250 FIRST YEAR COURSE IN GENERAL SCIENCE 



the water sinks into the ground, leaving a hollow where 

 the ice was. This may fill and form a pond or small lake. 

 Such hollows, if they are not filled with water, are known 

 as " kettle holes," because of their rounded form., 



269. Other Causes of Lakes. Some lakes like Lake 

 Superior, lakes in Central Asia, and lakes in Africa were 

 formed in basins left when elevations were made in the 

 earth's crust. Others, like Crater Lake in Oregon, fill 

 the craters of extinct volcanoes. Still others were formed 

 by the damming of a river by a landslide or a lava 

 flow. 



270. Lakes as Sources of Rivers. When the basins, 

 formed as described, have filled with water up to the lowest 

 part of their walls, the water of course flows out at that point 

 and the lake then becomes the source of a stream. If a lake 

 " has no outlet/' as is sometimes said, the water will eventu- 

 ally become salt. Many lakes which apparently have no 

 outlet, discharge their water through passages from the 

 bottom or sides of the lake into underground streams, and 

 thus remain fresh water lakes. 



271. Uses of Lakes. Many lakes furnish abundant sup- 

 plies of fish for food. They also add greatly to man's 

 pleasure by their beauty and by their facilities for boating 

 and bathing. The large lakes modify the extremes of heat 

 and cold in surrounding land, as do the oceans. If a lake 

 is situated in an elevated region, its water may be carried 

 long distances in pipes, to be used for power or for supply- 

 ing domestic needs in cities. 



272. Reservoirs. The commercial use of lakes has led 

 to the making of artificial lakes or reservoirs by building 

 dams across the outlets of a basin and thus holding back 

 the water that enters it. There may be many millions of 

 gallons of water flowing daily into the reservoir from con- 

 stant springs and streams draining many square miles of 

 territory. 



