IRRIGATION 259 



Water is sometimes conducted for many miles, passing through 

 hills aud over valleys and gorges, lii the case of the (Junuison 

 tunnel of Colorado, the Gunnisoii river is diverted from its course 

 and carried through a tunnel almost six miles long, pouring into 

 the Uucompahgre Valley, where it is used to irrigate 140,000 acres. 



(b) Reservoirs. In many places in the arid regions of this 

 and other countries dams have been built across gorges or narrow 

 valleys, producing lakes or reservoirs whose water is used in the 

 irrigation of tillable land farther down the valley. In this way 

 the rains and snows of winter, which would otherwise be lost, are 

 held for the use of crops at a time when the water of the stream 

 is entirely insufficient for the purpose. The Roosevelt dam across 

 the Salt Jiiver in Ari/omi is a good illustration (Fig. 115). Here 

 sufficient water is stored for irrigating 2 11), 000 acres. This dam, 

 curved upstream, is 284 feet high and 1)10 feet long, with a thick- 

 ness at its base of 108 feet and 20 feet at the top. It forms a lake 

 or reservoir 25 miles long and from one to two miles wide and con- 

 tains 1,367,000 acre-feet of water. Many similar systems have heen 

 constructed by the government, or arc under way, that will irrigate 

 from 10,000 to 225,000 acres each, making a total of over 3,000,000 

 acres irrigated by these projects ( Fig. 1 Hi). 



(c) Pumping from Some Subterranean Supply. In some 

 localities in arid regions extensive underground reservoirs of water 

 occur sufficiently near the surface to be pumped for irrigation pur- 

 poses. In other regions artesian wells may furnish a bountiful 

 supply. Where irrigation is practiced in humid regions pumping is 

 the usual method. The rice fields of Arkansas and Louisiana are 

 irrigated in this way. 



(d) Pumping from Streams or Canals. In Egypt, India, 

 China and Japan much of the water for irrigation is pumped on the 

 land by means of hand or foot power. Sometimes cattle or donkeys 

 are used for this purpose. 



Preparation of the Land for Irrigation. The first step in 

 preparing the land for irrigation is the removal of the vegetation 

 (Fig. 117). The character of this varies with the amount of rain- 

 fall from stunted grass, sage brush, greasewood. and mosquite to 

 the remains of heavy forest*. The cost of clearing varies from two to 

 five dollars per acre for most lands to as much as one hundred and 

 fifty dollars per acre for forests. After the vegetation if? removed 

 the land must be graded so that the water may be uniformly ap- 

 plied. Many tracts are so flat that very little grading is necessary. 



