THE WORK OF STREAMS 



99 



Fig. 79. — Block diagram showing the manner in which 

 the divide between two streams is narrowed. 



soon finds depressions where it accumulates into streams. Even 

 though a slope were perfectly uniform, a slight heterogeneity of soil 

 or rock would permit the water to remove more material in one 

 place than in another 



— 



and thus begin the 

 excavation first of a 

 gully, and later, by 

 prolonged erosion, of 

 a ravine which still 

 later would develop 

 into a broad valley. 



A valley is length- 

 ened at its upper end 

 and is cut back by the 

 water which flows in 

 at its head (Fig. 78), 

 the direction being determined by the greatest volume of water 

 which enters it. This is called headzvard erosion. A valley is 

 widened by rainwash, lateral erosion (Fig. 79), and in other ways (p. 

 89). Its length depends upon the distance to which its stream can 

 cut inland. At the beginning a valley has running water only 



during and immediately after rains, 

 but later, when it has cut below the 

 water table (p. 56), a permanent 

 stream flows through it (Fig. 32, p. 

 57). Tributary streams tend to turn 

 in the direction of their main (Fig 

 80), a feature which is often most 

 pronounced late in their history. 



Valleys Formed in Ways Other 

 than by Stream Erosion. — Although 

 the great majority of valleys are de- 

 veloped by stream erosion, some were 

 already formed for the streams which 

 Map showing the usual flow through them. The popular 

 relation of tributary streams to the ^^ ^ canyons such as that of 

 main stream into which they flow. . . . 



the Colorado River in Arizona, were 



formed by great cataclysms which rent the earth and produced 

 the deep fissures now occupied by streams, is without foundation. 

 Streams, however, do occasionally flow into fissures formed by 



Fig. 80. 



