CHAPTER V. 



THE LIFE HISTORIES OP LAKES." 



Lakes, like many other features of the eaith's surface, as stated 

 in our introductory chapter, have their periods of growth, adolescence, 

 maturity, decadence, and old age leading to extinction. 



The lives of most lakes are so long that human records cover only a 

 small portion of their histories, hence their growth and decadence can 

 seldom be traced by observing a single individual. By studying many 

 examples, however, in various stages of development and decline, we 

 are enabled to obtain separate links in the chain of their existence, and 

 may determine, at least in outline, the general course that they run. By 

 having the theoretical history of a normal lake in mind, one is enabled to 

 determine the period of life attained by any special example that may 

 be studied. 



The histories of all lakes are far from uniform. There are various 

 accidents, as they may be termed, which introduce new conditions, and 

 may renew their youth or hasten their decline. In general, lakes may be 

 grouped in two great classes, in each of Avhich the role they play is in 

 the main the same. The differences in the lives of these two classes 

 depend mainly on climatic conditions, and have been noticed in describ- 

 ing fresh lakes and terrestrial saline lakes. The destiny of a lake born 

 beneath humid skies runs in a somewhat definitely prescrilied channel and 

 departs in a marked way from the more varied life of a lake originating in 

 an arid region. The general outline of the history of each of the two 

 classes referred to is briefly as follows : 



Lakes o.f Humid Reg-ious. — The normal lakes of humid regions 

 are comparatively short-lived. The streams triljutary to them bring in 

 sediments which tend to fill their basins, to these are added the debris 

 of water-loving plants and the hard parts of animals, and at the same 

 time the streams flowing from them tend to cut down their outlets and 

 drain them at lower and lower levels. Two processes thus conspire to 

 diminish their volumes and shorten their existence. The deposition of 

 sediment on their bottoms usually leads to their extinction more quiekl}- 



