SOIL CHANGES DUE TO IRRIGATION 85 



The concentration of river waters, at least in the 

 United States, appears to vary with the rainfall. In 

 humid districts, where the soils are more thoroughly 

 water-washed, and where the run-off is large, the quantity 

 of dissolved material is small. In arid districts, with 

 soils richer in soluble matter, the concentration of the 

 river waters increases. While the annual rainfall is not 

 the only factor determining the concentration of river 

 waters, yet it determines, in large measure, the quantity 

 of soluble substances. The same general law may be 

 observed in the data dealing with the Canadian rivers. 

 In the St. Lawrence Basin, the proportion of dissolved 

 substances in the river waters is considerably smaller 

 than in the Saskatchewan Basin, which is more of a semi- 

 arid character. Similarly, the data from the river waters 

 of Europe shows a variation with the general climatic 

 conditions, especially with the rainfall. 



The Nile, famous in irrigation history, does not carry 

 a great abundance of soluble material. It stands in this 

 respect between the waters of the Mississippi and those 

 of the Great Basin. The data of the above table, which 

 are representative of the rivers of the world, show that 

 the quantity of dissolved substances in river waters is 

 not extraordinarily large. In most cases, the waters of 

 even long rivers in arid districts are less concentrated than 

 the ordinary drainage water of agricultural fields. 



The river waters of humid regions, with low total con- 

 centration, are comparatively rich in carbonates; those 

 of arid regions, on the other hand, with high concentra- 

 tion, contain more sulfates and chlorides than carbon- 

 ates. This is explained when it is recalled that, under 

 humid conditions, the native vegetation grows abundantly 

 and the proportion of soil humus is much larger than 



