SOIL WATER, DRAINAGE, AND IRRIGATION 211 



volume of water comes from the mountainous country hundreds 

 of miles away in Mexico and the United States. In the lower 

 Rio Grande valley there is much rich alluvial soil, needing 

 only water to make it fertile. Hundreds of miles of ditches 

 have been constructed to direct the water from the river to 

 the land. A main canal, looking like a river, carries water 



FIG. 103. 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 au area of 25.5 square 



miles. The impounded waters will be used to irrigate 190,000 acres of land. 



Photograph by United States Reclamation Service 



in one case for more than 100 miles. From this main canal 

 smaller ones branch off, and finally the smallest ditches run 

 to the individual farms. Floodgates are used in turning water 

 upon the fields when it is needed. In this way, with an abun- 

 dant supply of water and with constant sunshine, a rapid and 

 luxuriant growth of plants is made possible. 



