In comparison with the long life of the earth, lakes must be regarded as merely temporary bodies of water, which will sooner or later disappear, either by being drained of their waters or by being filled up with the sediments which are washed into them. The general term lake is employed for any inland body of water, which does not form part of the sea, but lakes are formed in very different ways and have correspondingly different histories. Most lakes occupy depressions below the general drainage level of the country, whether these depressions be due to movements of the earth's crust, to glacial excavations, to unequal decomposition by the atmospheric agencies, or to some other factor. Others, again, are held back by dams, such as lava streams, glacial moraines, or the glaciers themselves, by the debris of land-slips, or by the deltas of tributary streams which bring in more material than the main river can dispose of. Others still are enlarged basins cut out by rivers. Great lakes that persist for long periods of time are contained in basins, often of great depth, which were formed by movements of the earth's crust; the other kinds are more evanescent and usually of rather small size.
Fig. 80. - Wave-cut bluff on Lake Ontario. (U. S. G. S).
Small lakes accomplish very little in the way of rock destruction, but are rather places of accumulation. The waves, even in storms, are not heavy enough effectively to cut back the shores, while the current of water through the lake is too slow and the sediment transported too small and light to erode the bottom as a river does. In great lakes, such as those which drain into the St.
Lawrence, storms develop a very heavy surf, and such lakes eat into their shores as the ocean does, but the very small tide confines the work of the waves within narrower limits, and the lighter breakers are less effective. Lakes are subject to various accidents which cause great fluctuations of the water-level. Deserted shore-lines are marked by ' beaches and terraces. The method of denudation by lakes is the same as that of the sea, but the modes of accumulation of material are characteristically different.