Playa lakes.—It is the law of the desert that the arid region be walled in by mountains. This encircling rampart forces the clouds to rise, and by robbing them of their moisture leaves the desert dry and barren. Those waters which fall upon the inner margin of the ranges drain toward the interior of this pan-like depression and are not returned to the sea—the desert is without an outlet. Infrequent though they be, the desert rains are of the cloudburst type and in the hills develop torrents whose waters, emerging upon the desert floor, develop lakes in the space of a few minutes or at most hours. In the hot and dry atmosphere the waters of these shallow basins may be sucked up in the space of a few hours but reappear in the same basins at the time of the next succeeding cloudburst. Such ephemeral lakes are known as playas.
Salines.—Desert lakes more favored in their supply of water may be relatively long lived and persist for periods measured in years or centuries. Such lakes are, however, extremely sensitive to climatic changes (see [p. 198]).
For the reason that they have no outlet the waters of desert lakes become salt through continued evaporation. They are, therefore, spoken of as salines. Lake Bonneville, so long as it discharged its waters over the sill of the Red Rock Pass, must have remained fresh; but when the level of its waters had fallen below this outlet, its waters became salt and the content increased as the volume diminished.
The shallow basins upon the floors of desert lakes may have come into existence in various ways; but it would appear that the irregular removal of the soil by the winds, modified as this is by differences in composition of the rock materials and by vegetable growth, and the deposition of sand by the same agent, are by far the most important. Many of the types of tectonic and volcanic lakes which have been described are characteristic of humid and arid regions alike.
Alluvial-dam lakes.—Within the mountains upon the desert borders, the alluvial fans which form at the mouths of valleys, because of the characteristic cloudburst, sometimes obstruct a main valley at the junction with its tributaries. By this process the waters of the main river are impounded in essentially the same manner as are the rivers of humid regions by the deltas of their tributaries.
Résumé.—The types of lakes which we have now considered are arranged below in tabular form so as to show their relationship to important geological processes. While not complete, the list includes the more important classes, as well as others which, while not of common occurrence, are yet of interest in giving further illustration to the processes which have been treated in earlier chapters.
By giving careful attention to criteria which have been above suggested, it should be possible in the greater number of instances at least to determine whether any lake which is visited has had its origin in one or another of the processes described.
CLASSIFICATION OF LAKES
| Tectonic Lakes | Volcanic Lakes |
| Newland lakes Basin-range lakes Rift-valley lakes Earthquake lakes | Crater lakes Coulée lakes |
| Continental Glaciation Lakes | Mountain Glaciation Lakes |
| Morainal lakes Pit lakes Glint or colk lakes Ice-dam lakes Glacier-lobe lakes | Rock-basin lakes Valley moraine lakes Landslide lakes Border lakes |
| River Lakes | Strand Lakes |
| Ox-bow lakes Saucer lakes Crescentic levee lakes Raft lakes Side-delta lakes Delta lakes | Barrier lakes Dune lakes |
| Ground Water Lakes | Desert Lakes |
| Sink lakes Karst lakes—poljen | Playa lakes Salines Alluvial dam lakes. |