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[[Image:dam icon.png|right|80px|]]

[[Image:Catchment dam.jpg|thumb|right|200px| Turkana men and women build a dam on the river Lokitaung in northern Kenya. Photo by AFP/Simon Maina.]]

Water can be made available by damming a natural rainwater catchment area, such as a valley, and storing the water in the reservoir formed by the dam, or diverting it to another reservoir. Important parameters in the planning of dams are: the annual rainfall and evaporation pattern; the present use and runoff coefficient of the catchment area (e.g. bare rock catchments have high runoff coefficients, around 0.9); water demand; and the geology and geography of the catchment area and building site.