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, and the Itaipú Dam between Brazil and Paraguay, which at 623 ft (190 m) and more than 12,600,000 kW 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, producing 6,465,000 kW. 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.

Wilson Dam: see Tennessee Valley Authority.

W. A. C. Bennett Dam: see Peace, river, Canada.

Vaiont Dam, 858 ft (262 m) high, on the Vaiont River, a tributary of the Piave River, in Venetia, NE Italy, near Belluno. Vaiont Dam, one of the highest in the world, was completed in 1961 and is used to generate electricity. After heavy rains in 1963, landslides into the Vaiont reservoir caused the stored water to spill over the dam, sweeping away the village of Longarone and flooding nearby hamlets; some 2,000 people drowned.

Three Gorges Dam, 607 ft (185 m) high and 7,575 ft (2,309 m) long, on the Chang (Yangtze) River, central Hubei prov., China, 30 mi (48 km) W of Yichang. The largest concrete structure in the world, the dam was constructed from 1994 to 2006. Its hydroelectric station, which will not be fully operational until 2009, will have 26 turbines that will generate 18,200 MW, making it the largest in the world. In 2003 sluice gates were closed to begin flooding the scenic gorges for which the dam was named and filling a reservoir that will be 410 mi (660 km) long and will hold as much water as Lake Superior. Some 1.4 million people were moved in the process of creating the dam and reservoir.

The Dalles Dam, 260 ft (79 m) high and 8,875 ft (2,705 m) long, on the Columbia River between Oregon and Wash., NE of The Dalles, Oreg.; built 1952-57 by the U.S. Corps of Engineers. The dam, a major link in the development of the Columbia basin, provides hydroelectric power and improves navigation. Located at the head of the slackwater pool created by Bonneville Dam, it impounds a reservoir that provides ship passage 25 mi (40 km) upstream to John Day Dam. Fishways permit salmon and other migratory fish to pass the dam.

Shasta Dam, 602 ft (183 m) high and 3,460 ft (1,055 m) long, on the Sacramento River, N Calif.; built 1938-45. One of the largest concrete dams in the world, it is a major unit in the Central Valley project. The dam restores navigable depths to the Sacramento River, provides flood control, electric power, and irrigation and reclamation development. Its reservoir, Shasta Lake, 46 sq mi (119 sq km), is formed by the impounded waters of the Sacramento, Pit, and McCloud rivers. The lake is included in Whiskeytown-Shasta-Trinity National Recreational Area (see National Parks and Monuments, table).

Ross Dam, Wash.: see Skagit, river.

Roosevelt Dam: see Salt River valley.

Pensacola Dam, 145 ft (44 m) high and 6,500 ft (1,980 m) long, on the Grand River (local name of the Neosho), NE Okla., NE of Tulsa; built 1938-41 by the state of Oklahoma. The dam impounds Lake of the Cherokees, one of the largest reservoirs in the United States; it is a major recreation area.

Parker Dam, at the Ariz.-Calif. line, on the Colorado River; completed 1938. It is 320 ft (98 m) high and 856 ft (261 m) long. The dam impounds water for Los Angeles and other coastal cities, has a power plant, and supplies some water for irrigation. It also diverts water to Arizona.

Oroville Dam, 770 ft (235 m) high and 7,600 ft (2,317 m) long, on the Feather River, N Calif., near the city of Oroville. The largest unit of the Feather River project, the dam was built (1957-68) to provide electric power, drinking water, and irrigation for central and S California.

Oahe Dam, major unit of the Missouri River basin project, 242 ft (74 m) high and 9,360 ft (2,853 m) long, on the Missouri River, central S.Dak., near Pierre; built 1948-63 by the U.S. Corps of Engineers. The reservoir impounded by the dam extends c.250 mi (400 km) upstream and has one of the largest reservoir capacities in the United States. The dam provides hydroelectric power, flood control, improvement of navigation, and irrigation.

O'Sullivan Dam, Wash.: see Columbia basin project.

