LAKE. 700 LAKE. by the land. In the case of the Caspian, Dead, and Aral seas, the term sea is applied to wliat are true lakes. Ge.nehal Characteristics. Lakes vary from eacli oilier in form, depth, and source of water- ••iupply. They are usually elongateil in the di- rection of the inlet and outlet, owing to tiie fact (hat 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 cases the water rises over low divides, forming many islands and a very irregular coast, iis in Champlain, Win- nijiiseogee, and other lakes. Others may have straight shore-lines following the valley walls. or may occu])y small circular basins. In dejitii lakes vary greatly; thus Great Salt Lake lias a depth of less than 2.5 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. Kxcepting in ery 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. Imt re- ceive their waters from the rains and from inderground : others have many trilnitarics ; all receive a notable supply from underground. Usu- ally the largest feeder or inlet is at the upper cnii of the lake. The surface of the water slopes slightly toward the outlet, which in most cases 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 jicriod that the connection is not readily discovered. There is also a slight tide on the larger lakes, though ordinarily unnotice- able; but in V-shaped bays its height may be so increased as to be easily detected. Steadily blowing wind, drifting the water befoif it, causes well-defined currents of water. On smaller lakes. wh'U heavy rains and melting snows coincide with strong wind, floods occur on Uic deltas. The lake-level is then raised by the wind, and the water of the streams cannot How off, conse(|uently 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 imder 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 'seiches' is applied. Lakes Without Outlet.s. In all countries there is evaporation from the surface of lakes, so that less water flows out than enters ; there may also be loss through seepage into the earth. On account of this, even in moist climates, lakes with a small drainage area may not rise to the point of outflow. This is true of small basins in sand-dune regions, or in sandy glacial de- posits, where the water speedily soaks into the loose soil. Slowly, however, the washing in of clay and the growth of vegetation forms a more impervious bottom, so that first swamps, then ponds, are caused. Little kettle-shaped basins in terminal moraines often have such a small drainage area that they do not rise to overflow, or ])ossibly overflow only in periods of heavy rains or melting snows. Deep lakes in volcanic craters, like Crater Lake in Oregon, also fail to reach the point of overflow, because the rainfall and drainage arc not sullicient to fill the basins. Where the climate is very arid, the lack of abundant rain, the rapid evaporation, and the seepage, commonly keep the lake waters below the rim of the basin; and in deserts these condi- tions may completely dry up the basins, or permit them to have lakes or swam])y Ixittoms only dur- ing rains. These effects of aridity are illustrated in the Great Salt Lake, the Dead Sea, and the Caspian Sea. As the streams entering lakes bring a load of mineral matter in solution, and as this is not carried ofl' in the vapor, lakes without outlet have a .steadily increasing load of mineral matter. Among the mineral substances thus brought, salt, gypsum, and carl)onate of lime are usually the most abundant. In time, therefore, these substances may be present in such quan- tity that no more can be held by the water, and then some of tin* mineral load must be de- ]iosited. Thus carhonatc of lime is being precipi- tated on the bottom and sliores of the Great Salt Lake, and salt and gypsum have been pre- cipitated in many lakes in recent times and in past ages. Origin of Lakes. Tlie causes of lakes are as various as their forms. In general they may be considered as consequence of natural inter- ference with drainage. They may exist on a new land surface, when they may be called orig- inal conacriucnt laLcK. since they are formed in conse(iucnce of original irregularities in the land; they may result from the normal develop- ment of rivers, and may then be called lakes of }tonnul (lev clop 1)1 rn i : and they may be due to some accidental interference with preexisting drainage, when they may be called lakes of acci- dental orifiin. .ll lakes fall into one of these three great classes. Original conscipient lakes are illustrated by the shallow lakes of Florida, which exist in de- pressions on a raised sea bottom ; the same condi- tion exists in the Siberian [ilains, and in the .r- gentine plains. Consequent lakes are also found in shallow basins on the beds of extinct lakes. Thus the Great Salt Lake is in a depression in the deposits of a much larger lake that once existed there. . y other new land surface, as a lava flow, or a tliick sheet of glacial drift which obscures the old land, may have depres- sions in which ponds or lakes develop. There are many small lakes of this origin in the glaciated belt of . ierica and Europe. Of lakes of normal development there are also numerous illustrati<ins. Abandoned meanders of rivers, forming ox-how lakes, and abandoned river channels on deltas, shut off from the river by the deposit of river silt, are instances. The growth of alluvial fans by streams coming from a mountain into a more level valley sometimes dams the river in the main valley. Thus Tulare Lake in California is made by a broad, low, alluvial fan made by King River, which comes down from the Sierra Xcvada. Still another kind is found where stream development is tak- ing place in a region of limestone or other solu- ble rock. Under these conditions some of the drainage is underground, the surface settles hero
