THE WORK OF RUNNING WATER 85 



slope. The term fan is more appropriate than cone for most alluvial 

 accumulations at the bases of steep slopes. 



Nearly all young rivers descending from mountains build fans 

 where they leave the mountains. Thus the rivers descending from 

 the Sierras to the great valley of California build great fans at the 

 foot of the range, and most of the rivers coming out of the Rockies 

 to the plains east of them do the same thing. The fans of streams 

 descending from the mountains are often many miles across. 



The fans made by neighboring streams often grow until they 

 unite. The union of such fans makes a compound alluvial fan, or 

 a piedmont alluvial plain (Fig. 42 and PL XXIII, p. 86). Such 

 plains exist at the bases of most considerable mountain ranges. 

 The depth of alluvial material in such situations is often scores and 

 sometimes hundreds of feet. 



Alluvial fans and piedmont alluvial plains are often valuable 

 for farming. In some parts of California, for example, the alluvial 

 lands are so valuable that farms are small and highly improved. 

 Even in semi-arid regions they are often cultivated, the water being 

 supplied (1) by wells, through which the debris of the fan is made 

 to yield up the water it has absorbed, or (2) by irrigation ditches 

 which connect with a stream or reservoir at a greater height. 



2. In valley bottoms. The gradient of a stream generally be- 

 comes less as it flows on, and so it happens that sediment is dis- 

 tributed for great distances 

 along valley bottoms. Some 

 of it is left in the channels, / \ 



and some of it is spread 

 over the low lands along the 

 streams, making them alluvial 

 plains. Deposition in a val- 

 ley which has no flat tends to Fi S- 86. Flat developed by aggrada- 

 tion diagrammatic, 

 develop one (Fig. 86). 



When a stream deposits sediment in its channel, the channel 

 becomes smaller. In time it may become too small to hold all the 

 water, and a part then breaks out, and follows a new course in the 

 valley flat. This process may be repeated again and again (Fig. 87). 

 The departing streams may or may not return to the main. If they 



w* 



1 T 



