30 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1917. 



the type is smooth, with a very gentle gradient toward the stream. 

 It is flood-swept during periods of high water and during tlie rainy 

 season, and at all times the water table is high. The type is of no 

 present agricultural importance. 



IRRIGATION. 



Irrigation is necessary in the Middle Gila Valley area because of 

 the low rainfall and the long dry seasons. The Indians practiced 

 irrigation before the coming of the white man, using a rude system 

 of canals to take water from the river and distribute it over the river 

 bottoms and near-by desert slopes. The 1910 census reports a total 

 of 89,400 acres in Pinal County embraced in irrigation projeets, but 

 only 25,431 acres actually irrigated, of which 13,831 acres were sup- 

 plied with water by partnership and individual enterprises, 3,500 

 acres by cooperative enterprises, and 8,000 acres under the Indian 

 Reservation project. The irrigated acreage for Pinal County is 

 practically all Avithin the limits of the present survey. Nearly all 

 the water for irrigation is drawn from the Gila River by gravity 

 canals. In 1917, 9,000 acres were under irrigation on the units of 

 the Indian Reservation within the present survey. 



The Gila River is a broad stream, with poorly defined banks 

 throughout most of its course. During the rainy season gi'eat 

 volumes of water fill the channel and overfloAv the banks, but during 

 the dry season its sandy bed is bare. In the absence of a storage 

 supply, irrigation from the river is impossible during parts of the 

 year. 



The water of the Gila River is of good quality for irrigation, the 

 flood Avaters containing only small amounts of soluble salts. A 

 sample of such water taken at Florence contained 68 parts of soluble 

 salts per 100,000, but during normal flow the water is slightly higher 

 in salts. The river water is very high in silt, and the flood waters 

 carry in addition much organic matter washed from mountain slopes 

 by the torrential rains. These sediments are deposited on the soils 

 by irrigation and overflow waters, and while some inconvenience is 

 caused by the rapid and frequent silting up of canals, much benefit 

 results from such deposits, especially in the case of the desert soils of 

 light texture and Ioav organic content. The continued use of the 

 silty irrigation water even improves the texture of the lighter soils. 



During the last few years many wells have been sunk for irriga- 

 tion purposes in various parts of the river bottoms, and a few in the 

 adjacent areas of old valley-filling soils of the desert. In most in- 

 stances a good supply of water has been obtained, the shallower 

 wells in the stream bottoms furnishing a more abundant su]i])ly than 

 the wells in the desert areas. Water obtained in the desert sections 

 generally is of good quality, usually containing less than 50 parts 



