Geoscience Reference
In-Depth Information
(a)
(b)
Figure 16.39
(a) Landsat 7 satellite image of the Mississippi just south of Memphis, Tennessee, in August, 2012. The large sandbars along
the river were exposed due to much lower discharge associated with the drought in the central United States. (b) Barge traffic on the Mis-
sissippi River. Barges such as this one are used to ship corn and soybeans, as well as coal, oil, and other chemicals, up and down the river.
This shipping method is threatened when river levels are low, such as during the recent drought.
protected areas are rarely flooded. These systems
work, but increase the overall power of the stream at
high discharge by confining it to a narrow channel rather
than allowing gradual flooding.
KEY CONCEPTS TO REMEMBER ABOUT
HUMAN IMPACTS ON STREAMS
Most streams in the world have been impacted by
humans in some way.
4.
Dams are used to control downstream discharge on
rivers by impounding water (in reservoirs) and releasing
it slowly. Dams are used for flood control and hydro-
electric production, and to facilitate channel navigation.
Asphalt and concrete structures in cities increase storm
runoff. As a result, the lag time of a stream decreases,
peak discharge increases, and flash floods are more
likely.
5.
Although dams fill a definite societal need, they have
significant environmental impact because the natural
rhythm of seasonally variable stream flow is lost. In ad-
dition, they are a barrier to migrating river species.
Artificial levees are ridges built along stream chan-
nels to increase the height of the stream bank so that
SUMMARY OF KEY CONCEPTS
1.
Streams consist of channels in which water flows down-
hill under the force of gravity. They are fed by runoff or
groundwater and may have a large lake as their source.
Drainage basins are separated topographically from one
another by bordering high ground called divides and
interfluves.
During this shaping process, the slope of the stream de-
creases. Once the stream reaches this stage of evolu-
tion, it is said to be “graded.” The behavior of the stream
is controlled in large part by base level and climate.
4.
As streams approach a graded state, they begin to erode
horizontally through meandering. This process produces
a wide valley because bordering bluffs are eroded.
2.
Channel flow is defined by its hydraulic variables, includ-
ing width, depth, velocity, slope, and discharge. These
variables are interrelated. For example, stream discharge
is calculated by the equation
Q
â
w
ñ
d
ñ
v
.
5.
Streams produce a variety of erosional and deposi-
tional landforms, including gullies, cutbanks, point bars,
deltas, and terraces.
3.
In the course of their evolution, streams gradually sculpt
the landscape so that the slope of the stream is suffi-
ciently steep to carry the average discharge in the river.
6.
Given the importance of streams to human societies,
they are heavily managed.
