THE PRESENT SITUATION IN FOEESTRY 99 



vegetative cover of a given watershed is only one of a number of important 

 factors governing the flow of water. There has been so much discussion of 

 this subject and the position of the foresters has been so repeatedly mis- 

 interpreted that I shall digress from my main subject for a moment to state 

 my own position in the matter. 



First, the quantity of water in streams, and the regularity of their flow, 

 are affected by precipitation, temperature, topography, vegetation, character 

 and condition of soil (including cultivation, etc.), and rock character and 

 position. The interplay and relative importance of these factors vary greatly 

 in different localities and regions. In general, the most important of them 

 all is the amount, character, and distribution of the precipitation. 



Secondly, ample evidence is furnished by prolonged European experiments 

 and investigations of unimpeachable scientific authority, as to the fact that 

 a forest cover exerts, under most conditions, a very important influence upon 

 streams. This influence, however, has a limit and may be overbalanced entirely 

 by other factors, such as heavy rains and sudden thaws. Forests can not, 

 under prolonged precipitation or other exceptional conditions, prevent large 

 floods, but they tend to diminish both the number and the violence of floods. 



Thirdly, while forests transpire, and growing forests consume more water 

 than other forms of vegetative cover, and so may lessen the aggregate volume 

 of stream discharge in the course of a year, they tend to make more water 

 available by regulating this discharge and by modifying the distribution of 

 rainfall. 



Forests regulate stream discharge (a) by converting surface run-off into 

 underground seepage, and (b) by checking erosion. 



Forests convert surface run-off into underground seepage by checking 

 the force and prolonging the period of rainfall, through the action of the 

 tree tops, and by accumulating snow and retarding its melting; by checking 

 surface run-off through the action of roots, leaf litter, twigs, and fallen 

 trees; by shortening the period during which the ground is frozen and im- 

 permeable to water, and by creating and maintaining a permeable and 

 absorptive soil. 



Forests check erosion by the same means by which they convert surface 

 run-off into underground seepage, and also by the binding effect of tree roots 

 upon the soil. The less the volume of water which runs over the surface of 

 the ground, and the more slowly this water moves, the less is its wearing 

 effect. On steep slopes, or on friable soils, surface run-off creates gullies, 

 torrents, and consequent rapid and permanent physiographic change. 



The results of forest destruction are both to make run-off progressively 

 more sudden, tending to increase the violence of floods, and to load the 

 streams with silt and coarser material. The degree to which the removal of 

 forests or any other vegetative cover increases erosion varies according to 

 the completeness of its destruction and its recuperative power. 



The conversion of run-off into underground seepage and the checking of 

 erosion are the two essential forest influences which act together to control 

 flood conditions. 



