LAKE, a mass of still water situated in a depression of the ground, without direct communication with the sea. The term is also applied to widened parts of rivers, and to bodies of water which lie along sea-coasts, and in direct communication with the sea. The terms, pond, tarn, loch and mere are applied to smaller lakes according to size and position. Lakes are nearly universally distributed, but are more abundant in high latitudes, and in mountain regions, especially those which have been recently glaciated. They are frequent along rivers which have low gradi ents and wide flats, being clearly connected with the changing channel of the river. Low lands in close proximity to the sea, especially in wet climates, have numerous lakes, e.g., Florida. Lakes may be either fresh or salt, some being more salt than the sea itself. They occur at all altitudes: e.g., Lake Titicaca (S. America) 12,500 ft., Yellowstone lake (U.S.A.), 7,741 ft. above sea-level ; and on the other hand the surface of the Caspian sea is 86 ft., the Sea of Tiberias 682 ft., and the Dead Sea 1,292 ft. below the level of the Mediterranean. The primary source of lake water is atmospheric precipitation, which may reach the lakes by rain, melting ice and snow, springs, rivers and immediate run-off from the land-surfaces. Lakes, owing to their isolation from the great oceans, have peculiar physical, chemical and biological features.
a. Lakes due to Glacial Erosion. Of these the following types are recognized :—(1) Lakes in terminal basins. These are common in the lower valleys of the Alps and occupy the sites of ancient glaciers, e.g., Lakes Lucerne, Ziirich, Neuchatel, etc. (2) Lakes in valleys deepened by glacial erosion. Many lakes in glaciated mountain regions can only be explained by this means. (3) Lakes in cirques. (4) Lakes in hanging valleys. (5) Lakes on roches
moutonnees. The three types last mentioned are also common in glaciated mountain regions. Many are now completely filled with alluvium.
b. Barrier Lakes are of great importance and are formed in the following ways: (I) Landslides. These occur in mountain regions where a fall of rock may dam back the water of a stream. Lakes thus formed have not a permanent character, for the effluent will rapidly lower its bed in the loose material of the landslide. (2) A valley glacier. Below the snow-line a valley glacier may block a tributary stream, e.g., Lake Marjelen, dammed by the Aletsch glacier (Switzerland) and Lake Castain (Alaska) held up by the Malaspina glacier. (3) The lateral moraine of a glacier may obstruct the flow of a river thus forming a lake, e.g., Lake Mattmark (Switzerland). (4) Frontal moraines frequently hold back the water of the river of ter the glacier has disappeared. Such lakes are abundant in high latitudes, e.g., the great Cana dian Lakes. (5) Irregularities in glacial drift, boulder clay and other deposits left by a retreating ice-sheet may cause the forma tion of lakes. (6) The alluvium laid down by a main stream may dam back the waters of a tributary stream or vice versa, e.g., the lakes formed by the levees of the Mississippi. Barrier lakes are also formed by the obstruction of the flow of a river in the following ways: (7) by a lava-flow, (8) by the movement of a sand dune, (9) by the accumulation of beach material or the growth of a storm beach across the mouth of a river.
c. Crater Lakes. The craters of ancient and dormant volcanoes are frequently filled with water. They vary from a few yards to several miles, are usually circular and rarely have a visible outlet.
d. Lakes of Tectonic Origin occur in natural structural features of the crust of the earth, e.g., L. Joux (Jura) in a syncline, Lakes Tanganyika, Nyassa, etc., lying in the Great Rift Valley of North Africa.