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reservoir behind the dam provides habitat for the worm,
which spends part of its life cycle in the water. Humans are
infected by the worm through bathing, swimming, walking
barefoot along water banks, and drinking infected water.
In arid regions, the creation of a reservoir results
in greater evaporation because it has a larger surface
area in contact with the air than did the stream or river.
As a result, serious water loss and increased salinity of the
remaining water may occur.
When an area behind a dam is flooded, the trees
and other vegetation die and are decomposed, releas-
ing the large quantities of carbon that were tied up in
organic molecules in the plant bodies as carbon diox-
ide and methane. These gases absorb infrared radiation
and are therefore associated with global climate change.
The construction of large dams involuntarily dis-
places people from lands flooded by reservoirs. The Three
Gorges Project, the largest dam ever built, was recently com-
pleted on the Yangtze River in China (see Chapter 10).
The reservoir behind this dam is 632 km (412 mi) long.
The tops of as many as 100 mountains have become small
islands, fragmenting habitat and threatening 57 endan-
gered species. The reservoir has displaced almost 2 mil-
lion people, the largest number for any dam project.
The environmental and social impacts of a dam may
not be acceptable to the people living in a particular
area. Laws prevent or restrict the building of dams in cer-
tain locations. In the United States, the Wild and Scenic
Rivers Act prevents the hydroelectric development of cer-
tain rivers, although this law protects less than 1 percent
of the nation’s total river system. Other countries, such as
Norway and Sweden, have similar laws.
Dams cost a great deal to build but are relatively inex-
pensive to operate. A dam has a limited life span, usually
50 to 100 years, because over time the reservoir fills in
with silt until it cannot hold enough water to generate
electricity. This trapped silt, which is rich in nutrients,
is prevented from enriching agricultural lands down-
stream. For example, Egypt must rely on heavy applica-
tions of chemical fertilizer downstream from the Aswan
Dam to maintain the fertility of the Nile River Valley.- What is biomass, and how is it used?
- What are the advantages and disadvantages
of using wind to produce electricity? of using
hydropower to produce electricity?
slow- moving rivers or from streams with small flow capacities.
These new technologies have the potential to increase the
amount of hydroelectric power generated by existing dams.
Building a dam changes the natural flow of a river:
Water backs up, flooding large areas of land and forming
a reservoir, which destroys plant and animal habitats. Na-
tive fishes are particularly being harmed by dams because
the original river ecosystem is so altered. The migration
of spawning fish is also altered (see Chapter 10). Below
the dam, the once-powerful river is reduced to a relative
trickle. The natural beauty of the countryside is affected,
and certain forms of wilderness recreation are made im-
possible or less enjoyable, although the dams permit wa-
ter sports in the reservoir.
With at least 200 large dams around the world,
earthquakes may occur during and after the filling of the
reservoir behind the dam. The larger the reservoir and
the faster it is filled, the greater the intensity of seismic
activity. An area need not be seismically active to have
earthquakes induced by reservoirs.
If a dam breaks, people and property downstream
may be affected. In addition, waterborne diseases may
spread through the population. Schistosomiasis is a tropi-
cal disease caused by a parasitic worm. As much as half
the population of Egypt suffers from this disease, which re-
sulted when Egypt dammed the Nile River. First, the Low
Aswan Dam was built on the Nile River in 1902 to control
flooding. Lake Nasser was created in 1971 when the High
Aswan Dam was completed (Figure 18.11). The largeAswan High Dam construction, 1964
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The Aswan High Dam was completed in 1971, creating Lake Nasser.