• 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 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 BCE 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 sandrockdirt, 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.


    Reservoirs can be located below ground, at ground level (like this one) or above the ground as water towers.

    Into Thin Air
    Evaporation is a common problem with reservoirs. In wet areas, the water that evaporates often falls again as rain. But in hot, dry areas, evaporation can result in a huge loss of water. The level of reservoirs in desert areas can drop 1.5 meters (5 feet) in a single year because of evaporation. Scientists and engineers are looking for ways of controlling evaporation. Such experiments in evaporation reduction include attempting to use palm leaves in Saudi Arabia, or inventing various other types of floating shade covers.

  • Term Part of Speech Definition Encyclopedic Entry
    bank Noun

    a slope of land adjoining a body of water, or a large elevated area of the sea floor.

    boating Noun

    sport and recreational activities involving watercraft.

    cavern Noun

    large cave.

    cistern Noun

    manmade container for storing large amounts of water. Also called a service reservoir.

    crop Noun

    agricultural produce.

    Encyclopedic Entry: crop
    dam Verb

    to block a flow of water.

    desert Noun

    area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

    Encyclopedic Entry: desert
    dirt Noun

    dry earth or soil.

    downriver Adjective

    toward the mouth, or ending point, of a river.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    engineer Noun

    person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

    evaporation Noun

    process by which liquid water becomes water vapor.

    Encyclopedic Entry: evaporation
    farmer Noun

    person who cultivates land and raises crops.

    fish Verb

    to catch or harvest fish.

    flood Verb

    to overflow or cover in water or another liquid.

    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    lake Noun

    body of water surrounded by land.

    Encyclopedic Entry: lake
    lake volume Noun

    total amount of water in a lake.

    palm Noun

    type of tree with a tall trunk, no branches, and a leafy crown.

    reduce Verb

    to lower or lessen.

    reservoir Noun

    natural or man-made lake.

    Encyclopedic Entry: reservoir
    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    rock Noun

    natural substance composed of solid mineral matter.

    sand Noun

    small, loose grains of disintegrated rocks.

    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    service reservoir Noun

    manmade container for storing large amounts of water. Also called a cistern.

    snow Noun

    precipitation made of ice crystals.

    sport Noun

    athletic activity.

    surface area Noun

    amount of exposed land, water, or other material.

    vary Verb

    to change.

    water tower Noun

    elevated structure used for storing water.