A NATIONAL PLAN FOR AMERICAN FORESTRY 433 



future prosperity of the basin depends in large part upon safe- 

 guarding the irrigation-water supply and the storage reservoirs. 

 While the main irrigation projects have developed large storage 

 facilities, the small enterprises in the mountain valleys have only 

 slight storage facilities or none; hence their effectiveness depends 

 upon sustained stream flow throughout the irrigating season. 



Numerous power plants and many communities throughout the 

 basin are dependent upon sustained stream flow to meet their water 

 needs. Of outstanding importance to the future of southern Cali- 

 fornia is the Hoover Dam project. This stupendous project with its 

 700-foot dam will impound 30,500,000 acre-feet of water, irrigate 

 more than 2,000,000 acres, develop more than $6,500,000 worth of 

 power annually, and furnish the supplementary water supply needed 

 for Los Angeles, San Diego, and other southern California cities and 

 communities. 



The flow of the Green River and its tributaries in Colorado, Wyo- 

 ming, and Utah illustrates rather clearly the fact that the perennial 

 flow for sustaining irrigation, power, and domestic supplies, comes 

 principally from the higher mountain areas. On that watershed 

 about 60 percent of the 18 to 30 inch yearly precipitation comes as 

 snow, which above 7,000 or 8,000 feet elevation accumulates from 

 October until mid-April. During the late spring months heavy sur- 

 face run-off from this melting snow swells the streams to a normal 

 high-water stage. Stream-gage records of the United States Geolog- 

 ical Survey show that 74 percent of the annual run-off occurs in the 

 4-month period April to July. The low-water stage is maintained 

 rather uniformly through the remainder of the year by the flow of 

 hundreds of springs scattered throughout the higher mountain areas 

 and by that of many mountain lakes. Summer rainfall at any eleva- 

 tion, and snowfall and springs at low elevations, make only a rela- 

 tively small contribution to stream flow. The snowfall at high ele- 

 vations and the conditions under which its transformation into water 

 takes place are the important factors in water production. 



EROSION 



In the Colorado River Basin abnormal erosion, since white settle- 

 ment has removed from 1 to 7 inches of the fertile topsoil from exten- 

 sive slope and even plateau areas, is still occurring on far too high a 

 percentage of the forest land, and is cutting out valuable alluvial soil 

 along nearly all water courses below the dense timber belt. 



The most serious erosion conditions on forest lands are found in the 

 pinon-juniper, ponderosa pine, and mountain-brush types, especially 

 on the heavy clay or adobe soils and sometimes on sandy loams. 

 These soils are ordinarily deficient in humus, are more or less alkaline, 

 do not readily absorb water, support only a thin stand of vegetation, 

 and readily disintegrate when thoroughly wet. Thus under the 

 influence of semitorrential rains, if inadequately protected by vegeta- 

 tion, they are eroded at a rapid rate. The serious effect of vegetative 

 depletion is exemplified by the extensive areas (largely privately 

 owned) in the pine forests near Pagosa Springs, Colo., that were logged 

 beginning in the nineties and continuing until about 1920. The 

 timber was heavily cut and the areas have been overgrazed, mainly 

 by sheep. Throughout these areas there is excessive sheet and gully 

 erosion, in practically all stages of intensity and activity. 



