Weather Horoscopes E-mail Visit our sponsors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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. Earth dams may be either simple embankments of earth or embankments reinforced with a core of cement or with an upstream surface made watertight.
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 have been constructed from early times to provide a ready supply of water for irrigation and other purposes. 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. The Grand Coulee Dam, located near Spokane, Wash., is the largest hydropower dam in the United States, producing 6,465,000 kW. The 20th cent.