1510 A NATIONAL PLAN FOR AMERICAN FORESTRY 



TABLE 1. Watershed-protective value of forests in the United States 



The fact that the extent and character of the forest cover, as well 

 as stream flow and erosion, are controlled in part by the quantity and 

 distribution of precipitation makes it difficult to draw deductions 

 from gross acreages such as are given in table 1. It may be noted 

 that in the Pacific Cascade drainages, with steep slopes and heavy 

 rainfall but with about 90 percent of the total area hi forest, mostly 

 dense, floods and erosion are no great cause for concern, while in the 

 Colorado River Basin, with much lower rainfall but with less than 

 one third of its area in forest of a lighter type, floods and erosion are 

 serious. More localized comparisons are given in the watershed 

 description section. The effect of forest destruction on run -off is 

 indicated by studies at the Red Plains Erosion Experiment Station 

 in Oklahoma, where a plot from which the forest litter had been 

 burned produced more than 100 times as much run-off as a similar 

 unburned plot; its effect on erosion is indicated by a study of Hoyt 

 and Troxell in California, in which the flood flows from burned water- 

 sheds were found to contain 20 to 67 percent of ash and silt. 



The Great Basin, with only 14 percent of its area forested and only 

 28 percent of this classed as of major influence, developed a serious 

 flood and erosion situation only after the forest and other vegetative 

 cover was reduced by overgrazing and fire. Similarly, in the Ohio 

 River Basin, 35 percent of which is in forest, the silting problem and 

 increased frequency of floods have followed misuse of the land by man. 



Erosion is a geologic phenomenon older than the hills, yet in each 

 region the original vegetative cover was usually sufficient for soil 

 building. Reduction of the cover through timber cutting, fire, over- 



