94 PROCEEDINGS OE THE 



region. The contention is made that when other con- 

 ditions are similar, it is the physiographic form which 

 most largely determines the amount of run-off in 

 proportion to the precipitation and the fluctuations in 

 stream-flow as well. 



I wish to emphasize the fact that the physiographic 

 form of the drainage basin, more particularly those 

 features which most largely influence stream-flow, have 

 been brought about by forest growth acting through 

 long periods of time. 



In checking wind and water erosion at the sources 

 of our mountain streams, the forest produces a much 

 greater effect upon physiographic detail than generally 

 recognized. On the summits of mountains and on 

 ridges, where the forest has a density of .8 or greater, 

 and where the forest floor has been undisturbed by 

 fire and grazing, the wealth of litter, humus, and min- 

 eral soil takes up practically all of the precipitation ; 

 which, seeping through the soil, reappears on the sur- 

 face at lower elevations without bringing silt and other 

 eroded material with it. Erosion, therefore, in such 

 regions is very slow as compared with non-forested 

 regions. 



Vertical corrasion in the channels of the intermit- 

 tent and permanent streams is also a slower process, 

 because there is but little grinding material carried 

 by the moving water. 



On the other hand, when summits and ridges have 

 been without forest cover for long periods, there is not 

 only an almost total absence of litter and humus, but 

 a scant covering of mineral soil as well. The absence 

 of an absorbing medium causes the larger part of the 

 rainfall to flow over the surface from the place of fall- 

 ing. This surface flow causes rapid erosion. 



The forest, in preventing the transportation of soil 



