PRACTICAL FOEESTBY. 



37 



not sink into the ground more than half as readily as in the forest, as 

 experiments have shown. The result is that a great deal of water 

 reaches the streams in a short time, which is the reason why floods 

 occur. It is therefore true that forests tend to prevent floods. But 

 this good influence is important only when the forest covers a large 

 part of the drainage 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 



frg^l^^^^^^. 



^^■w. » n«r .-^.-y ^i^r«.^.^. wm- 



Fig. 22.— Beginnings of erosion in soil tramped bare by stock. Sierra Nevada Mountains, Calilornia. 



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 saturated runs into the streams 

 almost as fast as it would over bare ground. 



An unforested drainage basin in the San Bernardino Mountains of 

 southern California was found by the Forest Service to discharge the 

 rain it received more than twice as rapidly as similar forested basins 

 near by. In consequence the stream in the former went dry, while 

 the streams in the latter were still flowing abundantly. 



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. 



358 



