Lake (Lat. lacus) is a portion of water surrounded by land. Lakes are of two kinds—fresh-water and saline—and have been formed in various ways. Taking first the fresh-water lakes, these may be grouped as follows: (1) Obstruction Lakes.—Some of these are more or less temporary sheets of water, such as the lake-like expansions of certain rivers, and the deserted loops of river-channels. Other temporary lakes are due to the operations of the beaver; to the choking of the narrower passages of a river-channel by drifted vegetable debris or river-ice; or to the advance of a glacier across the mouth of a lateral valley. Now and again rock-falls and landslips obstruct the drainage of valleys and give rise to lakes; and similar results have been brought about by the advance of lava across a valley. (2) Crater Lakes.—These occupy the craters of extinct or quiescent volcanoes. (3) Sink Lakes.—These lie in hollows caused by subsidence of the surface consequent upon the removal of underlying soluble rocks, such as rock-salt, and calcareous and gypseous rocks. (4) Earth-movement Lakes.—Unequal movements or warping of the earth's crust have occasionally originated hollows by direct subsidence. It is possible also that local elevation by affecting the lower ends of valleys may sometimes have obstructed the flow of rivers, and thus given rise to lakes. (5) Glacial Lakes.—These consist of (a) hollows of erosion or rock-basins, which have been excavated by glacier-ice, and (b) hollows caused by the unequal distribution or accummulation of glacial detritus during the glacial period. (6) Subterranean Lakes.—These are found chiefly in calcareous regions, where they occupy the underground channels which have been excavated by the chemical and mechanical action of water (see CAVES). They are met with also in volcanic regions, filling, or partially filling, the cavities which are sometimes seen in lava-flows (see LAVA).
Fresh-water lakes are very unequally distributed. They are most numerous in those regions which were overflowed by land-ice during the glacial period, as in the British Islands, Scandinavia, Finland, &c., Canada, and the adjoining United States. Lakes occur at all heights above the sea; the most elevated being Lake Tsana in Abyssinia (7500 feet), Lake Titicaca in the Bolivian Andes (12,500 feet), and Askal Chin in Tibet (16,600 feet). The largest lake in the world is Lake Superior, which covers an area of 31,200 sq. m., and has a mean depth of about 475 feet. Lake Baikal, in central Asia, is the largest and deepest mountain-lake, its area being 13,500 sq. m., and its mean depth 850 feet, but in places it reaches a depth of more than 4000 feet. Some of the mountain-lakes of Europe also attain great depths; thus, Lake Geneva is 1000 feet, Lago Maggiore 1158 feet, and Como 1358 feet.
Salt Lakes.—Two kinds are recognised: (a) portions of the sea cut off from the general oceanic area by epigene or hypogene agencies; (b) lakes, originally fresh-water, which have been rendered saline by evaporation and concentration. Those of the first group range in size from mere pools and lagoons up to inland seas, such as those of the great Aralo-Caspian depression. The Dead Sea and the Great Salt Lake of Utah are good examples of the second group of saline lakes, which might be defined shortly as lakes which have no outlet to the ocean. The Caspian Sea is 97 feet below the level of the Black Sea, has an area of about 170,000 sq. m., and is from 2500 to 3000 feet deep in the deepest parts. A still more depressed area is that of the Dead Sea, the surface of which is 1292 feet below the level of the Mediterranean Sea.