FEESHENING OF LAKES BY DESICCATION. 227 



for the reason that the inflow and the evaporation now counterbalance each 

 other. There are no obsei-vations on the rate of evaporation in this region, 

 but in a similar calculation relating to Great Salt Lake, Mr. Gilbert, after 

 considering all the data available, has assumed 6 feet per annum as the loss 

 by evaporation. In the comparatively fresh waters of Walker Lake the 

 rate of evaporation under the same atmospheric conditions must be consider- 

 ably greater than in the nearly saturated brine of the Utah Lake. In order 

 not to overestimate, however, we will assume the same rate of evaporation 

 in Walker Lake that has been adopted in the case of Great Salt Lake, viz., 

 6 feet per annum. The area of the lake is 118 square miles, or 365,516,800 

 square yards; an annual loss of 6 feet by evaporation gives 731,000,000 

 cubic yards as the total annual evaporation. This estimate was made inde- 

 pendently of the former, but the data in each case are necessarily indefinite, 

 and the close approximation in results is not an indication of accuracy. 



The average depth of Walker Lake is 118 feet, and as the waters are 

 19.66 times as saline as those of the river, omitting calcium carbonate from 

 each, it would evidently require the evaporation of a lake of fresh water 

 of the ])resent size and 2,320 feet deep to produce the amount of saline 

 matter now held in the lake. Evaporation taking place at the rate of 6 feet 

 per year, it would require 370 years to reduce this hypothetical lake to the 

 present volume of Walker Lake. Before drawing any conclusions as to the 

 length of time that the present lakes of the Lahontan basin have existed, 

 we may make a more general calculation of the same nature as the above. 



Let us suppose the three principal lakes of the Lahontan basin united, 

 and supplied by the three rivers of which we have, analyses, viz., the Hum- 

 boldt, the Truckee, and the Walker. We should have a lake 1,300 square 

 miles inarea, with an average depth of 1 17 feet, and containing 26.7 1 times the 

 percentage of salt held by the average of the tributary streams, not includ- 

 ing CaCOg. To obtain a water-body of this degree of salinity from the 

 concentration of the river- waters would require the evaporation of a lake of 

 the area of the assumed one and 3, 1 25 feet deep. Evaporation taking place 

 at the rate of six feet per year, it would I'equire 521 years for the waters to 

 be condensed to the degree represented by the present lakes. This estimate 