Norris Dam: see Tennessee Valley Authority.

Navajo Dam, 402 ft (123 m) high and 3,648 ft (1,112 m) long, NW N.Mex., on the San Juan River, near the Colo. line; built 1958-63 by the U.S. Bureau of Reclamation. The dam, a major unit of the Colorado River storage project, regulates the flow of the San Juan River and provides flood control. Water impounded by the dam is used by the Navajos to irrigate a significant portion of land on the Navajo Native American Reservation.

McNary Dam, 7,265 ft (2,214 m) long and 183 ft (56 m) high, on the Columbia River between Oregon and Washington, near Umatilla, Oreg.; built 1947-56 by the U.S. Corps of Engineers. Located at the head of the slack-water pool created by John Day Dam, it provides slack-water navigation 61 mi (98 km) upstream to the mouth of the Snake River; locks permit vessels to pass around the dam. McNary Dam is one of the largest hydroelectric power facilities in the United States.

Kettle Rapids Dam: see Nelson, river, Canada.

Kemano Dam: see Nechako, river, Canada.

Kariba Dam, hydroelectric project, in Kariba Gorge of the Zambezi River, on the Zambia-Zimbabwe border, S central Africa; built 1955-59. One of the world's largest dams, it is 420 ft (128 m) high and 1,900 ft (579 m) long. The Kariba project's generators supply electricity to the Copperbelt in Zambia and to parts of Zimbabwe. Kariba Lake, the vast reservoir created by the dam, extends c.175 mi (280 km) and has a maximum width of 20 mi (32 km). The creation of the lake forced resettlement of about 50,000 people living along the Zambezi. In 1960-61, Operation Noah captured and removed the animals threatened by the lake's rising waters.

John Day Dam, 219 ft (67 m) high and 5,640 ft (1,719 m) long, on the Columbia River between Oregon and Wash.; built between 1959 and 1968 by the U.S. Army Corps of Engineers. It is an extremely large generator of hydroelectric power. The dam's reservoir regulates navigation upstream; locks provide ship passage from The Dalles Dam reservoir to McNary Dam (see Columbia basin project).

Hungry Horse Dam, 564 ft (172 m) high and 2,115 ft (645 m) long, NW Mont. on the southern fork of the Flathead River. A major unit in the development program for the Columbia River basin, it was built (1948-53) to provide hydroelectric power, flood control, and irrigation. Hungry Horse Reservoir, formed by the dam, extends c.35 mi (55 km) upstream.

Hoover Dam, 726 ft (221 m) high and 1,244 ft (379 m) long, on the Colorado River between Nev. and Ariz.; one of the world's largest dams. Built between 1931 and 1936 by the U.S. Bureau of Reclamation, the dam is named for President Herbert Hoover; from 1933 to 1947 it was known as Boulder Dam. A key unit on the Colorado, the dam is a major supplier of hydroelectric power and provides for flood control, river regulation, and improved navigation. Hoover Dam impounds Lake Mead, the largest reservoir in the United States; water is used to irrigate more than 650,000 acres (263,000 hectares) in S California and Arizona, and c.400,000 acres (162,000 hectares) in Mexico. Hoover Dam is part of Lake Mead National Recreation Area (see National Parks and Monuments, table). Boulder City, Nev., was built to house workers on the project.

Great Salt Plains Dam, on Salt Fork, a tributary of the Arkansas River, NW Okla., near Enid. The dam was authorized in 1936 as a federal project and completed in 1941. In a salt-encrusted plains area, it provides flood control and impounds a large reservoir that is part of a national wildlife refuge.

Grand River Dam: see Pensacola Dam.

