Dam

A dam is a structure built across a river, valley, or watercourse to control or reduce the flow of water. Water held back by a dam forms a reservoir. Common purposes include irrigation of fields, provision of drinking water, flood control, and hydroelectric power generation.

Dams are assessed by their height, the area of their base, and their water storage capacity. The structure depends on the type of bedrock below, the shape of the valley, and the water pressure from above. Planning therefore requires geological and engineering surveys to evaluate ground stability and the risk of erosion.

Types

Several major types exist. A gravity dam resists water pressure through the weight of its material or stone. An arch dam transfers pressure to the valley sides and suits narrow valleys with solid rock. An embankment dam is constructed from earth, soil, and stacked stone, and is the most common type worldwide. There are also small, temporary barriers used to reduce flooding or shore erosion.

Dam components

Most large dams include a spillway—an overflow channel to prevent water from breaching the body and destroying it. They also have gates and controls for releasing water, and where hydroelectric generation occurs, turbines and generators. Some include fish ladders to mitigate impact on fish migration.

Benefits and impacts

Dams can provide water supply during drought and reduce flood severity during heavy rains. Hydroelectric power generates electricity without direct carbon emissions. However, reservoirs flood farmland and settlements, often requiring population resettlement. They also trap nutrient-rich sediment that would otherwise flow downstream, affecting soil fertility and the form of river mouths. In cross-border rivers, water allocation can become a matter requiring international agreement.

History

Simple dams were built by early societies that depended on irrigation, particularly around major rivers. Iron reinforcement and modern engineering in the nineteenth and twentieth centuries made it possible to construct large dams for hydroelectric generation. Today, dams under construction are assessed for environmental and social impact before authorization is granted.