14 What the NATIONAL FORESTS Mean to the WATER U SER 



large quantities of water which in turn are passed on to the great reservoir 

 of mineral soil beneath; and finally, the surface cover of stumps, fallen 

 twigs, branches, and even whole trees acts as a mechanical obstruction 

 to prevent rapid run-off. The surface run-off from forest areas is less, 

 both in total amount and in velocity, than that from similarly situated 

 unforested areas. The steeper and more rugged the topography, the more 

 marked is this contrast. 



In hilly country some erosion is, of course, inevitable under any con- 

 ditions. When the soil cover of trees, underbrush, and litter is kept intact, 

 however, this is more often beneficial than otherwise, since only the lighter 

 soil particles are washed away, to be later deposited in the more level lands 

 below, adding to their fertility. But when this protective cover is inter- 

 fered with, whether by fire, destructive lumbering, overgrazing, or inju- 

 dicious clearing of land for agriculture, the proportion of coarser, infertile 

 materials washed away increases greatly and transforms erosion from a 

 constructive into a dangerously destructive force, difficult of control and 

 capable of doing untold damage. 



From the standpoint of the water user, the tendency of the mountain 

 forests to prevent erosion is of the utmost importance. Wherever storage 

 reservoirs must be used, whether for municipal supplies, irrigation, or water 

 power, they are exposed to the ever-present danger of silting up. Every 

 bit of soil brought down by the streams and deposited in them reduces their 

 capacity and consequently their effectiveness by just so much. This sedi- 

 mentation is serious under any condition, but doubly so when, as not in- 

 frequently happens, no other satisfactory dam sites are available and the 

 reservoir can not be replaced at a reasonable cost. 



Water heavily laden with eroded material often decreases the effi- 

 ciency and increases the cost of maintaining diversion dams, pipe lines, 

 flumes, canals, and other irrigation works. Sometimes such water dam- 

 ages the crops to which it is applied, and not infrequently it seriously 

 injures or even ruins the land by burying it under a mass of sand, gravel, 

 bowlders, and other infertile debris. Excessive erosion may interfere seri- 

 ously with navigation by filling the streams with material which is deposited 

 in their lower reaches and in the harbors into which they empty. 



The action of the forest in reducing surface run-off tends also to regu- 

 late the flow of streams. Instead of rushing away in uncontrollable tor- 