Grand Coulee Dam, 550 ft (168 m) high and 4,173 ft (1,272 m) long, on the Columbia River, N central Wash., NW of Spokane; built 1933-42 as a key unit in the Columbia basin project of the U.S. Bureau of Reclamation. Grand Coulee Dam, one of the world's largest concrete dams, is used for flood control, river navigation, irrigation, and power production that services the varied manufacturing in the area. The dam has the largest power-producing capacity (6,465 MW) in the United States. The dam impounds Franklin D. Roosevelt Lake, 130 sq mi (337 sq km), which extends to the Canadian border; it is one of the largest reservoirs in the United States. Power generated at the dam is used to pump water into Grand Coulee, a vertical-walled gorge, c.30 mi (48 km) long, carved by the Columbia River through the Columbia Plateau. The coulee, dammed at each end, is used as a reservoir (Banks Lake); it supplies water to more than 500,000 acres (202,343 hectares) on the plateau and acts as a backup against pump and power failures. Franklin D. Roosevelt Lake is part of Lake Roosevelt National Recreation Area. Located on the Pacific flyway, a chief north-south migratory route, the area has a great variety of waterfowl and land birds. Grand Coulee (1990 pop. 984) and Coulee Dam (1990 pop. 1,087) were founded by the U.S. government in 1935-36 as construction, operational, and housing bases for the dam.

Glen Canyon Dam, 710 ft (216 m) high, 1,560 ft (475 m) long, NE Ariz., on the Colorado River. The key unit of the U.S. Bureau of Reclamation's Colorado River storage project, it is one of the world's largest concrete dams (larger in bulk, though not in height, than Hoover Dam). The dam, completed in 1963 and dedicated in 1966 after completion of its power-generation facilities, regulates the flow of the upper Colorado and its tributaries and produces hydroelectricity (since 1964). The dam sharply reduced the seasonal flow of the Colorado downstream, dramatically altering the ecology of the river in the Grand Canyon. Changes in water releases have been experimented with in an attempt to ameliorate the effects of the dam.

Lake Powell, formed by the dam, extends 186 mi (299 km) upstream into S Utah. The lake was named after the American explorer John W. Powell, who mapped and named the canyon in 1870. This lake is the nucleus of the Glen Canyon National Recreation Area (see National Parks and Monuments, table). Downstream is the Glen Canyon Bridge, 1,271 ft (387 m) long and 700 ft (213 m) high, one of the world's longest and highest steel-arch bridges.

Garrison Dam, c.11,300 ft (3,400 m) long and 210 ft (64 m) high, on the Missouri River, near Riverdale, W central N.Dak.; one of the world's largest earth-filled dams used for irrigation power. Built by the U.S. Army Corps of Engineers (completed 1956), it is a key unit in the Missouri River basin project. It impounds Lake Sakakawea.

Furnas Dam, c.390 ft (120 m) high, at the junction of the Rio Grande and Sapucaí rivers, SE Brazil, in Minas Gerais state; completed 1962. It has a 620-sq mi (1,606-sq km) reservoir. It includes large hydroelectric facilities and is the center of an extensive regional electrical grid serving the industrial centers of São Paulo, Rio de Janeiro, and Belo Horizonte.

Fort Peck Dam, 21,430 ft (6,531 m) long and 250 ft (76 m) high, on the Missouri River, NE Mont.; one of the world's largest earth-filled dams. The dam was built (1933-40) by the U.S. Army Corps of Engineers as a flood-control and navigation-improvement project. In 1944 it became part of the Missouri River basin project and is used for irrigation and the generation of hydroelectricity. Fort Peck Lake, 189 mi (304 km) long, is one of the largest artificial lakes in the United States and an important recreation area of the N Great Plains.

Fontana Dam, N.C.: see Little Tennessee, river.

Flaming Gorge Dam, in a deep canyon of the Green River, NE Utah; built 1958-63 by the U.S. Bureau of Reclamation as a major unit in the Colorado River storage project. The dam regulates the flow of the upper river and produces hydroelectricity. Flaming Gorge Lake, extending 91 mi (146 km) upstream, is part of Flaming Gorge National Recreation Area, administered by the U.S. Forest Service. The canyon was named in 1869 by the U.S. explorer John Wesley Powell because the brilliant red gorge, from a distance, looked as if it were on fire.

