94 LAKES OF NORTH AMERICA. 



As their basins are filled, however, the waters expand and offer a greater 

 surface to the atmosphere, thus promoting evaporation. A continuance 

 of this process results in so enlarging the water surface that in time evap- 

 oration equals the suj^ply and the water body passes to the condition of a 

 playa lake. Sedimentation may raise the water surface so that an outlet 

 is found before the playa stage is reached, thus transferring an enclosed 

 and saline lake to the class normal to humid regions, already considered. 



The existence of lakes in countries where there is a close adjustment 

 between precipitation and evaporation, is also controlled largely by topo- 

 graphic conditions. It may be said that this is the primary condition that 

 determines whether lakes shall exist in arid regions or not. This is true, 

 if we consider the origin of the lake basins, and also important if the ex- 

 istence of lakes in ready-formed basins is discussed, since the topography 

 has a direct and frequently controlling influence on rainfall and on evapo- 

 ration. The influence of topography is also marked in determining the 

 ratio of the area of a basin to the area of the lake in its lowest depres- 

 sion. The hydrographic basins of enclosed lakes as a rule are large in 

 reference to their water surfaces, when compared with the ratio of catchment 

 areas to lake areas in humid regions. Any change tending to diminish 

 the area tributary to an enclosed lake, as the sapping of the head waters of 

 its tributary streams, would have a marked influence on its history. 



Episodes of another character also occur in the lives of enclosed lakes. 

 The salts slowly accumulated in them may not only be flooded out by 

 overflow consequent on changes in topography, or on an increase in rain- 

 fall, or on a decrease in evaporation, but may be eliminated by reason of a 

 reverse change in the ratio of inflow to evaporation. A decrease in humid- 

 ity or an increase in evaporation, or what is probably more frequent, a 

 combination of these two processes, may reduce a lake to the playa stage. 

 When this occurs, its salts will be precipitated and may become buried or 

 absorbed by sediment, so that when a new lease of life is granted and the 

 waters expand and form a perennial lake, they are fresh, or essentially 

 so, and start anew in the process of concentration. Still other changes 

 that beset the lives of enclosed lakes might be enumerated, to show that 

 they are subject to greater vicissitudes than their sister lakes in more 

 favored lands. 



When the lakes of arid regions become extinct, either by reason of 

 evaporation or sedimentation, the evidence of their former existence 

 remains inscribed on the inner slopes of their basins or concealed in the 

 strata deposited over their bottoms. These records as a rule are much 



