Geoscience Reference
In-Depth Information
downstream. A well-known example of a dam is Hoover Dam,
which straddles the Colorado River on the border of Arizona
and Nevada (Figure 16.35). Formed behind this dam is Lake
Mead, which is one of the largest artificially created bodies of
water in the world, with an area of 603 km 2 (233 mi 2 ). Lake
Mead will be examined again in Chapter 20.
Historically, dams have been built for three primary rea-
sons: (1) to enhance river navigation, (2) to provide hydroelectric
energy, and (3) to provide flood control. The reservoirs behind
dams are also extensively used for recreation and as a source
of irrigation in many places. Dams improve navigation because
the amount of water in the river downstream of a dam can be
controlled. For example, Figure 16.36 is a hydrograph from the
Chattahoochee River at Norcross, Georgia, which lies down-
stream of Buford Dam. Note how the discharge of the stream
was held much more constant following dam construction than
would have occurred naturally before. As a result of this control-
ling effect, rivers such as the Ohio, Mississippi, and Missouri are
important transportation corridors because water depth is gener-
ally maintained at a consistent level that is safe for barge traffic.
Dams can also be used to produce hydroelectricity. This form
of renewable energy is produced when water from the upstream
side of the dam (from the reservoir) flows down and through tun-
nels in the dam to its outlet into the channel. As the water flows
through the dam, it spins turbines within generators that produce
electricity. The hydroelectric generators at Hoover Dam, for exam-
ple, are capable of supplying nearly 1.5 million kW of power and
provide electricity to Arizona, Nevada, and Southern California.
The third major function of dams is to provide flood pro-
tection for downstream areas because they can be used to store
excess runoff from upstream tributaries during periods of heavy
rainfall. In theory the gates of a dam can be shut completely
during a particularly wet period, which would cause all the
Figure 16.34 An example of engineered levees. This artificial
levee along the Mississippi River at New Orleans, Louisiana,
keeps the Mississippi River (right) from flooding the city. The U.S.
Army Corps of Engineers built and maintains this structure, as
well as many others like it, along the Mississippi.
low-level floods that naturally occur along the stream and bring
a fresh supply of sediment and nutrients into the wetlands. This
relationship is lost, however, along a stream that is heavily pro-
tected by levees. Another problem with levees is that they may
actually increase the intensity of flooding when major floods
do occur. Under natural flood conditions, a stream will slowly
spill out of its banks and spread across its floodplain. Although
this slow spread can certainly be damaging, the gradual nature
of the flood decreases the stream power and has a cushioning
effect on locations downstream. In a levied section of river,
however, the water cannot slowly spread horizontally because
it is confined between the levees. As a result, the depth of the
water increases beyond what it naturally would, and the associ-
ated stream power grows. Much of this energy is transferred to
the levee system, which may then fail during especially large
floods like the 1993 and 2001 Mississippi River floods. If such a
failure occurs, water pours catastrophically through the breach
in the levee, resulting in tremendous damage to farmland and
human structures where the break occurs.
Dams and Reservoirs
Why Build Dams? Another way that humans interact with
rivers is through the construction of dams, which are among the
largest human structures in the world. A dam is an engineered
obstruction that is built across a river to control its flow. When
you were young, you may have built dams by piling dirt across
small streams to control them. In large dams, this control is ob-
tained through hydraulic gates in the dam that can be adjusted
to allow a specific discharge of water to proceed downstream.
Dams typically have behind them a large lake or reservoir that
forms by impounding stream water that would otherwise flow
Figure 16.35 Hoover Dam on the Colorado River. This dam
was constructed in the early 1930s to provide flood control and
 
 
Search WWH ::




Custom Search