee
responses of a graded
d stream to cchanges altering its equ
uilibrium. A change in
n base level,,
for instancce, would cause
c
a streaam to adjusst as previo
ously discusssed. Increaased rainfalll
in a streaam's drainag
ge basin w
would result in greater discharge and flow velocity.
v
Inn
short, the stream wo
ould now ppossess greaater energy--energy thhat must bee dissipatedd
within thee stream sysstem by, forr example, a change in
n channel shhape. A chaange from a
semicircullar to a bro
oad, shallow
w channel would
w
dissip
pate more eenergy by friction.
f
Onn
the other hand, the stream
s
mayy respond by
b active down
d
cuttinng and erod
de a deeperr
valley andd effectively
y reduce its gradient un
ntil it is oncee again gradded.
οDEVEL
LOPMENT OF STR
REAM VA
ALLEYS
Valleys arre common
n landformss, and with few excep
ptions they form and evolve
e
as a
consequennce of stream
m erosion, although otther processses, especiaally mass wasting, alsoo
contributee. The shapes and sizees of valley
ys vary conssiderably; ssome are sm
mall, steep-sided gulliees, whereas others are bbroad and have
h
gently sloping vallley walls. Some
S
steep-walled, deeep valleys of
o vast propportions aree called canyyons, such ass the Grand
d Canyon off
Arizona. Particularly
P
narrow andd deep valleeys are gorgees.
A valley may
m begin where
w
runofff has sufficcient energy
y to dislodgee surface materials andd
excavate a small rilll. Once forrmed, a rill collects more
m
surfacee runoff an
nd becomess
deeper and wider until a full-fledged valley develops (~ Figure 12-19). Several
processes are involved in the origin and evolution of valleys, including down cutting,
lateral erosion, mass wasting, sheet wash, and headward erosion.
Down cutting occurs when a stream possesses more energy than it requires to transport its
sediment load, so some of its excess energy cuts its valley deeper. If downcutting were
the only process .operating, valleys would be narrow and steep sided, as in Figure 1219a. In most cases, however, the valley walls are undercut by the stream. Such
undermining, termed lateral erosion, creates unstable conditions so that part of a bank or
valley wall may move downslope by anyone or a combination of mass wasting
processes (Figure 12-19b). (a) Furthermore, sheet wash and erosion of rill and gully
tributaries carry materials from the valley walls into the main stream.
In addition to becoming deeper and wider, stream valleys are commonly
lengthened as well. Valleys are lengthened in an upstream direction by headword
erosion as drainage divides are eroded by entering runoff water (~ Figure 12-20a). In
some cases headward erosion eventually breaches the drainage divide and diverts
part of the drainage of another stream by a process called stream piracy (Figure 1220b). Once stream (b) piracy has occurred, both drainage systems must adjust; one
now has more water, greater discharge, and greater potential to erode and transport
sediment, whereas the other is diminished in all of these aspects.
STEAM 'TERRACES
Adjacent to many streams are stream terraces, erosional remnants of floodplains formed
when the streams ";ere flowing at a higher level. These terraces consist of a fairly flat
upper surface and a steep slope descending to the level of the lower. present-day
floodplain (~ Figure 12-21). In some cases, a stream has several steplike surfaces above
its present-day floodplain, indicating that stream terraces formed several times.
Although all stream terraces result from erosion, they are preceded by an episode of
floodplain formation and deposition of sediment. Subsequent erosion causes the stream
to cut downward until it is once again graded (Figure 12-21). Once the stream again
becomes graded, it begins eroding laterally and establishes a new floodplain at a lower
level. Several such episodes account for the multiple terrace levels seen adjacent to
some streams.
Renewed erosion and the formation of stream terraces are usually attributed to a
change in base level. Either uplift of the land over which a stream flows or lowering of
sea level yields a steeper gradient and increased flow velocity, thereby initiating an
episode of down-cutting. When the stream reaches a level at which it is once again
graded, down-cutting ceases. Although changes in base level no doubt account for many
stream terraces, greater runoff in a stream's drainage basin can also result in the
formation of terraces.
meanders. The San Juan River in Utah occupies a meandering canyon more than 390
meters deeep (~ Figurre 12-22). S
Such stream
ms, being restricted byy solid rock
k walls, aree
generally ineffective in eroding laterally; ass a result, th
hey lack a ffloodplain and
a occupy''
the entire width of the canyon flooor.
o understannd how a strream can cu
ut downwar
ard into soliid rock, butt
It is nott difficult to
forming a meanderin
ng pattern inn bedrock is
i another matter.
m
Beccause laterall erosion iss
inhibited once down
n-cutting beegins, one must
m
infer that the m
meandering course
c
wass
establishedd when the stream flow
wed across an area cov
vered by allu
luvium. Sup
ppose that a
stream neaar base leveel has estabblished a meeandering pattern.
p
If thhe land overr which thee
stream floows is uplift
fted, erosionn is initiated
d, and the meanders
m
beecome incissed into thee
underlyingg bedrock.
Ansswers
Additiona
al Readingss