Streams gain flow in two ways. One is by the organization of runoff water
(called overland flow) from thin sheets running off of upland surfaces. Invariably,
the water moves downhill via gravity and becomes channelized as the water
increases its turbulence with increases in speed on its downslope run. Organization
into channels is simple but profound because it allows the multiplication of ero-
sional force many times that of overland flow. The second way in which streams
gain flow is through the intersection of their channels with the water table (top of
thegroundwater); a stream might flow year round even through dry seasons.
Erosion within streams takes place in four ways. The first is by direct assault by
force of the moving water (hydraulic force). Moving water is quite powerful as can
be attested to by those who have toppled attempting to wade through a fast-moving
mountain creek in shin-deep water. The second type of erosion is caused by the
propensity of streams to erode laterally. The erosion on the outside of meander
bends undercuts the banks, the bank materials fall into the stream and are trans-
ported away. The third type of in-channel erosion is performed by the materials
in transport. Analogous to sandblasting performed by sediments carried during
wind transport, stream materials are able to abrade the channel sides and them-
selves through myriad collisions. The net result is that materials become smaller
and smoothed and, ultimately, are mostlysilt.Evenacursoryexaminationnear
streams shows this evidence. The fourth type of channel erosion is corrosion—
the chemical assault on rocks. Chemical weathering is elaborated on in theweath-
ering and mass wastingarticle. Stream corrosion occurs primarily via solution
and hydrolysis. In most situations, erosion via corrosion is minor compared to
the other three erosion mechanisms.
Streams have widely varied rates of erosion depending on the stream’s dynamics.
Increased erosion is caused as water turbulence increases as a result of either rough-
ness of the channel or increase in the flow velocity. Of course, the total amount of
material removed by stream erosion is also related to the volume of water.
Of considerable interest is the erosional profile of a stream. Given long enough,
and in the absence of significant tectonic forces, a stream will form a concave lon-
gitudinal profile. That is, the slope of the stream with the horizontal is steeper near
the headwaters than near the mouth. This results in more erosion near the head-
waters and more deposition near the mouth. A smooth profile is known as a graded
profile and, before a stream achieves a graded profile, it erodes away the breaks in
profiles represented by falls and rapids.
Even without rocky channels, flow within streams is anything but smooth. Chan-
nel sides and bottoms are not perfectly smooth and this unevenness causes water to
be turbulent and flow in swirls known as eddies. The noses of these eddies concen-
trate kinetic energy and provide increased force for the accomplishment of erosion.
Stream Erosion and Deposition 327