432 A NATIONAL PLAN FOR AMERICAN FORESTRY 



are not readily eroded, they are considered to exert a moderate water- 

 shed protective influence. A level forested area of porous volcanic 

 soil about 45,000 acres in extent on the Colorado Plateau of northern 

 Arizona is classed as having practically no watershed-protection value. 



The pinon-juniper woodland, occupying about half the forest area, 

 occurs principally near the lower limits of forest growth. The quan- 

 tity of usable water obtained from this type is small. ^ At these levels 

 snow seldom accumulates. Most of the summer rains are so mild 

 that there is little surface run-off from them and their effect on the 

 water supply is negligible. Semi torrential rains are rather common 

 and sometimes reach the proportions of " cloudbursts. " Such storms 

 cause very rapid run-off accompanied by severe erosion especially on 

 steep and unstable slopes that are inadequately protected by a plant 

 cover. In the woodland type there is normally but little undercover 

 on slopes and the tree cover itself is seldom dense enough to form 

 a closed canopy and heavy litter. This cover, such as it is, should 

 be maintained and improved where it is depleted. As previously 

 mentioned, studies by the Southwestern Forest and Range Experi- 

 ment Station have indicated that on most of the soils on which the 

 woodland type occurs in Arizona, which are fairly representative of 

 woodland soils throughout the basin, a stand of herbaceous and 

 shrubby vegetation can be maintained which with the trees is suffi- 

 cient to check run-off and abnormal erosion. Growth conditions are 

 so severe that the protecting cover can easily be seriously depleted 

 by overgrazing, fire, or too heavy cutting. 



Woodland areas have been classed as of major watershed-protection 

 influence if their potential forest cover would aid materially in check- 

 ing run-off and erosion on slopes, a heavily silt-laden run-off from 

 which is resulting, or would result in undue damage to irrigation 

 developments or other property. Woodland areas the topography 

 of which is level, rolling, or moderate, and the soil of which is not 

 readily eroded, have been classed as of moderate watershed-protec- 

 tion value. 



Of the total forested area within the Colorado River Basin approxi- 

 mately 36,196,000 acres has been classed as of major watershed- 

 protection influence, 8,829,000 acres as of moderate influence, and 

 45,000 acres as of scant influence. 



WATER SUPPLY FOR AGRICULTURE, POWER, AND DOMESTIC 



USE 



In the Colorado River Basin agriculture, the most important 

 industry, is almost wholly dependent upon irrigation. More than 

 2,700,000 acres of land is already under irrigation from Colorado 

 River water. Rural homes and villages dot the irrigated valleys, 

 and large urban centers have gained added impetus in growth from 

 irrigation farming. The irrigated portion of the Salt River Valley, 

 for example, contains one fifth the population of Arizona. In the irri- 

 gated Imperial Valley of southern California unusually large values are 

 present, and in the upper portion of the Colorado River Basin valley 

 after valley owes its present development to irrigation. The United 

 States Bureau of Reclamation has estimated 69 that the irrigated 

 area within this basin could be expanded to 6,930,000 acres. The 



6 S.Doc. 142, 67th Cong., 2d sess., 1922. 



