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A reservoir (from French réservoir – a " tank ") is a storage space for fluids. These fluids may be water , hydrocarbons or gas. A reservoir usually means an enlarged natural or artificial lake , storage pond or impoundment created using a dam or lock to store water . Reservoirs can be created by controlling a stream that drains an existing body of water . They can also be constructed in river valleys using a dam . Alternately, a reservoir can be built by excavating flat ground or constructing retaining walls and levees. Tank reservoirs store liquids or gases in storage tanks that may be elevated, at grade level, or buried. Tank reservoirs for water are also called cisterns. Underground reservoirs are used to store liquids, principally either water or petroleum , below ground.