THE WORK OF STREAMS 173 



rivers, like the lower Mississippi, all the above causes, and 

 perhaps other less important ones, are in operation. 



The principal features produced by stream deposition are 

 described in the following paragraphs. 



Fans and cones. Alluvial fans are so called because 

 they are half-circular in ground plan when developed typi- 

 cally, and are composed of alluvial material (Figs. 179 and 

 180). Cones are relatively steep fans. Alluvial fans vary 

 in diameter from a few feet to several miles. Some of the 

 California rivers have built fans some forty miles across. 

 Fans are developed best at the bases of steep slopes in dry 

 regions, where streams of diminishing volume leave the rela- 

 tively high gradients of their mountain valleys to enter low- 

 lands. The deposit in such a situation chokes the channel 

 of the stream, and some of the water spreads around the 

 obstruction. The process being repeated many times, and 

 the stream meanwhile extending the deposits in the direction 

 of its flow, they presently acquire more or less of the " fan " 

 shape which suggested their name. The main water chan- 

 nels of many large fans give off branches that in turn di- 

 vide repeatedly downstream. These branching channels are 

 called distributaries, and their explanation is involved in 

 what has already been said. The deposits in a given chan- 

 nel reduce its size until some of the water breaks over the 

 side and follows a new course to the margin of the fan. 

 The new channel, becoming choked, gives off other distribu- 

 taries, which divide again. The spreading of the water 

 flowing over the fan becomes an important cause of deposi- 

 tion, since it increases the friction of flow, and therefore 

 decreases the velocity. Deposition may be caused also by 

 much or all of the water sinking into the porous material 

 of the fan. Thus the growth of fans is due to deposition 

 brought about by (1) decrease in gradient, (2) increase in 

 friction of flow, and (3) often by decrease in volume. Plate 

 VIII shows a portion of a large fan, together with waste, 

 channels, tributaries, and distributaries. 



