reservoir

A reservoir (; from French réservoir [ʁezɛʁvwaʁ]) is an enlarged lake behind a dam, usually built to store fresh water, often doubling for hydroelectric power generation.
Reservoirs are created by controlling a watercourse that drains an existing body of water, interrupting a watercourse to form an embayment within it, excavating, or building any number of retaining walls or levees to enclose any area to store water.
The term is also used technically to refer to certain forms of liquid storage, such the "coolant reservoir" that captures overflow of coolant in an automobile's cooling system.

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