Geoscience Reference
In-Depth Information
downstream of dams on the Saline and Solomon rivers, respec-
tively, within the Kansas River basin. In both graphs the blue
line represents the simulated discharge at each town if the dams
had not been present, whereas the red line shows the actual
discharge. Although actual peak discharge certainly increased
during the flood interval, it was about 80% less than what it is
simulated to have been. As a result of these two dams, the small
towns of Niles and Tescott were protected from the flood.
ecosystem is adapted to this variable discharge and can be
pressured because stream flow is largely held constant to meet
the various societal needs. A variety of animal species, for ex-
ample, are currently threatened by dams in the United States.
In the Pacific Northwest, dams impact salmon because they of-
ten form a barrier that stops fish from migrating upstream to
spawning sites. Along the Missouri River, the pallid sturgeon
is threatened because it requires high spring flows to trigger
its reproductive cycle; these flows are moderated significantly
by the numerous dams along the river. At this point in time,
environmentalists and government agencies are working toward
reducing the environmental impact of dams.
The regulation of stream discharge by dams can also be
controversial during dry periods when tension arises between
the needs of people living near reservoirs and those farther
downstream. A good example of such tension occurred in as-
sociation with the major drought that baked the central United
States from 2011 to the early part of 2013. This drought was
especially intense in the northwestern part of the Mississippi
River drainage basin, particularly in the upper part of the Mis-
souri River watershed. As a result, stream discharge in this
portion of the Mississippi drainage basin was dramatically
down.
Lower flow in the upper Missouri watershed would naturally
have the effect of reducing discharge in the Mississippi River
below St. Louis (where the Missouri River and Mississippi
River join) because less water would reach the trunk stream.
Such a natural reduction in downstream discharge was magni-
fied during the drought, however, because the Army Corps of
Engineers reduced the flow out of the large reservoirs in the
upper Missouri watershed (Figure 16.38a). This decision was
made in an effort to stabilize water levels in those lakes, which
were already 20% below capacity because of the drought. At
the Gavins Point Dam (Figure 16.38b), for example, flow out
of the dam was reduced from 1060 m
3
/sec (37,500 ft
3
/sec) to
750 m
3
/sec (26,500 ft
3
/sec) in the latter part of fall 2012.
This lower controlled discharge from the upper Missouri
River basin occurred at the same time that the Mississippi River
was already low due to the drought in much of the overall wa-
tershed. As a result, there was even less water in the Mississippi
River below St. Louis (Figure 16.39a) than would be the case
if dams were not present in the upper Missouri River basin.
Low flows in the Mississippi River are problematic because
the stream is a major transportation conduit for agricultural
products, with more than 90% of U.S. corn and soybean ex-
ports moving to the Gulf of Mexico on barges in this fashion
(Figure 16.39b). Mississippi River barges also transport a large
amount of coal, oil, and other chemicals. This kind of trans-
port is highly efficient, with the amount of material carried in
15 river barges being equal to 215 rail cars or 1050 large tractor
trailers on a highway.
Given lower water levels on the river, barge companies had
to reduce efficiency by lowering the load weight on their ships
so that they float higher in the river. In an effort to deepen the
river, the Corps aggressively dredged in key places. Neverthe-
Controversial Aspects of Dams
Although dams have
a clear benefit to society because they can be used to control
flooding, produce electricity, and enhance shipping, they can
also be controversial because of a variety of societal and envi-
ronmental costs associated with them The most obvious societal
cost is that dams are extremely expensive. One example of ex-
traordinary cost to build a dam is the Itaipu Dam on the border
of Paraguay and Brazil. This structure is currently the largest
hydroelectric dam in the world, producing up to 12,600 mW of
electricity, and provides 78% of the electricity used in Paraguay
and 26% of that consumed in Brazil. Construction of this dam
began in 1975 and was finished in 1983. It cost between US$20
billion and US$25 billion to complete and was such a large fi-
nancial drain that it contributed to the downturn of the Brazilian
economy in the 1990s. Similar costs occurred during construc-
tion of the Three Gorges Dam on the Yangtze River in China.
This controversial dam cost US$30 billion.
Another negative effect associated with dams is specifi-
cally related to the reservoirs they impound. In the course of
their creation, reservoirs flood large areas upstream of the dam
site. If this flooding occurs in heavily populated areas, many
people need to be relocated. Upstream of the Three Gorges
Dam, for example, approximately 632 km
2
(244 mi
2
) of land
will ultimately be inundated. As a result, up to 1 million people
have been relocated to avoid being flooded. Such forced relo-
cation has often been controversial, particularly in the United
States where individuals have a strong sense of property rights.
In a related vein, ancient cultural artifacts are sometimes lost
in this way. Upstream of the Aswan Dam in Egypt, which was
constructed across the Nile River, impoundment of Lake Nasser
resulted in the loss of numerous classical Egyptian sites and
artifacts.
In addition to the societal costs of reservoirs, they also
have negative environmental impacts. These impacts are most
closely related to the loss of ecosystems and spectacular scen-
ery. Prior to construction of the Itaipu Dam, for example, the
Parana River flowed through a spectacular gorge that contained
a variety of beautiful waterfalls. Many of these waterfalls, along
with much of the habitat unique to the plants and animals of the
region, are now submerged beneath the reservoir. Similar scen-
ery and habitat loss occurred in the canyon land region of the
southwestern United States when dams such as Glen Canyon
Dam and Hoover Dam (Figure 16.35) were constructed.
In the context of ecosystem impacts, dams also have nega-
tive environmental costs because of the way they regulate
the discharge of streams, when, in fact, stream flow is natu-
