A NATIONAL PLAN FOR AMERICAN FORESTRY 453 



area, but on much of the affected area sufficient grass remains to 

 retard abnormal erosion. 



The subalpine forest, usually scattered growth or patchy stands of 

 alpine fir and white-bark pine with intermingled grass or brush lands, 

 occurs at the higher elevations, extending to timber line where winter 

 snow depth often exceeds 10 feet. The patches of tree growth, 

 together with good stands of herbaceous and shrubby vegetation in 

 the openings, serve very well for erosion control and bring about a 

 rather satisfactory delivery of the heavy snow blanket. When the 

 herbaceous cover is broken the value of the type for erosion control 

 ordinarily is impaired. 



Brush fields, often the result of fires, are intermixed with areas of 

 dense timber. The dense growth of brush, the understory of grasses 

 and other herbs, and the litter formed within the brush clumps control 

 erosion with unusual effectiveness unless the cover is depleted. Forest 

 Service studies in southern Idaho have shown that on extremely steep 

 brush slopes the dense vegetation and the loose soil maintained under 

 the brush cover facilitates rapid absorption of moisture, and erosion 

 is negligible. 



WATER-SUPPLY PROBLEMS 



How to obtain adequate water for irrigation on the Columbia River 

 drainage without excessive cost for storage is a great problem, which 

 becomes more intense when rainfall is subnormal for several years, as 

 has recently been the case on much of the area. The large quantity 

 and high values of the irrigated land, as well as the high average 

 annual returns from the land, make irrigation agriculture a dominant 

 industry. According to the 1930 census irrigated land in the basin 

 totals 3,389,000 acres and represents an investment in lands, buildings, 

 irrigation enterprises, and implements of several hundred million 

 dollars. 



Many large irrigation projects are found in the basin, including the 

 Twin Falls and Boise projects of Idaho and such important apple- 

 producing areas as the Yakima and Wenatchee of Washington and the 

 Hood River of Oregon. Of outstanding importance is the projected 

 Columbia Basin project of Washington, which involves irrigation of 

 some 1,200,000 acres. Many other areas are capable of irrigation 

 development ; in southern Idaho, for example, there is more than 2,600- 

 000 acres of irrigable land. 



Water power, also, is of great importance in the Columbia River 

 Basin. On the Snake River and its tributaries of southern Idaho, for 

 example, 166,000 horsepower has already been developed. The Lake 

 Chelan development alone has a present capacity of 125,000 horse- 

 power. Smaller plants are in operation on many rivers, and the 

 undeveloped possibilities in the basin run into several million horse- 

 power. The domestic water supplies of numerous cities and towns, 

 also, originate chiefly on forested watersheds of the basin. 



The influence of forest cover in regulating stream flow in the Colum- 

 bia River Basin is indicated by a preliminary Forest Service study in 

 the Clearwater River drainage in northern Idaho, previously dis- 

 cussed. The burning of some 535,424 acres, or 17.7 percent, of the 

 watershed in 1919 caused the following changes in stream flow in the 5 

 subsequent years as compared with the 5 years prior to the fire: (1) 

 An average advance of 14 days in the date of peak flow; (2) 9.5 percent 



