reservoir

A reservoir is an artificial lake used to store water. Reservoirs are often created by building a reinforced dam, usually out of concrete, earth, rock, or a mixture across a river or stream. Once the dam is completed, the stream fills the reservoir. When a reservoir is predominantly man-made (rather than being an adaptation of a natural basin) it may be called a cistern. The term reservoir is also often used to describe underground reservoirs such as an oil or water well.