reservoir,an open-air storage area (usually formed by masonry or earthwork) where water is collected and kept in quantity so that it may be drawn off for use.

Changes in weather cause the natural flow of streams and rivers to vary greatly with time. Periods of excess flows and valley flooding may alternate with low flows or droughts. The role of water-storage reservoirs, therefore, is to impound water during periods of higher flows, thus preventing flood disasters, and then permit gradual release of water during periods of lower flows. Simple storage reservoirs were probably created early in human history to provide water for drinking water and water for irrigation. From southern Asia and northern Africa the use of reservoirs spread to Europe and the other continents.

Reservoirs ordinarily are formed by the construction of dams across rivers, but off-channel reservoirs may be provided by diversion structures and canals or pipelines that convey water from a river to natural or artificial depressions.

When streamflow is impounded in a reservoir, the flow velocity decreases and sediment is deposited. Thus, streams that transport much suspended sediment are poor sites for reservoirs; siltation will rapidly reduce storage capacity and severely shorten the useful life of a small reservoir. Even in larger reservoirs, sedimentation constitutes a common and serious problem. Because removal of the deposited sediments from reservoirs is generally too costly to be practical, reservoirs on a sediment-laden stream are characteristically planned to provide a reserve of storage capacity to offset the depletion caused by sedimentation. Despite this, the life expectancy of most reservoirs does not exceed 100 years at present sedimentation rates.

An associated problem is erosion of the stream channel below a reservoir when water is released. Because the sediment load is deposited in the reservoir, the released water has renewed transporting capacity, and channel erosion results.

Water in a reservoir may be lost by surface evaporation, by seepage into the surrounding soil or rocks, and by seepage through dam foundations. Seepage losses ordinarily can be reduced, but evaporation losses are often of major consequence. Gross evaporation from water surfaces in the temperate and tropical climates may amount to a few metres a year. In humid regions this loss is offset by direct precipitation, and the net surface loss may be moderate or negligible; but in . In regions of lower rainfall the net loss may be substantial, amounting to 1.5 m metres (5 feet) or more annually in some desert areas. Considerable success has been achieved in experimental programs for reducing evaporation losses, notably in Australia, Africa, and the United States, by the application on the water surface of a liquid monomolecular layer of a saturated, long-chain fatty alcohol such as hexadecanol.

Water reservoirs range in size and complexity from small single-purpose impoundments of only a few acre-feet (1 acre-foot = 1,233 cubic m, or 43,560 cubic feet) to huge and complex multiple-purpose impoundments of millions of acre-feet. A single-purpose reservoir is designed to fulfill only one function, such as irrigation, power generation, navigation, flood control, water supply, recreation, or low-flow regulation. The prevailing modern trend has been toward construction of multiple-purpose reservoirs designed to serve at least two principal functions.

Reservoirs can cause adverse environmental impacts—e.g., destruction of fish habitats and ecosystems)—and large-scale reservoir projects may require the submergence of cities and towns. For example, construction of the Three Gorges Dam on the Yangtze River (Chang Jiang) in China, designed primarily for flood control and as a source of electric power, displaced almost two million people from their homes. Social and environmental impacts must be taken into account during the planning stages of new reservoirs.

Relatively small enclosed reservoirs are familiar sights in most towns and cities. Often constructed on hills or supported in steel tanks on towers, these reservoirs are integral parts of most local water distribution systems. They usually provide a storage volume equal to the community’s average water demand for a single day. The tanks are elevated to provide adequate water pressures in the pipelines. The water is used for emergencies such as power blackouts, pump station failures, and fire control. In addition, these tanks serve to satisfy the peak hourly water demands in the community. When water demand exceeds the average daily demand, water flows out of the tanks into the distribution network. When water demand is very low (i.e., late at night), high-lift pumps refill the tanks.