A NATIONAL PLAN FOR AMERICAN FORESTRY 27 



Regular stream flow, freedom from silt that will fill reservoirs are 

 important factors in all water power development and use. 



The importance of watershed protection is indicated by the fact 

 that the Federal Government has been willing during the past 50 

 years to expend for the improvement of our rivers and harbors in 

 excess of $2,000,000,000. Again regularity of flow and freedom from 

 silt are vital factors in the availability, use, and cost of maintenance. 



Floods occur in every part of the United States and damages range 

 downward from the great Mississippi disaster of 1927, estimated at 

 246 lives and $300,000,000, to those caused by local floods so small 

 as to escape notice. Every year has its quota. 



Expenditures for major engineering works for flood control have 

 run into the hundreds of millions of dollars, but the problem is far 

 from solution. There is ample reason to believe that fully satisfac- 

 tory control must utilize all means at our disposal, including the forest. 



Excessive erosion, either spectacular or so inconspicuous as to go 

 unnoticed, is common to nearly all parts of the United States. It 

 occurs on agricultural, range, and forest land. It loads streams with 

 silt, clogs irrigation works, navigable channels and harbors, fills 

 reservoirs, increases the height of floods, and adds enormously to 

 their destructive power. 



Because it first removes the fertile top layers of soil it is a primary 

 cause of land abandonment. It is undoubtedly the most destruc- 

 tive agency affecting our greatest basic resource, the soil. 



THE RELATION OF THE FOREST TO WATERSHED PROTECTION 



General observational studies in the United States have substan- 

 tiated both the popular conception and European experience that the 

 destruction of the forest cover leads to erosion and that the presence 

 of such cover is the most effective means for erosion control. 



Furthermore, they have shown that the forest will rebuild the soil. 



Intensive research makes the relationship between forest cover and 

 erosion still more conclusive. Results in different parts of the 

 country on different soils with varying precipitation, etc., show that 

 the ratio of erosion between barren forest land and that with forest 

 cover may vary all the way from 15, 1,000, and 3,920 to 1. 



It has been estimated that 1,000 years may be necessary to build 

 up an inch of soil, an amount which often is removed by erosion in 

 1 year. 



Observational studies have shown that destruction or deteriora- 

 tion of the forests is one of the major contributing causes of exces- 

 sively rapid run-off and destructive floods, and that the presence of 

 the forest retards the rate of run-off, puts the water into the soil and 

 underground channels, reduces the height of floods, increases summer 

 flow, and delivers water free from sediment. 



Intensive research shows that the ratio of run-off between denuded 

 and forest-covered soils varies from 3, 110, and 187 to 1, with inter- 

 mediate ratios for partially destroyed forest. 



Among the chief causes of forest destruction in relation to water 

 protection are fire, logging, overgrazing, and smelter fumes. The 

 most critical watershed conditions in the United States have, how- 

 ever, resulted from clearing for agriculture. 



The best classification possible with the data available indicates 

 (fig. 16) that 308 million acres or half of the forest area of the United 



