SOIL WATER, DRAINAGE, AND IRRIGATION 301 



that purpose also. The failure to introduce underground 

 drainage in irrigated districts is now beginning to prove a 

 limiting factor in the production of plants, and doubtless 

 such drainage will soon become usual there. 



304. Source of water for irrigation. The larger part of our 

 irrigation projects consists in diverting mountain streams into 



FIG. 145. An irrigation dam 



The Roosevelt dam on Salt River in Arizona is 280 feet high and 1080 feet long. 



The water which it holds back forms an artificial lake with an area of 25.5 square 



miles. The impounded waters are expected to irrigate 190,000 acres of land. 



Photograph by the United States Reclamation Service 



near-by valleys. Sometimes streams are dammed (fig. 145) 

 either to assist in diverting the water or to store it for later 

 use. Tunnels have been made through mountains, and ele- 

 vated aqueducts have been built over valleys to carry these 

 streams. In some regions irrigation by use of deep wells is 

 practiced. Water is pumped into reservoirs, and from these 



