Lake

Lake, large, inland body of fresh or salty standing water. Lakes are distinguished from bodies of water such as bays and gulfs, and some seas, that have an interchange with the ocean and are subject to tides. Lake basins are formed by many geologic processes, such as buckling of stratified rock into large folds, displacement of large masses of rock by faults (see Fault), and blocking of valleys by landslides. Volcanic craters may fill with water, resulting in lakes such as Crater Lake in Oregon or Lake Avernus in Italy.

Lakes also form by glaciation. Glaciers carve out large basins by scooping up bedrock and redistributing loose material. Many of the lakes of North America formed this way, including the Great Lakes and New York’s Finger Lakes. Lakes have also been detected under ice sheets and glaciers in Antarctica, Greenland, and Iceland. Temporary melt-water lakes can also form on top of ice sheets.

At the end of the ice ages, huge melt-water lakes dammed by glaciers formed at the edge of giant ice sheets. One of these lakes, called Lake Agassiz, covered a large area of North America until about 11,000 years ago. In some cases, the ice-age lakes caused catastrophic floods when the ice dams broke. The so-called scablands in eastern Washington state formed in this way when the ice dam on ancient Lake Missoula burst, sending huge walls of water from Montana across the Columbia Plateau.

The source of lake water is atmospheric precipitation that reaches the lake directly and by means of springs, brooks, and rivers. Lakes form and disappear over the course of varying lengths of geologic time (see Chronology). They may evaporate, as the climate becomes more arid, or they may fill up with sediment, leaving a bog or swamp in their place. In arid regions where precipitation is slight and evaporation great, lake levels rise and fall with the seasons and sometimes dry up for long periods.

In lakes where evaporation prevents the water from overflowing the basin rims, substances dissolved in the water become concentrated. The dissolved matter, brought by tributary streams, varies in composition with the nature of the rocks in the local drainage system. The primary mineral constituent of salt lakes is common salt; bitter lakes contain sulfates; alkali lakes contain carbonates; borax lakes contain borates; and some lakes contain combinations of these substances. Salt flats may be left when large lakes evaporate. Examples include the Bonneville Salt Flats in Utah, left by ancient Lake Bonneville; and Death Valley in California.

Lakes form at all altitudes and are distributed throughout the world. Almost one-half of the world’s lakes are in Canada. Lakes are abundant in high latitudes, particularly in mountain regions subjected to glacial action. Many lakes are important commercially as sources of minerals and fish, as shipping arteries, and as vacation resorts. Some lakes have distinctive ecosystems or unusual species of plants or animals such as the Baikal seal. Environmental issues affecting lakes include pollution, acid rain, and invasive species. Diverting rivers that replenished the Aral Sea, a saltwater lake, caused major ecological damage.

The largest lakes in the world include the Caspian Sea, Lake Superior, and Lake Victoria. The Dead Sea is the world’s lowest lake, 408 m (1,340 ft) below sea level. The Caspian, the world’s largest lake, covers an area of 370,998 sq km (143,243 sq mi). Lake Baikal is the deepest freshwater lake in the world, with a maximum depth of 1,637 m (5,371 ft).

Artificial lakes called reservoirs can be created by building dams to control the flow of rivers. The stored water can be used to generate see hydroelectric power, to irrigate crops, or as a water supply for cities. Notable artificial lakes include Lake Mead in Arizona and Nevada, formed by Hoover Dam; and Lake Volta in Ghana, formed by Akosombo Dam.