172 



the case over large areas?) Diminished volume means 

 reduced velocity and carrying power, and hence deposition. 



(c) Many rivers deposit 

 at their mouths where 

 the current is checked. 



(d) Deposition is brought 

 about also by changes in 

 the shapes of river chan- 

 nels. If, for example, 

 water charged with sedi- 

 ment leaves a narrow, 

 straight, and smooth sec- 

 tion of the channel to 

 enter a wide, crooked, and 



FIG. 179. An alluvial fan in the Illinois 

 Valley. The velocity of temporary, 

 wet-weather streams is reduced as they 

 leave the gulley in the background, 

 and they are forced to deposit the 

 sediment which they carry. (III. Geol. 

 Sun.) 



irregular one, the friction 

 of the current with the 

 bed and banks is increased, its velocity is therefore decreased, 

 and deposition may result. (2) Tributaries with high gradi- 

 ents often deliver to their sluggish main streams more sedi- 

 ment than the latter can wash forward, resulting in deposits 

 along the floor of the main valley. In many large depositing 



FIG. 180. Alluvial fan at mouth of Aztec Gulch, Dolores Valley, south- 

 western Colorado. (U.S. Geol. Surv.) 

 Account for the small fan in front of the large one. 



