PROTECTION 165 



instead of a few hours, and the nearby rivers have 

 a steady, equable flow, instead of alternate stages 

 of floods and low water. 



Closely bound up with the protection of water- 

 sheds is the erosion problem. Without a forest 

 cover, rain runs off mountain slopes very rapidly, 

 often carrying with it silt and sand, and, in severe 

 floods, even rocks and bowlders. A well known 

 physical law states that the carrying capacity of 

 a stream increases as the sixth power of its velocity. 

 In other words, double the velocity of a stream and 

 you have multiplied its carrying power by 64; in- 

 crease its velocity ten times, and you multiply its 

 carrying power by a million. The delay caused by 

 the forest cover in each raindrop's journey do^vn a 

 mountain side not only prevents floods, but also 

 preserves the fertility of the fields in the valleys 

 below. 



Many streams in the West carry such enormous 

 amounts of silt that the storage capacity of reser- 

 voirs has been seriously impaired, even within a 

 comparatively short time. Then, also, there is the 

 added difficulty and expense of keeping the diver- 

 sion works — the ditches and canals — free from an 

 excess of this material. Studies which have been 



