A barrier built on the course of a river, usually concrete, behind which a reservoir is formed. The retained water can then be used for irrigation, as a source of drinking water, for flood control or for electricity generation. Dams can be divided into gravitational, where the water is retained by the sheer weight of the dam, and arch dams. In addition, the dam can be equipped with spillways, movable gates, locks for ships or a system of pipes that lead water to a hydroelectric power plant or a drinking water treatment plant.