72 A PRIMER OF FORESTRY. 



the bottom, the heavier pieces first. Where it falls 

 the beds of rivers are filled up and fertile lands are 

 covered with pebbles and sand. 



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 ofl' in a few days. This 

 may be quite as serious a matter for the farmers as 



Fig. 47. — Plantation of European alder in the bed of a torrent controlled by- 

 dams. This torrent is now extinct. Alps of southern France. 



the destruction of their fields, as for example in places 

 like southern 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 alread}^ car- 

 ried stones and sand into the orange groves and even 

 into the towns of the San Gabriel Vallev. Before the 



