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 otf 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 

 irrio-ation 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 



