Dams are built across streams and rivers to create reservoirs. The purpose of the reservoirs is capturing floodwaters to provide a consistent water supply for domestic use, irrigation, and power generation, and also to create lakes for recreation.

Very often a single reservoir is used for multiple purposes, and when water levels are low there is a conflict for the division of the water. There are many examples. Shall water be released for irrigation or to save downstream fish and habitats? For drinking purposes or to enable locks to operate to aid navigation? To keep water levels high for recreation use (swimming, boating, fishing) or to prevent salt water from entering valleys adjacent to coasts? People living around reservoirs want water levels to remain high, because exposed mud flats are esthetically unpleasant.

Dams built for flood control can give people living downstream from the dam a false sense of security. During very large episodes of flooding, a dam may not be able to hold the large volumes of water; dams may ultimately collapse, possibly killing large numbers of people. For example, in the Italian Alps, after the Vaiont Dam, the fourth highest dam in the

world, was built in 1960, occasional slippage of rock material occurred on the slopes behind the dam. In 1963 a mass of unstable rock debris, with a volume estimated to have been 700,000 cubic meters, broke away from the high valley walls and plunged into the reservoir, creating a wave that washed over the dam. Within two minutes a town 1 km downstream from the dam and a few smaller villages were inundated, killing 2,600 people.

As population increases, especially in urban areas, more dams will be needed to provide water for drinking, fire fighting, cooling air conditioners, decorative fountains, and lawn sprinklers. It is estimated that there are 40,000 large dams in the world, and the United States alone has almost 100,000 small dams.

More often than not these new dams flood wilderness areas and places considered sacred to native peoples, drown scenic landscapes and river valleys, cover productive farms, prevent the migration of fish to their habitats, and inundate forests and sites of historic, archaeological, and geological interest (for example, Three Gorges Dam, Yangtze River, China).

Dams and their reservoirs change river systems completely and for all time. Sediments that normally would be carried downstream are locked up behind the dam, even

Norris Dam, Tennessee. Completed in 1936, Norris Dam was the first dam built by the Tennessee Valley Authority (TVA) and has the largest flood control storage of any TVA dam on a tributary of the Tennessee River. (Tennessee Valley Authority)

tually reducing the volume of water the reservoir can hold and thereby its efficiency. The additional weight of the sediments and water bearing down on the crust can cause earthquakes, as has happened at Lake Mead behind Hoover Dam, in Arizona. In Egypt little sediment is being carried to the Nile delta, making what was once an area fertilized naturally with silt carried by the river dependent on artificial sources. Even the fisheries off the coast have diminished because of the reduced amount of nutrients carried into the Mediterranean Sea. Impounded waters change chemically, especially in areas of aridity, where evaporation increases salinity, making the water less useful for irrigation. Surface evaporation off large lakes can alter local weather patterns by setting up convection currents (such as at Lake Nasser, behind the Aswan Dam, in Egypt). Not only are large volumes of water lost to evaporation, but, in addition, reservoirs sited in sandstone terrain lose large volumes of water laterally into the surrounding bedrock. Dams also change land use patterns and biological habitats downstream from the dam.

Because of the decrease of sediments, streams develop a greater capability to erode the valleys. Rapids become more dangerous, with large boulders no longer removed because of the elimination of large floods. Changes in river flow also cause changes in the distribu tion of vegetation, helping to establish new plants. Flooding no longer removes plants that have grown on the riverbed, and the more or less constant level of water changes the patterns of vegetation on the valley walls.

Building dams may have unexpected consequences. For example, in Suriname a dense virgin forest was flooded by a reservoir, resulting in the death and decomposition of a large number of trees. Their decomposition produced hydrogen sulfide that enveloped the people in the region. Workers at the dam had to wear gas masks for several years, until all of the trees decomposed. If that wasn't bad enough, the reservoir water became acidified, which corroded the metal works of the dam. Water hyacinth, which had been relatively rare, began to spread over the lake because of the nutrients released into the lake by the decomposed trees. Navigation was slowed because of the dense growth. Floating plants provide food, oxygen, and breeding sites for carriers of two devastating diseases, malaria and schistosomiosus. Mosquitoes breed in the puddles on the plants, and aquatic snails thrive in the slow-moving water and play host to the flatworm that causes schistosomiosus. Finally, the floating vegetation blocks out sunlight, diminishes the oxygen levels of the bottom waters, and kills plants that fish depend upon.

—Sidney Horenstein

See also: Freshwater; Lakes; Rivers and Streams Bibliography

Caulfield, Catherine. 1983. "Dam the Amazon, Full Speed Ahead." Natural History 92, no. 7: xxx; McPhee, John. 1989. The Control of Nature. New York: Farrar, Straus and Giroux; Montgomery, Carla. 1999. Environmental Geology, 5th ed. Dubuque, IA: Wm. C. Brown; Morgan, Arthur E. 1971. Dams and Other Disasters. Boston: Porter Sergeant; Petersen, Keith C. 1995. River of Life, Channel of Death: Fish and Dams on the Lower Snake. Lewiston, ID: Confluence.

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