Reservoir Water Sources

A reservoir is an artificial lake where water is stored. Most reservoirs are formed by constructing dams across rivers. A reservoir can also be formed from a natural lake whose outlet has been dammed to control the water level. The dam controls the amount of water that flows out of the reservoir.

Service reservoirs are entirely manmade and do not rely on damming a river or lake. These reservoirs, sometimes called cisterns, hold clean water. Cisterns can be dug in underground caverns or elevated high above ground in a water tower.

People build reservoirs because the amount of water in a river varies over time. During very rainy times or when mountain snow is melting, the water in a river rises and sometimes overflows its banks. By limiting the amount of water allowed to continue downriver, reservoirs help control flooding.

During droughts, or extended dry periods, the water level in a river may be very low. Under these conditions, more water is released from the reservoir so farmers can water their crops and homes and businesses can function normally.

The water in reservoirs is very still. Because of this, bits of sand, rock, dirt, and other material, called sediment, sink to the bottom, leaving the water quite clear. Over time, this sediment builds up, and adequate filtration is required to ensure adequate water quality and protection of downstream equipment.