dam, barrier, commonly across a watercourse, to hold back water, often forming a reservoir or lake; dams are also sometimes used to control or contain rockslides, mudflows, and the like in regions where these are common. Dams are made of timber, rock, earth, masonry, or concrete or of combinations of these materials. Timber is seldom used in dams because timbers are impermanent and their height is limited. Rock-fill dams consist of an embankment of loose rock with either a core impervious to water (e.g., clay) or a watertight face on the upstream side. Earth dams may be either simple embankments of earth or embankments reinforced with a core of cement or with an upstream surface made watertight. Masonry and concrete dams are either gravity dams or arch dams (either single-arch or multiple-arch). Gravity dams are dependent upon their own weight for resistance to the pressure of the water. Single-arch dams are curved upstream and are usually constructed in narrow canyons or gorges where the rocky side walls are strong enough to withstand the tremendous lateral thrust of the dam that is caused by the pressure of the water. Dams of the multiple-arch type consist of a number of single arches supported by buttresses. Dams may also be constructed with roller-compacted concrete, in which thin layers of concrete are compacted as if they were earth layers; this produces a far stronger dam, without the need for full forms.

Dams have been constructed from early times to provide a ready supply of water for irrigation and other purposes. One of the earliest large dams for this purpose was a marble structure built c.1660 in Rajputana (Rajasthan), India. A dam used only to impound water is often called a barrage; the largest such barrage is the Syncrude Tailings Dam in Canada, which impounds 540 million cubic meters of water.

Most modern dams are constructed for multiple purposes, e.g., to provide for irrigation, to aid flood control and hence improve the navigability of waterways, and especially to furnish power for hydroelectric plants. Notable dams built to provide hydroelectric power include the Aswan Dam in Egypt, the Kariba Dam in Zambezi, the Daniel Johnson Dam in Canada, the Guri Dam in Venezuela; the Itaipú Dam between Brazil and Paraguay, and the Three Gorges Dam in China, which is the largest hydropower dam in the world. The Grand Coulee Dam, located near Spokane, Wash., is the largest hydropower dam in the United States. The 20th cent. witnessed many great dam projects in the United States (see Central Valley project; Missouri River basin project; Tennessee Valley Authority). The Oroville Dam, located in California, the tallest in the United States, is 770 ft (235 m) high; the Rogun Dam, in Russia, the tallest in the world, is 1,100 ft (335 m) high. A large dam in Panama forms Gatún Lake, the key to the Panama Canal system.

See A. H. Cullen, Rivers in Harness: The Story of Dams (1962); N. Smith, A History of Dams (1972); D. Jackson, Great American Bridges and Dams (1988); A. H. J. Dorsey, ed., Large Dams: Learning from the Past, Looking at the Future (1997).

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