When the speed and direction of
eddies are averaged over minutes,
streams are known to exhibit helical
flow, which is the corkscrewing of
water downstream. Helical flow leads
directly to the meandering of streams.
The largest materials that can be
transported at a particular time and
place are pushed along the bottom
via traction. Slightly smaller pieces
skip along the bottom in parabolic
arcs (saltation). The smallest pieces
are transported relatively rapidly
and for longer distances by suspen-
sion. Finally, the corroded materials
are moved in solution. Of these
modes, the suspended load is the
largest transported component in
most streams. The transportation of
eroded materials in streams is analo-
gous to the processes of wind erosion
except that wind is not capable of
transporting dissolved materials.
Especially when streams flow over their own deposits they are actively changing
their channels over time because helical flow does not allow water to flow straight in
a channel for more than a dozen or so times its channel width. The fastest, deepest
flow in a channel is known as the thalweg and swings from side to side in the chan-
nel. The thalweg is usually near the outside of stream curves and this concentration
flow helps to erode the stream bank in that direction. Over time, the meanders
themselves migrate, increase their amplitudes, and cut themselves off, and new
meanders form.
Sometimes streams flood sending water out of the channel. The water flowing
over the general landscape is not as fast as the in-channel flow and the water loses
some of its capability to move materials. So, materials are deposited near streams
and given the general name of “alluvium.” Many times alluvium is composed of
silt. The flat areas along channel peripheries are floodplains and they are depositio-
nal features resulting from repeated flooding. In a sense, floodplain deposits are
alluvial materials waiting to be transported by a larger flood at some later time.
As streams meander and flood along floodplains there is a large variety of land-
forms created out of the depositional materials. These include natural levees caused328 Stream Erosion and Deposition
This small river in southern Utah illustrates the
erosive power of water in an arid environment.
(Pmccall/Dreamstime.com)