CHAPTER XVIII 



ADJUSTMENT OF RIVERS 



Rivers are among the most powerful of the agents of topo- 

 graphical development, and it is important to understand some- 

 thing of their modes of change and adjustment. These changes 

 are sometimes exceedingly complex and puzzling, for rivers do the 

 most unexpected things in what seems an utterly capricious and 

 whimsical way. We often see rivers breaching hills and even vast 

 mountain ranges, cutting their way through enormous obstacles, 

 which a slight deviation from their course would, seemingly, have 

 enabled them to avoid. They apparently choose the difficult and 

 shun the easy path. The general explanation of these paradoxi- 

 cal results is, that the river began its flow when the topography 

 was entirely different from its present state of development. It 

 is this fact which renders the rivers such valuable aids to the 

 geologist in his attempts to reconstruct the past, for the apparent 

 whims and caprices are really the necessary results of law. 



A river has its stages of development, youth, maturity, and old 

 age, just as has a land surface, each stage displaying its character- 

 istic marks. When a new land is upheaved, the first drainage 

 lines established upon it are, as we have already learned, deter- 

 mined entirely by the slopes of the new surface and are called 

 consequent streams. In its earliest stages a river can drain its 

 territory or basin in only imperfect fashion, and whatever depres- 

 sions exist in the surface of the new land are filled up with water 

 and form lakes. Tributaries are much fewer than in later stages 

 of development ; the divides between the tributaries are obscurely 

 marked, and in plains these divides are broad areas, not lines. 

 The Red River of the North is an example of a stream in a very 

 youthful stage, which flows across the level floor of an abandoned 

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