B. Map of Yosemite Falls and its earlier site near Eagle Peak (after F. E. Matthes).
CHAPTER XXIX
A STUDY OF LAKE BASINS
Freshwater and saline lakes.—Lakes require for their existence a basin within which water may be impounded, and a supply of water more than sufficient to meet the losses from seepage and evaporation. If there is a surplus beyond what is needed to meet these losses, lakes have outlets and remain fresh; their content of mineral matter is then too slight to be detected by the palate. If, on the other hand, supply is insufficient for overflow, continued evaporation results in a concentration of the mineral content of the water, subject as it is to continual augmentation from the inflowing streams.
As we have seen, there are in areas of small rainfall special weathering processes which tend to bring out the salts from the interior of rock masses, these concentrated salts generally first appearing as a surface efflorescence which is ultimately transferred through the agency of wind and cloudburst to the characteristically saline desert lakes.
Lake basins may be formed in many ways. Depressions of the land surface may result from tectonic movements of the crust; they may be formed by excavating processes; but in by far the greater number of instances they result from the obstruction in some manner of valleys which were before characterized by uniformly forward grades. In relatively few cases loose materials are heaped up in such a manner as to produce fairly symmetrical basins.
Fig. 430.—Map and diagram to bring out the characteristics of newland lakes.