A reservoir (etymology: from French réservoir a "storehouse" ) is a natural or artificial lake, storage pond, or impoundment from a dam which is used to store water. Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to describe naturally occurring underground reservoirs such as those beneath an oil or water well.