reservoir, storage tank or wholly or partly artificial lake for storing water. Building an embankment or dam to preserve a supply of water for irrigation is an ancient practice; India and Egypt have many old and large reservoirs. In building artificial lakes for a municipal water supply it is necessary to consider all the aspects of a catchment area, including the amount and distribution of rainfall, evaporation, runoff, soil or rock conditions, and elevation (for its effect upon precipitation and upon the pressure in the conducting pipes). The ground of the reservoir may be naturally impervious enough to prevent excessive seepage, or a clay or other lining may have to be built. The embankments or retaining walls may be of earth, loose rock, or masonry. Earth forms a good embankment but must be sealed by a core of clay, and the face must be covered with masonry or a similar substance to prevent erosion. Distributing reservoirs in towns are sometimes built of masonry or of reinforced concrete. They serve to cope with fluctuations of demand and with interruptions of supply from the source. Reservoirs are also built on the headstreams of or along the courses of rivers to aid in flood control, on canals to maintain water level for navigation, and to ensure water supply for hydroelectric plants. Some reservoirs are built on the tributaries of large rivers to act as catch basins for silt. In addition to seepage, the major loss of water from a reservoir is by evaporation; chemicals that form a film on a water surface are used to minimize such losses. Covered tanks made of prestressed concrete are used for limited local water supply.

Wachusett Reservoir, on the South Branch of the Nashua River, central Mass., NE of Worcester; built 1897-1905. Impounded by Wachusett Dam (completed 1906), it receives some of its water from Quabbin Reservoir and supplies the Boston area.

Rybinsk Reservoir, artificial lake, c.2,000 sq mi (5,200 sq km), NW European Russia. It was formed in 1941 between the upper Volga River and its tributaries, the Mologa and Sheksna rivers, with the completion of the dam and hydroelectric station at Rybinsk. The reservoir is the southernmost component of the Volga-Baltic Waterway.

Quabbin Reservoir, 39 sq mi (101 sq km), in the Swift River valley, central Mass., NE of Springfield. The reservoir, formed by Winsor Dam and Quabbin Dike, is the largest reservoir in Massachusetts. It is fed by the Swift and Ware rivers. The water flows to the Wachusett Reservoir through Quabbin Aqueduct (25 mi/40 km long) and supplies the Boston area.

Kensico Reservoir, c.4 sq mi (10 sq km), SE N.Y., N of White Plains, formed by Kensico Dam (completed 1915) on the Bronx River. A principal unit in the New York City water supply system, the reservoir receives water from sources in the Catskill Mts. and from the Delaware River.

Hume Reservoir, c.70 sq mi (180 sq km), on the Murray River, near Albury-Wodonga, on the Victoria-New South Wales border, Australia. It is the largest water-storage area in Australia. Impounded by Hume Dam (completed 1937), the reservoir irrigates most of the Murray basin. It receives additional water from the Snowy Mts. Hydroelectric Scheme.

Ashokan Reservoir, 13 sq mi (34 sq km), SE N.Y., completed 1912. It is supplied by the Esopus and Schoharie watersheds and provides part of New York City's water supply. Water is carried to the city via the 92-mi-long (148-km) Catskill Aqueduct. Completed in 1917, the aqueduct delivers water to Kensico Reservoir near White Plains and Hillview Reservoir in Yonkers, from where it is distributed to parts of New York City through tunnels cut in solid rock. The aqueduct passes 1,114 ft (340 m) under the Hudson River at Storm King Mt. A steel pipe under the Narrows of New York Bay carries water to Silver Lake, Staten Island, 120 mi (193 km) from Ashokan Reservoir.

Lake, on the upper Volga River, northwestern Russia. It was created by two dams on the Volga and its tributary, the Sheksna. When the project was completed in 1947, a lake of 1,768 sq mi (4,580 sq km) in area was formed; it was the world's largest artificial body of water. It regulates the flow of the Volga, provides power for Moscow and other cities, and is part of the Volga-Baltic Waterway.

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