•EFFECT OF CLIMATIC CHANGES ON GLACIATION OO i 



suppose that the two basins are so similar that a decrease of 5 or 10 per 

 cent in the rainfall causes the same diminution in the amount of water 

 reaching each lake. Under such circumstances each will eventually con- 

 tract to the same degree, for the size of a lake in an arid region is deter- 

 mined by the point at which the supply of water is balanced by the 

 evaporation. If the supply and the evaporation are equal, the lakes must 

 have the same area without respect to their depth or to the surrounding 

 topography. This statement is so axiomatic that it would not be neces- 

 sary were it not that it is often overlooked.* 



Supposing, then, that both lakes contract so that their shores retire 

 an average distance of 500 feet, what sort of new strands will be formed? 

 The first lake will fall only one foot. It will be bordered by shores so 

 flat that the waves will have little opportunity to cut bluffs or form 

 beaches. What little work they accomplish will be so nearly at the same 

 level as that of the original strand that in later times, if the lake disap- 

 pears, it will be impossible to distinguish the one from the other. In the 

 other lake, on the contrary, the fact that the bottom of the lake slopes at 



* This statement must not be understood as meaning that two lakes having the same 

 water supply and climate, but differing in topography, are always of the same size. 

 When climatic conditions remain stable for a considerable time such lakes must become 

 of essentially the same size, but during the process of change they may act differently. 

 The matter may be illustrated by supposing that under certain climatic conditions we 

 have two similar laJves (A and B), nearly square in form, and having an area of 100 

 square miles. Suppose that climatically both are subject to precisely the same condi- 

 tions, and that their only point of difference is that A is extremely shallow and its 

 bottom slopes only 1 foot in 600, while B is of a more common type, with a slope of 

 1 in 24. Suppose that the climate has been stable so long that the water supply and 

 the evaporation are exactly equal. Then let the climate suddenly change so that the 

 water supply of each lake is reduced 10 per cent. Let the new rate of evaporation be 

 40 inches per year in each. Supposing the climate to i-emain uniform under the new 

 conditions, the course of events would be as shown in the following table : 



^n Pv^U! nV P*;!^nnrntfn'.f Lv^ Area after level has been Total reduction of 



. *° %"?e'c%lSonthesr" -duced (square miles). level (inches). 



A. B. A. B. A. B. 



First year 4.00 4.00 98.41 99.92 4.00 4.00 



Second year 3.42 3.98 97.02 99.87 7,42 7.98 



Third year.. 2.89 3.9.5 96.04 99.81 10.31 11.93 



Fourth year 2.50 3.93 95.26 99.75 12.81 15.86 



Fifth year 2.21 3.91 94.47 99.68 3 5.02 19.77 



If this table were continued, avc should see that the shallow lake (A) would in a 

 few years be reduced to an area of approximately 90 square miles, the limit toward 

 which it would tend under the assumed conditions. It would fall about 2 feet (26 

 inches). The other, although its rate of fall is much the faster of the two, would be 

 far slower in reaching a condition near the point of equilibrium, for a vast amount of 

 water would have to be evaporated before the level could be reduced to such a point 

 that the evaporation would be no greater than the supply. Absolute equilibrium from 

 the mathematical point of view would only be attained after an infinite series of years ; 

 but for the practical purposes of geology, with which alone we are concerned, essential 

 equilibrium would be reached in a few centuries at a level 50 to 55 feet below the 

 original surface. These are the conditions referred to in the text, 

 XI. — Bull. Gko'l. Soc Am., Vol. 25, 1913 



