A dam is a barrier built across a watercourse for impounding water. By erecting dams, humans can obstruct and control the flow of water in a basin. A reservoir is an artificial lake, usually the result of a dam, where water is collected and stored in quantity for various uses. The major function of reservoirs is to smooth out the variability of surface water flow through control and regulation and make water available in case of necessity. Water may flow out of a reservoir through various outlets such as derivations (to draw water for irrigation or other consumption), spillways (for flood protection) and penstocks (to produce electricity). Also, there may be water losses due to evaporation and seepage into the ground.