Salt Lakes are especially characteristic of arid climates, in which the rainfall is light and evaporation great. They may be formed in either of two ways: (1) through the separation of bodies of water from the sea. This is exemplified by the Salton Sink in the Colorado desert of southeastern California; the bottom of the Sink is considerably below sea level and has recently been converted into a lake by the influx of the Colorado River. Originally the Salton Sink was the head of the Gulf of California and old beaches, shell banks, etc., still remain to indicate this. The Colorado River, which enters the Gulf from the east some distance below the head, built its delta across the Gulf and thus cut off the zipper portion into a salt lake, which subsequently disappeared by evaporation. Beds of salt, which demonstrate the lacustrine stage, occur in the Sink. (2) By the long-continued concentration of river water in basins that have no outlet, where the influx of water is disposed of by evaporation from the surface of the lake. In either case an arid climate is requisite to maintain the salinity; in a moist region the large rainfall and slower evaporation would cause the lake to rise until it found an outlet, and then the water, if originally salt, would become fresh.
The history of Lake Bonneville exemplifies the change from fresh to saline conditions. As long as the water level was maintained above the outlet, the lake was fresh, but when the advancing aridity of the climate diminished the rainfall and increased the rate of evaporation, the water level sank until it fell below the outlet. Then the lake became saline, reaching its maximum salinity in the intensely bitter waters of Salt Lake, which is the remnant of the large lake.
Fig. 104. - Mountains nearly buried under old lake deposits; plain of Salt Lake, Utah. (Gilbert).
All river water contains greater or less quantities of dissolved substances, and of these one of the commonest is ordinary salt (NaCl). When such waters are evaporated, the solids remain behind, and thus the water becomes more and more saline till it reaches saturation. The salt is, however, not entirely derived from the rivers, for salt is a very wide-spread constituent of desert soils and surfaces, and in the Pampas of Argentina, salt crusts form repeatedly. Wind and rain thus bring into the lake quantities of salt in addition to that carried by the tributary streams. Other substances occur also, as will be seen below.
The mechanical deposits formed in salt lakes do not differ in any very important manner from those of fresh lakes. The finer clays settle more rapidly in brine than in fresh water, which makes strongly saline lakes extraordinarily clear and limpid. The organic deposits of salt lakes are practically nothing, for brackish water is not favourable to many organisms and in dense brines very few animals or plants can exist, and those that can are not the kinds which give rise to peat, or to siliceous or calcareous deposits. For the same reason, the deposits of whatever kind, laid down in salt lakes, are almost barren of fossils, except of land animals and plants, such as are washed into the lake by flooded streams.