Dams and Impounded Rivers - River Dynamics and Integrated River Management - page 391

7 Dams and Impounded Rivers
Abstract
Construction of dams on numerous rivers throughout the world must be the greatest achievement of human
beings in river development and the greatest disturbance to stream ecology. The dams impact the riparian
vegetation, invertebrates and fish in both the reservoir and downstream reaches. Reservoir sedimentation
has changed the fluvial processes. The downstream reaches are changed from aggradation to degradation,
which causes ecological problems. Various strategies for reservoir sedimentation management have been
applied, such as flushing, restoring clear water and releasing turbid water, and releasing turbidity (density)
currents. The risk of dam failure is studied. The advantages and disadvantages of dam removal for habitat
restoration are discussed. The Three Gorges Project on the Yangtze River has been the greatest hydro-project
in China. The construction of the Three Gorges Dam and management strategies for the reservoir are also
discussed in this Chapter.
Key words
Ecological impacts of dams, Reservoir sedimentation, Dam failure, Dam removal, Three Gorges Dam
7.1 Impacts of Dams on The Environment and Ecology
The Hoover Dam, an astonishingly graceful curve of concrete with a height of 221 m pushing back the
Colorado River to fill its deep canyon on the Arizona-Nevada border, unleashed the big dam era. President
Franklin D. Roosevelt spoke highly at the inauguration ceremony at Hoover Dam on 30 September 1935:
“The power and the water, I came, I saw, and I was conquered.” To many people, including leaders,
engineers, and scientists, big dams have been potent symbols of both patriotic pride and the conquest of
nature by human ingenuity. Providers of electric power, water, and food; tamers of floods; greeners of the
desert; for most of the last century, dams, the largest single structures built by humanity, have symbolized
progress. However, large dams also cause many problems, especially sedimentation in the reservoir, erosion
in the downstream reaches, and disturbance to the stream and terrestrial ecology.
Impounded rivers change the flow conditions, and, therefore, change the water quality in the reservoirs
and the downstream reaches, which affects the wildlife in the reservoir and downstream reaches (Petts,
1984). Freshwaters, because of a host of human assaults, but especially because of dams, are the most
degraded among the major ecosystems. A dam tears at all the interconnected webs of river valley life.
Swedish ecologists concluded in 1994 that nearly four-fifths of the total discharge of the largest rivers in
the U.S., Canada, Europe, and the former USSR is 'strongly or moderately affected' by flow regulation,
diversions, and the fragmentation of river channels by dams (Dynesius and Nilsson, 1994) The most
extreme illustration of the downstream impacts of water diversions is the Aral Sea in Central Asia. Once
the world's largest body of freshwater outside North America, the sea has shrunk to less than half its
previous area and separated into three hypersaline lakes. The dams on the Yellow River in China have
reduced the flow into the Bohai Sea and resulted in many cutoffs of flow in the lower reaches of the river
(Chapter 6). With an increasing number of such kind of events the negative impacts of large dams on
ecology have become a public concern.
7.1.1 Dam Construction
According to the estimates of the International Commission on Large Dams (ICOLD), the leading
dam-industry association, the world's rivers had been obstructed by more than 40,000 large dams by the