LAND WATERS STREAMS 



123 



competition still goes on. If a' and /' (Fig. 85) develop faster than 

 c r , its prospective drainage territory will be pre-empted by its rivals 

 (compare Figs. 85 and 86) . Thus as the result of the unequal rate 



Fig. 85. 



Fig. 86. 



Figs. 84, 85, and 86. Diagrams illustrating successive stages in the struggle 

 for existence among streams. 



at which valleys are lengthened, the larger number of those which 

 come into existence are arrested in their development. 



Piracy. Streams do not always hold the courses which they 

 establish for themselves in youth. Thus the valley of the 

 Potomac River across the Blue Ridge (Fig. 87) was deepened faster 

 than that of Beaverdam Creek. The head of the young Shenan- 



