Environmental Effects of Dams and Impoundments

Most of primary civilizations of world emerged in or near river valleys. The construction of dams and other hydraulic structures is, therefore, one of oldest branches of engineering (e.g. 11, 105). The earliest dams were probably built for purposes of irrigation, flood control, and water supply. Later, water was impounded so that its subsequent controlled release could provide a source of energy, first by use of waterwheels and later by use of hydroelectric generators. Other purposes include maintenance of an adequate river flow through year for navigation, and provision of facilities for recreation. Most modem reservoirs are designed for two or more of these purposes. Usually the role of water storage reservoirs . . . is to impound water in periods of higher so that it may be released gradually during periods of lower flows (135), but sometimes sole purpose of impoundment is to provide a new body of standing water for use as such; for example, for fishing or boating, or for waste-heat dissipation from a thermoelectric generating plant. The earliest dams were probably constructed by blocking stream with earth. Such dams are still constructed. In its simplest form. an earth-fill dam is a pile of compacted earth extending across a stream with a fairly gentle slope both upstream and downstream. Similar to earth-fill dams are rock-fill dams composed of quarried rock or natural boulders or gravel with a layer of impervious material on upstream face. A later development in dam construction was invention of masonry dam, probably in Spain (150). The earliest masonry dams were of gravity type. Such dams are held in place by their own weight pressing against their foundations, and usually have a long sloping downstream toe to prevent overtipping.