STREAMS AND THEIR WORK 173 



conditions: first, the swiftness of the current; sec- 

 ond, the volume of the water; third, the amount of 

 sediment it carries; fourth, the character of the sur- 

 faco oyr-r which it flows. At flood time the erosive 



Permission of U. S. Geological Survey. 



FIG. 66. Alluvial fan at the mouth of Aztec Gulch, south- 

 western Colorado. The deposit at the base of this slope is left 

 by the running water when its velocity is suddenly checked. 



power of a stream is greatest, for then it floAvs with 

 greater velocity, as it has a larger volume of water. 

 As a result, we find it carries much more sediment then 

 than at any other time. This sediment in turn makes 

 it possible for the stream to have a great erosive effect. 

 By being rolled along the bed and against the banks, 

 the sediment scours the bed deeper and broader. The 

 carrying power of a stream varies as the sixth power 

 of the velocitv. If the velocitv is doubled at flood 



