434 A NATIONAL PLAN FOE AMERICAN FORESTRY 



Erosion has greatly increased in practically all the tributary water- 

 sheds since settlement. Much of the pifion- juniper type is still in 

 the unreserved public domain, open to unregulated grazing use. 

 Wherever water is available for livestock this land is invariably 

 overgrazed and badly abused; furthermore, cutting and fires are 

 widespread. Vegetative depletion has accentuated the erosion and, 

 if we may judge from C. K. Cooperrider's studies of the Southwestern 

 Forest and Range Experiment Station in the woodland and brush 

 types of Arizona, the loss of soil productivity through erosion, in turn, 

 has made maintenance of even the reduced vegetation more difficult. 

 Thus is established a trend toward destruction that is difficult to 

 check. 



In the Verde River Valley near Jerome, Ariz., smelter fumes have 

 killed tree growth on a considerable area. Where the grass and other 

 vegetation, as well as the trees, have been practically eliminated 

 erosion has become extremely serious. Over most of the affected 

 zone, however, the soil is still protected by a good growth of grass. 



At the higher elevations the soils are gravelly loams, sandy loams, 

 or sandy silt loams. They are fertile, dark, and high in organic 

 content. Typically they are rather thinly deposited on steep slopes, 

 but they attain a fair depth in depressions. They support abundant 

 forest cover and are relatively free from erosion where the cover is 

 not depleted, although they receive the greatest precipitation occur- 

 ring in the basin. Even heavy clay soils under the more humid 

 conditions at the higher elevations produce fairly abundant plant 

 cover which affords them good protection, although these soils erode 

 readily when the vegetation is depleted or on areas where they have 

 never reached stability. 



According to an extensive erosion survey made by the Forest 

 Service on the Colorado River watershed above Grand Junction, 

 Colo., areas of heavy erosion compose 33 percent of the watershed, 

 areas of moderate erosion 27 percent, and areas of little or no erosion 

 40 percent. As a general rule the heaviest erosion occurs in the non- 

 forest types or in pinon-juniper woodland. Moderate erosion occurs 

 in pinon-juniper, brushland, and nonforest types where cover has been 

 somewhat depleted. Little or no erosion occurs on well-forested 

 areas or on non-forested areas where a good vegetational mantle is 

 maintained. 



One of the most serious effects of erosion is the silting which 

 threatens to shorten the usefulness of reservoirs. The Roosevelt 

 Reservoir on the Salt River of Arizona already has great silt banks in 

 its head. Portions of these are cut away and the material washed 

 closer to the dam by each big flood; then, as the lake refills, new 

 deposits are added. Such silt is made up of material cut from 

 watercourses and soil from slopes where the vegetative cover has 

 become depleted. According to settlers, serious destruction of the 

 vegetative coyer was general 30 to 40 years ago. With drought con- 

 ditions prevailing during many of the last 15 years, the slopes are but 

 slowly revegetating. 



Fortier and Blaney estimate 70 that the Colorado River carries 

 137,000 acre-feet of silt annually past the Hoover Dam site. If this 

 continues the reservoir will fill with silt in about 220 years, and its 



Fortier, Samuel, and Blaney, Harry P. "Silt in the Colorado River and its Relation to Irrigation." 

 U.S. Dept. of Agri. Tech. Bui. 67, 1928. 



