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 artificial 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 have been creating reservoirs for thousands of years. The oldest known dam in the world is the Jawa Dam in what is now Jordan. It was built in about 3000 B.C.E. to store water to use for irrigation, or watering crops.

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.

Reservoirs serve other purposes. They are used for boating, fishing, and other forms of recreation. Some of the dams that create reservoirs are used to generate electricity.

The largest reservoir in the world by surface area is Lake Volta, which was created by damming the Volta River in the African nation of Ghana. Lake Volta covers about 8,500 square kilometers (3,280 square miles), an area larger than the U.S. state of Delaware. Lake Volta ranks fourth in the world in terms of volume, the total amount of water in the lake. The world's biggest reservoir by volume is also in Africa. Lake Kariba lies on the border between Zambia and Zimbabwe. This lake, which was formed by damming the Zambezi River, stores 185 cubic kilometers (44 cubic miles) of water.

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. But over time, this sediment builds up, greatly reducing the total amount of water in the reservoir.