THE WORK OF STREAMS 



87 



gentle grade, it may drop its load. Decrease of volume due to evap- 

 oration and to the absorption of the water by the soil, such as takes 

 place when a river flows through a dry region, may reduce the stream's 

 velocity to such an extent that it is unable to carry its load of 

 sediment. The Platte River of Nebraska is a typical example of 

 such a river. Its 

 headwaters have a 

 small amount of sedi- 

 ment in proportion 

 to the volume of 

 water, and it is there- 

 fore able to cut a 

 deep canyon in its 

 upper course ; but in 

 passing over the dry 

 and thirsty plains it 

 loses so much water 

 that it is not only 

 unable to degrade its 

 bed, but even de- 

 posits much of its 

 load during the dry 

 season. A stream 

 which flows over 

 such sandy plains 

 has shallow, crooked 

 channels and is con- 

 stantly shifting its 

 course by cutting 

 away the banks in 

 some places and 



forming bars in others. When the load is so great that it is deposited 

 in the channel, the latter may become too small for the water of the 

 stream, in which case the water will break out and follow a new 

 course. If this is repeated many times a network of small, shallow 

 streams, called a braided stream (Fig. 62), may result. The Colorado 

 and Platte rivers have about the same gradient, but the former 

 receives less sediment in proportion to its volume and consequently 

 is able to cut a great canyon, while the lower Platte flows in a broad 

 and shallow valley. 



Fig. 62. 



Braided stream, Kandersteg valley, 

 Switzerland. 



