266 PHYSICAL GEOLOGY 



rock is resistant, because, as noted above, lakes act as settling 

 basins for the sediment brought in by their tributaries, so that 

 the issuing streams have at the outset few tools. Still other 

 less important changes are taking place in certain lakes. 



The fate of lakes. It is apparent from the foregoing dis- 

 cussion that lakes are short-lived features, and accordingly 

 that all existing lakes are of geologically recent origin. Since 

 deposits of every sort displace an equal volume of water, it is 

 evident that if continued long enough, deposition will com- 

 pletely fill a lake basin and obliterate the lake. Vegetation is 

 often a chief factor in the last stages of lake filling (Fig. 284)- 



FIG. 284. A pond nearly destroyed by encroaching vegetation. Yel- 

 lowstone Park. (Fairbanks.) 



Furthermore, the outflowing stream may cut the outlet of a 

 lake below the level of the bottom of its basin, and so drain off 

 all the water. Most lakes are being destroyed slowly in both 

 ways. It has been estimated that although Minnesota now 

 has perhaps 8000 lakes, large and small, it will contain in fifty 

 years fewer than 5000. Many of its lakes are really ponds, 

 well-nigh effaced. This, of course, does not mean that the 

 earth will presently be without lakes. Existing lakes will 

 be succeeded by others, just as they have been preceded by 

 many generations of earlier ones. 



