A reservoir is a body of water held by a dam on a river or stream, usually for use in irrigation , electricity generation, or urban consumption. By catching and holding floods in spring or in a rainy season, reservoirs also prevent flooding downstream. Most reservoirs fill a few miles of river basin, but large reservoirs on major rivers can cover thousands of square miles. Lake Nasser, located behind Egypt's Aswan High Dam , stretches 310 miles (500 km), with an average width of 6 miles (10 km). Utah's Lake Powell, on the Colorado River , fills almost 93 miles (150 km) of canyon.
Because of their size and their role in altering water flow in large ecosystems, reservoirs have a great number of environmental effects, positive and negative. Reservoirs allow more settlement on flat, arable flood plains near the river's edge because the threat of flooding is greatly diminished. Water storage benefits farms and cities by allowing a gradual release of water through the year. Without a dam and reservoir, much of a river's annual discharge may pass in a few days of flooding, leaving the river low and muddy the rest of the year. Once a reservoir is built, water remains available for irrigation, domestic use, and industry even in the dry season.
At the same time, the negative effects of reservoirs abound. Foremost is the destruction of instream and stream side vegetation and habitat caused by flooding hundreds of miles of river basin behind reservoirs. Because reservoirs often stretch several miles across, as well as far upstream, they can drown habitat essential to aquatic and terrestrial plants, as well as extensive tracts of forest. Humans also frequently lose agricultural land and river-side cities to reservoir flooding. China's proposed Three Gorges Dam on the Chang Jiang (Yangtze River) will displace 1.4 million people when it is completed; India's Narmada Valley reservoir will flood the homes of 1.5 million people. In such cases the displaced populations must move elsewhere, clearing new land to reestablish towns and farms. Water loss from evaporation and seepage can drastically decrease water volumes in a river, especially in hot or arid regions. Lakes Powell and Mead on the Colorado River annually lose 1.3 billion cubic yards (1 billion cubic m) of water through evaporation, water that both people and natural habitats downstream sorely need. Egypt's long and shallow Lake Nasser is even worse, losing as much as 20 billion cubic yards (15 billion cubic m) per year. Impounded water also seeps into the surrounding bedrock, especially in porous sandstone or limestone country. This further decreases river flow and sometimes causes slope instability in the reservoir's banks. A disastrous 1963 landslide in the waterlogged banks of Italy's Vaiont reservoir sloshed 192 million cubic yards (300 million cubic m) of water over the dam and down the river valley, killing almost 2,500 people in towns downstream. A further risk arises from the sheer weight of stored water, which can strain faults deep in the bedrock and occasionally cause earthquakes.
Sedimentation is another problem associated with reservoirs. Free flowing rivers usually carry a great deal of suspended sediment , but the still water of a reservoir allows these sediments to settle. As they accumulate in the reservoir, storage capacity decreases. The lake gradually becomes shallower and warmer, with an accompanying decrease in water quality . Even more important, sediment-free water downstream of the dam no longer adds sand and mud to river banks and deltas. Erosion of islands, banks, and deltas results, undermining bridges and walls as well as natural riverside habitat.
One of the losses felt most acutely by humans is that of scenic river valleys, gorges, and canyons. China's Three Gorges Dam will drown ancient cultural and historic relics to which travellers have made pilgrimages for centuries. In the United States, the loss of Utah's cathedral-like Glen Canyon and other beautiful or unusual environmental features are tragedies that many environmentalists still lament.
[Mary Ann Cunningham Ph.D. ]
Driver, E. E., and W. O. Wunderlich, eds. Environmental Effects of Hydraulic Engineering Works. Proceedings of an International Symposium Held at Knoxville, Tennessee, Sept. 12-14, 1978. Knoxville: Tennessee Valley Authority, 1979.
Freeze, R. A., and J. A. Cherry. Groundwater. Englewood Cliffs, NJ: Prentice-Hall, 1979.
Esteva, G., and M. S. Prakash. "Grassroots Resistance to Sustainable Development." The Ecologist 22 (1992): 45–51.
res·er·voir / ˈrezə(r)ˌvwär; -ˌv(w)ôr/ • n. a large natural or artificial lake used as a source of water supply. ∎ a supply or source of something: tapping into a universal reservoir of information. ∎ a place where fluid collects, esp. in rock strata or in the body. ∎ a receptacle or part of a machine designed to hold fluid. ∎ Med. a population, tissue, etc., that is chronically infested with the causative agent of a disease and can act as a source of further infection.
1. A surface body of water whose flow is artificially controlled by means of dams, embankments, or sluice gates in such a way that the water remains static until it is allowed to flow for a specific purpose (e.g. flood control or public water supply).
2. An underground rock formation that has sufficient void space to act as a store for water, natural gas, or oil.
1. A surface body of water whose flow is artificially controlled by means of dams, embankments, or sluice gates in such a way that the water remains static until it is allowed to flow for a specific purpose, e.g. flood control or public water supply.
2. An underground rock formation with sufficient void space to act as a store for water, natural gas, or oil.