FALLEN RAIN. 



69 



of the (Irainag'e basin of the stream. Even then the 

 forest may not prevent floods altogether. The forest 

 floor, which has more to do with the fallen rain water 

 than any other part of the forest, can affect its flow 

 only so long as it has not taken up all the water it can 

 hold. That which falls after the forest floor is satu- 

 rated runs into the streams almost as fast as it would 

 over bare ground. 



An unforested drainage basin in the San Bernardino 

 ^Mountains of south- 

 ern California was 

 found b}^ the Bu- 

 reau of Forestry to 

 discharge the rain 

 it received more than 

 twice as rapidly as 

 similar forested ba- 

 sins near by. In 

 consequence, the 

 stream in the former 



went dry, while the streams in the latter were still flow- 

 ing abundantly. (See PI. XVIII.) 



In these ways it happens that in mountain countries, 

 where floods are most common and do most harm, the 

 forests on the higher slopes are closely connected with 

 the prosperity of the people in the valleys below. 



Water in motion was nature's most powerful tool in 

 shaping the present sui-face of the earth. In places 

 where the slopes are steep, the structure of the ground 

 loose, and the rainfall abundant, water ma}^ work very 

 rapidly in cutting away the heights and filling the val- 

 leys. The destruction of the forest in such a region 

 exposes the surface to the direct action of falling rain 



Fig. 44.— Rapid erosion in deep fine soil. 



