PRACTICAL FORESTRY. 



39 



thaws swell the streams with marvelous quickness, and give them a 

 wonderful power to cut away their banks. Where the waterway is 

 very steep such a flood often carries with it many times its own weight 

 of earth and stones. As it nears the valley it breaks from its bed and 

 makes new channels, or spreads over the lowlands. The current loses 

 its swiftness, and its load of stones and sterile earth sinlcs to the bot- 

 tom, the heavier pieces first. Where it falls the beds of rivers are 

 filled up and fertile lands are covered with pebbles and sand. 



Fig. 24.— Plantation ol European alder In the bed of a torrent controlled by dams. This torrent is now 

 extinct. Alps of southern France. 



For a time after such a flood the streams are usually low, because 

 the water which should have fed them for weeks or months has run off 

 in a few days. This may be quite as serious a matter for the farmers 

 as the destruction of their fields, as, for example, in places like south- 

 ern California, where the crops depend on irrigation with the water 

 of streams which rise in the mountains. Torrents have begun to form 

 there in the San Bernardino Mountains, and have already carried 

 stones and sand into the orange groves, and even into the towns of the 

 San Gabriel Valley. Before the water of the San Gabriel River was 

 so largely taken out for irrigation it was rapidly cutting away the 

 fertile land on either side of its shifting bed, and it seemed likely that 

 serious loss of property would follow. This is the direct result of 

 fire and grazing in the mountains. 



358 



