6l 



The proportion of flood water to seepage is influenced by the 

 rapidity of the rainfall. It is well known from direct observation 

 that a slowly falling, prolonged rain, even on the naked soil of 

 steep slopes, is all taken up by the soil. On the other hand, a 

 heavy shower of short 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 accumu- 

 lation of drops is more rapid than the absorption, and the excess 

 moves over the surface to lower elevations. The forest canopy 

 very 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 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 influence of the forest in this respect is of the utmost 

 importance. So great is this influence that it exerts a marked ef- 

 fect upon topography. In mountainous regions particularly, the 

 repeated destruction of forests permits the soil formed by the de- 

 composition of the rocks at the sources of streams to be trans- 

 ported 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 elevations. 

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

 it flows over the surface. On the Qther 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 mini- 

 mum. 



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

 absorb the rain, but it must be of such a 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 

 rapidly than soil containing little or no organic matter ; hence, 

 the greater the amount of leaf mould and other litter, the more ra- 

 pidly will the rain be absorbed. Rapidity of absorption is also in- 

 fluenced 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 path- 

 way. 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 



