GENERAL CHARACTERISTICS. Lakes vary from each other in form, depth, and source of water supply. They are usually elongated in the di rection of the inlet and outlet, owing to the fact that they arise from some interference with the free drainage along a preexisting river valley. Often the outline is Irregular because the dam at the outlet causes the water to rise up into the tributary valleys. In some eases the water rises over low divides, forming many islands and a very irregular coast, as in Champlain, Win nipiseogee, and other lakes. Others may have straight shore-lines following the valley walls, or may occupy small circular basins. In depth lakes vary greatly; thus Great Salt Lake has a depth of less than 23 feet, and Crater Lake, in Oregon, of 2000 feet. They may occur at any elevation above the sea. and some, like the Dead Sea, are even below sea-level. Excepting in very severe climates, deep lakes do not freeze in win ter, because it is necessary to reduce the entire lake to 39° F. before the surface can freeze. Some lakes have no surface tributaries, but re ceive their waters from the rains and from underground; others have many tributaries; all receive a notable supply from underground. Usu ally the largest feeder or inlet is at the upper end of the lake. The surface of the water slopes slightly toward the outlet, which in most eases is limited to a single channel.
Great floods raise the level of all but the larger lakes, and wet seasons cause the surface to rise by the increased amount of water supplied from underground. This rising may come so long after the wet period that the connection is not readily discovered. There is also a slight tide on the larger lakes, though ordinarily unnotice able; hut in V-shaped bays its height may be so increased as to be easily detected. Steadily blowing wind, drifting the water below causes well-defined currents of water. On smaller lakes, whin heavy rains and melting snows coincide with strong wind, floods neor on the deltas. The hike-level is then raised by the wind, and the water of the streams cannot flow off, consequently flooding the delta. Still an other change of level of lakes is due to a dif ference in the air-pressure on the two ends of the lake. A low pressure on one end and a high on the other disturbs the equilibrium of the water: it is pushed down under the heavier air and rises under the low pressure. This starts an undulation of the lake water. which is analo gous to the rocking of a basin, and a wave passes up and down the lake, slowly dying out with each succeeding undulation. To such changes in lake-level the name `seiehes' is applied.