544 O. E. MELNZER LAKES OF THE BASIN-AND-RANGE PROVINCE 



its altitude, relief, and topography, and the latitude and climatic belt in 

 which.it lies. 



The map shows very plainly the relation of latitude to the size and 

 abundance of lakes. There is at present a general southward increase in 

 the rate of evaporation and decrease in precipitation. The distribution 

 of the Pleistocene lakes shows that at the time they existed there was a 

 comparable southward change in at least one of these factors. The north- 

 ern part of the province was crowded with Pleistocene lakes, most of 

 them of good size ; farther south the lakes were smaller and less abundant, 

 and some large basins contained no lakes of sufficient size and permanence 

 to leave distinct shore features. In southwestern Arizona lakes were ab- 

 sent, because there were no closed basins that could hold lakes, but in 

 southeastern California and trans-Pecos Texas closed basins occurred, and 

 the absence or scarcity of lakes in these parts of the province was appar- 

 ently due to aridity. 



The map also affords good examples of the influence of altitude, relief, 

 and topography on the existence and size of the lakes. The rapid transi- 

 tion from central Nevada, with its abundant Pleistocene lakes, to the 

 relatively desiccated region of southern Nevada and southeastern Cali- 

 fornia is largely due to rapid decrease m altitude with great increase m 

 temperature and evaporation. Lake Estancia, in central New Mexico, 

 whose strand is 6,200 feet above sealevel, was one of the highest of the 

 Pleistocene lakes, and consequently, though it lies far south, it occupied 

 22% per cent of its drainage basin. Lakes Bonneville and Lahontan evi- 

 dently owed their great size to the productivity of the lofty Wasatch 

 Mountains and Sierra Nevada, respectively. Lake Bonneville occupied 

 38 per cent of its basin and then overflowed. Lake Lahontan occupied 

 about 20 per cent of its basin, the smaller percentage being due largely to 

 the extensive area of relatively low, unproductive country east of the lake 

 that was included in its drainage basin. Aside from the effects of alti- 

 tude and relief in influencing evaporation and precipitation, the shape of 

 the drainage basin is important in concentrating the surface waters in 

 one locality, where they may accumulate to form a lake, or spreading 

 them to such an extent that they are dissipated without forming a lake. 

 The most striking example of the effect of concentration of the drainage 

 is afforded by the Salton Sea, which developed rapidly in the midst of an 

 intensely arid region when the Colorado, with its vast drainage basin, 

 became tributary to it. The ancient lake in Coal Valley, Nevada, is 

 another good example of this kind on a smaller scale. 



