. FIF'TH NATIONAL CONSERVATION CONGRESS 301 



soils the crown of trees, which break the violence of the rainfall, together with 

 a surface mulch of leaves and twigs, prevent the soil from becoming compact and 

 allow it to retain its granular structure, thus making it more permeable to water. 

 On a soil very permeable to water, such as sand, the influence of the forest in 

 decreasing surface run-off may be very insignificant, consisting chiefly in pre- 

 venting the soil from being washed away. 



The depth of soil has a bearing upon the amount of water which it can retain. 



No matter what its character may be, a thin soil can not retain much water. 

 The forest, however, tends to increase the volume of soil, and thus creates greater 

 reservoirs for water. It does this in two ways: (1) From above, by the addi- 

 tion of leaves and twigs, which, when decayed, become a constituent part of the 

 soil; and (3) from below, by inducing disintegration and decomposition of the 

 underlying rock. The forest, by constantly increasing the depth of the soil, 

 lessens the likelihood of it being washed away and enables it to remain where it 

 was formed. The addition of organic matter to the soil increases its water- 

 holding capacity. The tree roots at the same time enter the narrow fissures of 

 the rock, which they widen, thus producing many new openings into which the 

 water may sink. 



During the vegetative season the demand of the forest upon the water 

 stored in the ground is very great. In summer the forest, like other crops, con- 

 sumes more water than it receives in the form of precipitation. At the end of 

 the vegetative season, therefore, the level of the underground water is low. 

 As a result, the forest soil can absorb large quantities of water during the period 

 of vegetative rest, when there is an excess of water on the ground, either from 

 heavy rains or from the melting of snow. The forest soil, therefore, forms a 

 reservoir whose capacity is greatest when the excess of water on the ground and 

 the danger of floods is greatest. The water stored in the time of rest is used by 

 vegetation and for the flow of streams later on when there is usually a deficiency 

 of precipitation. 



FORESTS AND EROSION 



THE forest is the most effective agency for protecting the soil from erosion 

 because: (1) the resistance of the soil to erosive action is increased by 

 the roots of the trees which hold the soil firmly in place, and (2) at the 

 same time the erosive force of the run-off is itself reduced because the rate of its 

 flow is checked and its distribution over the surface equalized. 



Erosion has a bearing on the height of flood water in the rivers, since the 

 sediment carried by the rivers and the coarser detritus brought down by moun- 

 tain streams often increase streamflow to such an extent that the height of the 

 water is raised far beyond the point it would reach if it came free from detritus 

 and sediment. When the channel of a stream has become filled with waste 

 material, even a slight rainfall will cause a flood; while if the channel were 

 deep, it would have no perceptible effect upon the height of water in the stream. 

 The filling of mountain streams with waste not only increases the frequency of 

 floods but causes the streams to assume the character of torrents. A channel 



