SHADE TREES AND FORESTS 



4QI 



bringing water from neighboring rivers. The United States 

 government is undertaking large enterprises in this con- 

 nection. It is of special importance here that as much of 

 the water as possible be available during the dry seasons. 

 If no rains fall during this season, it is essential that the rain 

 which has fallen earlier in the season shall be stored up and 

 retained as long as possible on the mountain slopes so as to 

 feed the streams constantly during the dry season. In all 

 these cases forests are essential to regulating the water supply 

 of the streams. 



FIG. 193. Unforested watershed in the San Bernardino Mountains, California. 



Prevention of soil erosion. Another important use of the 

 forest is to prevent soil erosion. The mulch prevents the 

 rapid run-off which causes erosion, and the roots help to keep 

 the soil in place. Where forests on hillsides have been re- 

 moved, the floods wash down the soil, carrying it into the 

 streams, thus interfering with navigation and sometimes 

 spoiling the farms of the lowlands by deposits of gravel 

 and sand. Sometimes these floods cause great injury to 



