LAKES 337 



less and less salty, and may ultimately become fresh; otherwise 

 they remain salt. If evaporation exceeds inflow they diminish 

 in size and their waters become more and more salt or bitter. 



Indirect effects of lakes. Lakes tend to modify the climate of 

 the region where they occur, both by increasing its humidity and 

 by decreasing its range of temperature. They act as reservoirs 

 for surface-waters, and so tend to restrain floods and to promote 

 regularity of stream flow. They purify the waters which enter 

 them by allowing their sediments to settle, and so influence the 

 work and the life of the waters below. 



Origin of lake basins. 1 Lake basins arise in many ways, some 

 of which have been pointed out on preceding pages. Most of them 

 arise through processes of gradation. Some are formed by rivers 

 (p. 185), some by waves and shore currents (p. 313), some by glacial 

 erosion, some by glacial deposition (p. 268), and some by a com- 

 bination of glacial erosion and glacial deposition. Others are 

 formed by volcanic action, as when a lava flow dams a valley, or 

 when a volcano leaves its cone with a depressed crater. Still 

 others are formed by warpings of the earth's surface, and a few in 

 other ways. 



1 Salisbury's Physiography, larger edition, p. 303. 



