536 A NATIONAL PLAN FOR AMERICAN FORESTRY 



The importance of improving the vegetative cover is strikingly 

 shown by Forsling 5 in a study of two small subalpine watersheds in 

 central Utah. On one the vegetation has been maintained since 1915 

 with a cover of about 40 percent of the soil surface. The other sup- 

 ported a cover of only 16 percent from 1915 to 1920. This cover was 

 increased, partly by artificial reseeding, to 40 percent where it was 

 maintained from 1924 to 1929. Over 95 percent of the annual surface 

 run-off came from melting snow. Such run-off is the main supply for 

 irrigation, supplemented by a delayed drainage of percolated snow 

 water extending through the summer. Surface run-off from summer 

 rainstorms on the watershed in depleted condition (1915 to 1920) 

 swept away over 8 tons of sediment per acre, or for at least 3 years 

 of record about 85 percent of that removed yearly. After improve- 

 ment (1924 to 1929) the sediment removed annually by summer 

 rainstorms was only about 1 ton per acre. For the periods 1915 to 

 1920 and 1924 to 1929 the difference between the two watersheds in 

 surface run-off per inch of total rainfall was 0.042 and 0.011 inch, 

 respectively, and in sediment removed per inch of rainfall, 21.8 and 

 2.8 cubic feet. Thus, as a result of improvement in the vegetative 

 cover, a considerable reduction in the relatively small surface run-off 

 from rainstorms was accompanied by a marked reduction in erosion. 



On depleted areas of important watersheds, therefore, it is not suffi- 

 cient simply to maintain a thin stand of vegetation. Management 

 should be adjusted to facilitate as rapid recovery as is practicable. 

 Although grazing can usually be adjusted through improved range 

 management to meet watershed-protection requirements, there are 

 areas within watersheds from which vegetation has been almost 

 denuded or on which the natural balance is so insecure that any grazing 

 in an effort to secure the negligible quantity of feed available would 

 cause undue sliding of soil and prevent new vegetation from becoming 

 established. Grazing should be excluded from such areas, at least 

 until enough vegetation has been established to check the extreme 

 erosion of soil from the slopes. 



Large areas of western forest land are used for furnishing municipal 

 water supply. The extent of such use is indicated by the fact that 

 about two and one-half million people, living in over 700 cities or 

 towns in the Western States, obtain their water supplies from areas 

 within national forests. A pronounced public sentiment exists against 

 the grazing of livestock, and especially sheep, on watersheds from 

 which domestic supplies are derived. Sanitary engineers, however, 

 hold that danger of contamination is not from the livestock but from 

 the presence of human beings on the watersheds. Thus, the Wash- 

 ington State Board of Health, after an investigation of the watershed 

 furnishing Walla Walla with its domestic supply, advised the city 

 that removal of all stock would not insure purity and that filtration 

 was the only practical way to bring this about. The United States 

 Public Health Service in reporting on this case stated: " States have 

 not passed laws to prohibit grazing on watersheds, because it is 

 generally agreed among sanitarians that diseases are not transmitted 

 by water from animals to man." Nevertheless in many instances, 

 the Forest Service has entered into special agreements with munici- 

 palities for the more complete protection of their domestic supply of 



Forsling, C. L. A Study of the Influence of Herbaceous Plant Cover on Surface Run-off and Soil Erosion 

 in Relation to Grazing on the Wasatch Plateau in Utah. U.S. Dept. Agr. Tech. Bui. 220. 1931. 



