SURFACE WATER, DRAINAGE, AND IRRIGATION 313 



in these enterprises are enormous, but the returns have 

 usually justified the expense. In some places immense 

 dams are necessary. In other regions miles of concrete- 

 lined canals have been built. The large storage dams 

 retain the water that falls in certain stream basins during 

 the winter and early spring months. As this water is 



U. S. Reclamation Service. 

 FIG. 269. Salt River Project, Arizona. The Desert After Reclamation. 



needed by the farmers below the dam, it is allowed to flow 

 into canals which carry it to the tract to be watered. 



It will never be possible to irrigate all the arid land in 

 this way, since the total amount of rainfall of the dry 

 region is only enough to water properly one tenth of it. 

 Then too, some sections are so located that the 'expense 

 of bringing water to them would be too great to make 

 them profitable. 