Elephant Butte Dam, main unit of the Rio Grande project of the U.S. Bureau of Reclamation on the Rio Grande, SW N.Mex.; completed 1916. The dam, with its large reservoir, is used for flood control, hydroelectric power, and irrigation. The project was involved in a water-rights dispute between the United States and Mexico that was settled by a treaty in 1906.

Denison Dam, 17,200 ft (5,243 m) long, on the Red River along the Texas-Okla. border, NW of Denison, Tex. The dam, built by the U.S. Corps of Engineers for flood control and hydroelectric power, was completed in 1944 and impounds Lake Texoma (227 sq mi/588 sq km), one of the largest artificial lakes in the United States. The lake is a major recreation area and has two national wildlife refuges along its shores.

Dam, Henrik, 1895-1976, Danish biochemist. He identified vitamin K in 1934 and later investigated the role of vitamin E in nutrition. The 1943 Nobel Prize in Physiology or Medicine was awarded jointly to Dam and to E. A. Doisy for their work on vitamin K. In 1946, Dam became professor of biochemistry at the Polytechnic Institute, Copenhagen, and in 1956 he became head of the biology division of the Danish Fat Research Institute.

Coolidge Dam, 249 ft (76 m) high, 920 ft (280 m) long, on the Gila River, SE Ariz.; built 1927-28. It irrigates c.100,000 acres (40,470 hectares), half of which are Native American lands in San Carlos Reservation. San Carlos Reservoir, formed behind the dam, lies above old tribal burial grounds and the former camp of Geronimo.

Conowingo Dam, 4,648 ft (1,417 m) long, 102 ft (31 m) high, on the Susquehanna River, NE Md.; completed 1928. It is one of the largest nonfederal hydroelectric power plants in the United States. Conowingo Lake, formed by the dam, extends 14 mi (23 km) upstream.

Boulder Dam: see Hoover Dam.

Bonneville Dam, one of the major dams on the Columbia River where it passes through the Cascade Mts., between Oregon and Wash. The dam, 2,690 ft (820 m) long and 197 ft (60 m) high, was built between 1933 and 1943 by the U.S. Corps of Engineers and was one of the largest hydroelectric projects undertaken under the New Deal. It is used for navigation, flood control, and power production. Locks permit ships to pass around the dam; fish ladders allow salmon to spawn upriver.

Beaver Dam, city (1990 pop. 14,196), Dodge co., SE Wis., on Beaver Dam Lake, in a productive farm and dairy area; inc. 1856. Industries included food processing, metal and metal products fabrication, printing, and machinery manufacturing.

Akosombo Dam, Ghana: see Volta, river.

Barrier built across a stream, river, or estuary to conserve water for such uses as human consumption, irrigation, flood control, and electric-power generation. The earliest recorded dam is believed to be a masonry structure 49 ft (15 m) high built across the Nile River in Egypt circa 2900 BC. Modern dams are generally built of earth fill, rock fill, masonry, or monolithic concrete. Earth-fill (or embankment) dams, such as Egypt's Aswan High Dam, are usually used across broad rivers to retain water. The profile of an earth-fill dam is a broad-based triangle. Concrete dams may take various forms. The gravity dam uses its own dead weight to resist the horizontal force of the water. Concrete-buttress dams reduce material in the wall itself by using support buttresses around the outside base. An arch dam, such as Hoover Dam, is built in a convex arch facing the reservoir, and owes its strength essentially to its shape, which is particularly efficient in transferring hydraulic forces to supports.

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formerly Boulder Dam

Highest concrete arch dam in the U.S., built on the Colorado River at the Arizona-Nevada border. It impounds Lake Mead. The dam, completed in 1936, is used for flood and silt control, electric power, irrigation, and domestic and industrial water supplies. It is 726 ft (221 m) high and 1,244 ft (379 m) long (along the crest), has a power capacity of 1,345 megawatts, and a volume of 4.4 million cu yd (3.36 million cu m).

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