86 IRRIGATION PRACTICE 



under more arid conditions. Water passing through such 

 humid soils naturally takes up from the humus much 

 carbon dioxide. 



59. Salinity of lake waters. The waters of the great 

 lakes of the world, from which irrigation waters are 

 frequently taken, vary as largely as do the river waters. 

 The water of mountain lakes that are fed directly by 

 the melting snows contains little dissolved matter. For 

 example, the water of Lake Tahoe, in Nevada, contains 

 only 73 parts of dissolved substances to 1,000,000 parts 

 of water; whereas, the water of Sevier Lake, in Utah, con- 

 tains 86,400 parts, and, in the water of the Great Salt 

 Lake there are nearly 300,000 parts of dissolved sub- 

 stances. Ocean water, as another example, contains 

 about 39,000 parts of dissolved substances in 1,000,000 

 parts of water. Naturally the lakes that contain the 

 most concentrated solutions are in almost every instance 

 those of interior basins with no outlet to the ocean. The 

 water runs into these basins and as it is gradually evap- 

 orated it leaves behind its load of soluble materials to 

 concentrate the remaining water. In the course of time, 

 such waters become saturated with certain substances 

 which then crystallize out. This is the case with the Great 

 Salt Lake and many other well-known interior lakes of 

 western United States and other arid parts of the world. 



60. Salinity of mineral springs. The most heavily 

 charged waters, however, save those of interior basin 

 lakes, issue as mineral springs in many parts of the world. 

 The high degree of salinity of such waters seems to be 

 due, as already suggested, to the fact that the percolated 

 water has passed over subterranean layers of soluble 

 material which is brought up in solution when the spring 

 issues from the soil. The salinity of such waters varies 



