100 IRRIGATION PRACTICE 



irrigation season than either just before or after. In other 

 parts, where summer rains prevail, the irrigation water 

 is often heavily loaded with suspended matter. (Figs. 

 20, 21.) 



70. Suspended matter added to soil by irrigation. 

 Considerable quantities of sediment may be added to the 

 soil during a season's irrigation. If 2 acre-feet of water 

 are used annually for the production of crops, a calcula- 

 tion may be made similar to that which was made con- 

 cerning the soluble matter added to the soil. During the 

 time of summer floods, few waters contain less than 1,000 

 parts of suspended matter in 1,000,000 parts of water. If 

 this were continued throughout the season, it would 

 mean an addition to each acre of land of over 5,000 

 pounds of sediment. The southwestern rivers, which 

 carry, ordinarily, throughout the season much more sedi- 

 ment than this, add to each acre during each irrigating 

 season an extremely large total quantity. It has been 

 reported from Arizona that, frequently, the sediment 

 of one season's irrigations covers the land to a thickness 

 of 4 to 6 inches. In rivers with less sediment, these 

 effects are not so visible, but wherever irrigation is prac- 

 tised, especially in arid districts, a large quantity of solid 

 matter is deposited on and in the soil. This, continued 

 year after year, will certainly affect the productive power 

 of the soil. 



71. Suspended matters derived from surface soils. 

 The suspended matters in river waters come chiefly from 

 the surface washings of the lands near the headquarters of 

 the rivers. The character of the suspended matters car- 

 ried by rivers varies, therefore, with the surface nature 

 of the soils from which the sediments are derived. If the 

 contributing soils are sandy, the suspended matter will 



