The term lake is a comprehensive one and includes all continental bodies of water, not in tidal communication with the sea, in which the water is relatively stationary and not actively running like a stream. In lakes which have an outlet there is a movement of the water toward the outlet, but this movement is extremely slow, and in the Lake of Geneva a given particle of water requires more than 11 years to pass through the length of the basin. (Forel).

Lakes are formed in a great variety of ways, of which it is necessary to mention here only a few of the more important ones. We have: (1) Tectonic lakes, due to movements of the earth's crust, whether warping or faulting, by which basins, subsequently filled with water, are formed. In this class might be included volcanic lakes, which occupy ancient craters. (2) Erosion lakes, in basins which have been excavated by one or other of the erosive processes. In both of these classes the lakes occupy basins which are below the general drainage level of the country. (3) Barrier lakes, in which the water is retained by a built-up dam or barrier, such as a landslip, a lava stream, glacial moraines, a delta formed by a tributary in the main stream, etc. Such lakes are frequently above the general drainage level.

As compared with the sea, lake basins are but small and shallow, and, from the geological point of view, they are short-lived and ephemeral, because in course of time they are either drained by the outlet's cutting through the retaining barrier, or by filling up with the sediment brought in by tributary streams. The material transported by the Mississippi, which would require 11,000,000 years to fill the Gulf of Mexico, would fill the basin of Lake Superior in 66,000 years. (Barrell).

From another point of view we may speak of temporary and permanent lakes, the former usually in arid climates. Such temporary lakes are called playas and are formed occasionally or periodically, according to the amount and distribution of the rainfall, but the distinction between playas and river floods is rather arbitrary, and hence playa deposits have already been described in connection with flood plains.

Lakes are important places of sedimentary accumulation, for they act as settling basins and retain all the sediment brought in by streams. However turbid the inflowing streams may be, the outlet is beautifully clear, as is exemplified very strikingly by the Rhone, which enters the Lake of Geneva a muddy stream and leaves it in a state of exquisite clearness and brilliancy. The Yellowstone and Niagara rivers are other examples of the same kind. Occasional exceptions to this rule may occur when a shore current washes some sediment into the outlet, as happens in Lake Huron and the St. Clair River.