LAKES

A lake is a body of water that lies in a hollow on. the earth's surface and is entirely surrounded by land. It is unconnected with the sea except by rivers. In arid and semi­arid areas lakes may have no outlet to the sea and form the focus of an area of inland drainage. As a result the water of the lake becomes increasingly saline. Some inland seas are actually large lakes, e.g:, the Caspian Sea, Dead Sea artd Aral Sea.

CLASSIFICATION OF LAKES Though most lakes have been formed by the action of glaciers and icesheets, others have been formed by rivers, marine and wind action, by earth movement and vulcanicity, and a few lakes are also man-made. Some basic categories may be considered.

(i) Lakes produced by earth movements These are lakes formed by tectonic forces. They may be formed in two ways. (a) By crustal warping. These occupy basin-like depressions e.g., Caspian Sea, Lake Victoria (Africa), Lake Eyre (Australia). Lake Titicaca (Peru/Bolivia) is the highest tectonic lake in the world. (b) By faulting. These occur in rift valleys, e.g., Lake Nyasa (East Africa), the Dead Sea and Loch Ness (Scotland). These lakes are usually long, narrow and deep.
(ii) Lakes formed by erosion They may be of two types.
a) Both valley glaciers and ice sheets gouge out hollows and troughs on earth surface which later get filled with water to form lakes. These may be cirque lakes, forming in arm-chain-like depressions, also called tarns, found in moun­tain regions, or they may be trough lakes which occupy long hollows excavated in valley bottoms, also called ribbon lakes. They may also be rock basin lakes, formed by ice-scouring action of ice sheets resulting in formation of shallow hollows, as in Canada and Finland.
(b) Wind-eroded lakes: In the extensive depressions which are created by wind-erosion in the arid regions, if excavation is carried out below the water table, a lake may develop; e.g., the Qattara Depression in Egypt. Oases formed out of aquifers are more permanent lakes. Some desert lakes-palaya, for example-dry up be­cause of excessive evaporation.

(iii) Lakes produced by deposition These may be of the following kinds. (a) River deposits: Ox-bow lakes _are cut­offs of mature rivers, made in flood plains, which even­tually get separated from the river. They are common in lower valleys of the Mississippi. Deposits of alluvium in deltas may isolate a distributary, thus producing a delta lake, e.g., Etang de Vaccares in the Rhone Delta.
(b) Lakes produced by glacial deposits: Moraine-dammed lakes are caused by the deposition of terminal moraine across a valley, e.g., in the Lake District (England). Lake Grade (Italy) was formed like this. Depressions in boulder clay result in lake formation, such as the lakes in Northern Ireland.
(c) Marine deposits can form lakes: They are called Haffs. These lakes have been formed by sand bars extend­ing along a coast and cutting off indentations in the coast, thus producing lagoons. These are found in southern coast of Baltic Sea and along Les Landes coast of south-west France.
(iv) Lakes produced by vulcanicity
(a) Crater and caldera lakes: Craters are formed when the top of a volcano gets blown off in a violent eruption. A caldera is formed if a crater gets enlarged by subsidence. Examples are Crater Lake in Oregon (USA), Lake Toba in north Sumatra and Lake Knebel in Iceland.
(b) Lava-blocked lakes: River valleys get blocked by lava to form a lake; the Sea of Galilee was formed by lava-flow blocking Jordan valley.
(c) Lava­subsidence lakes: These are caused by depressions caused by collapse of the lava crust, e.g., Myvatn, in Iceland.

(v) Other Types
(a) Solution lakes: Some rocks are dissolved and removed by rain water. This produces underground caverns. Limestone caverns collapse to ex­pose lakes which are usually long and narrow, e.g., Lac de Chailexon in Jura Mountain. Sometimes, solution, aided by subsidence, produces large depressions called polje. Rock salt has been removed by solution in Cheshire (England) and the depressions have formed containing lakes, called 'flashes'.
(b) Barrier lakes: These are temporary lakes formed by ice, lava, moraine damming a valley, or damming by landslides, avalanches or screes.
(c) Beaver lakes: These are created by some animals like beavers which build dams across the streams to form lakes, e.g., Beaver lake in Yellow Stone National Park (USA).
(d) Mining ponds: These are caused by open cast mining, e.g., by tin mining in West Malaysia. (e) Man-made lakes: These are artificial lakes made by building dams across rivers.

Importance and Use Lakes play an important role. They are used for communication as natural routes; for hydro-electric power development; for regulating the flow of rivers; as a source for fishery; for water storage; for providing irrigating water. They also moderate the climate.