172 THE WORK OF RUNNING WATER 



The streams that flow into lakes are continually bring- 

 ing down the sand and mud they have gathered in their 

 course, and are thus filling up the lakes. 



The outlet to a lake tends to wear away its bed, but it 

 does this slowly, as it has little sediment with which to scour. 

 Thus lakes are being constantly filled and drained, and so 

 are comparatively short-lived features of the earth. 



Lakes are very important features to man. They filter 

 river water so that rivers emerging from lakes are clear. 

 Where the Rhone enters Lake Geneva, it is turbid and full 

 of silt, but when it emerges, it is clear and without sedi- 

 ment. Lakes also act as reservoirs for the water that pours 

 into them at the time of freshets. Rivers emerging from 

 lakes of considerable size vary little in the height of their 

 water at different seasons of the year. They are without 

 floods. The St. Lawrence illustrates this. On the other 

 hand the Ohio, with its frequent and terribly destructive 

 floods, shows the effect of unrestrained run-off. 



In some regions the rainfall is so small that the depres- 

 sions never fill up sufficiently to overflow their rims. The 

 water is evaporated from the surface as fast as it runs 

 into the lake. Thus all the salt and other soluble sub- 

 stances which have been extracted from the land and brought 

 into the lake by the rivers remain there, since only pure 

 water is evaporated. In this way lakes without outlet 

 become salt. Great Salt Lake in Utah is an example of 

 this. Some salt lakes, like the Caspian Sea, were probably 

 once a part of the ocean, so that they have always been 

 salt. 



As time goes on, more salt is brought to these lakes with- 

 out outlets, and they become more and more salty. Great 

 Salt Lake has something like 14 or 15 per cent of solid 



