284 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



duration, falling' on the same slope, may largely escape as run-off. In 

 the first instance each drop has time to be absorbed by the soil, while 

 in the latter the accumulation of drops is more rapid than the absorp- 

 tion, and the excess moves over the surface to lower elevations. The 

 forest canopy ver}^ perceptibly extends the period of time during which 

 the rain reaches the soil, and in this way lessens surface run -off. 



Again, forests, by checking the velocity of the wind and covering 

 the mineral soil with a thick layer of dead leaves and other forest 

 litter, effectively prevent soil transportation by both wind and water. 

 On high elevations, where streams generally have their birth, the influ- 

 ence of the forest in this respect is of the utmost importance. So great 

 is this influence that it exerts a marked effect upon topography. In 

 mountainous regions particular!}^, the repeated destruction of forests 

 permits the soil formed by the decomposition of the rocks at the sources 

 of streams to be transported to lower elevations, with a consequent slow 

 change in the details of the landscape. Such regions, if unforested, 

 are apt to have precipitous slopes and scanty soil on the higher eleva- 

 tions. In that case there is no adequate medium to absorb the rain, 

 and it flows over the surface. On the other hand, if such regions are 

 well wooded, the slopes are less precipitous, and a considerable depth 

 of soil usually covers the broad summits. As a result, the rain water 

 is absorbed and the surface flow is reduced to a minimum. 



Not only is it essential to have an adequate medium present to absorb 

 the rain, but it must be of such character as to absorb quickly. The 

 rapidity with which rain is absorbed is very largely governed by the 

 physical properties of the soil, the organic litter upon it, and the 

 vegetation. Decayed organic matter, by itself or in combination with 

 mineral soil, absorbs moisture much more rapidl}^ than soil containing 

 little or no organic matter; hence, the greater the amount of leaf mold 

 and other litter, the more rapidly will the rain be absorbed. Kapidity 

 of absorption is also influenced by the degree of looseness of the 

 mineral soil. In the forest the mulch of leaves and litter keeps the 

 mineral soil loose and in the best condition for rapid absorption. 



Not all the rain that is not absorbed by the soil where it falls reaches 

 the streams by flowing over the surface. Much of it is taken up in 

 passing from the place of falling to the stream. The amount taken 

 up depends upon the obstructions in its pathway. Where there are 

 no obstacles, as on barren ground, the moving water, by eroding 

 channels, forms small rivulets, and these larger and larger ones, which 

 flow with constantly increasing velocity. As a result, the water passes 

 rapidly over the surface, and but little gets into the soil. When the 

 soil is covered with obstructions, such as are offered by a forest with 

 its accumulation of litter and vegetable growth, the rain which is not 

 immediately absorbed is checked in its flow over the surface. The 

 water, being held back, is finally taken up by the soil and thus pre 

 vented from forming small rivulets through erosive action. 



