74 



Types of Aquatic Environment 



High and low water — Since the source of this water is 

 in the clouds, all lakes fluctuate more or less with varia- 

 tion in rainfall. The great lakes drain an empire of 

 287,688 square miles, about a third of which is covered 

 by their waters. They constitute the greatest system 

 of fresh water reservoirs in the world, with an 

 unparalleled uniformity of level and regularity of 

 outflow. Yet their depth varies from month to month 



Pig. 19. Diagram of monthly water levels in Lake Ontario for twelve years; 

 from the Report of the International Waterways Commission for 1910. 



and from year to year, as shown on the accompanying 

 diagram. From this condition of relative stability to 

 that of regular disappearance, as of the strand lakes of 

 the Southwest, there are all gradations. Topography 

 determines where a lake may occur, but climate has 

 much to do with its continuance. Lakes in arid regions 

 often do not overflow their basins . Continuous evapora- 

 tion under cloudless skies further aided by high winds, 

 quickly removes the excess of the floods that nm into 

 them from surrounding mountains. The minerals dis- 

 solved in these waters are thus concentrated, and they 

 become alkaline or salt. We shall have little to say in 



