176 



A YEAR IN SCIENCE 



Second, it has a rapid current. The velocity of flow 

 is largely determined by the steepness of the slope. 



Third, its course 

 is comparatively 

 straight. When a 

 stream originates, 

 the drainage runs 

 along the lowest 

 level of the 

 ground, which is 

 usually a straight 

 course. Streams 

 have a tendency 

 to become more 

 and more crooked 

 as they g r o w 

 older. 



Fourth, young 

 streams have nar- 

 row and V-shaped 

 valleys. The first 

 work a stream has 

 to do is to erode 

 its bed to a low level ; after the most of the downward 

 erosion has been accomplished it begins eroding the 

 sides. A V-shaped valley indicates very little lateral 

 erosion; hence, it may be concluded that the stream is 

 young. * 



Fifth, there is practically no flood-plain. By this 



FIG. 68. A gorge cut by the Colorado 



River. A young river valley is 



narrow, its sides are steep, and 



the stream forming it is swift. 



