CHAPTER XII 

 THE LIFE HISTORIES OF RIVERS 



The intricate pattern of river etchings. The attack of the 

 weather upon the solid lithosphere destroys the integrity of its 

 surface layer, and through reducing it to rock debris makes it the 

 natural prey of any agent competent to carry it along the surface. 

 We have seen how, for short distances, gravity unaided may pile 

 up the debris in accumulations of talus, and how, when assisted by 

 thaw water which has soaked into the material, it may accomplish 

 a slow migration by a peculiar type of soil flow. Yet far more 

 potent transporting agencies are at work, and of these the one of 

 first importance is running water. Only in the hearts of great 

 deserts or in the equally remote white deserts of the polar regions 

 is the sound of its murmurings never heard. Every other part of 

 the earth's surface has at some time its running water coursing 

 in valleys which it has itself etched into the surface. It is this 

 etching out of the continents in an intricate pattern of anastomos- 

 ing valleys which constitutes the chief difference between the land 

 surface and the relatively even floor of the oceans. 



The motive power of rivers. Every river is born in throes 

 of Mother Earth by which the land is uplifted and left at a higher 

 level than it was before. It is the difference of elevation thus 

 brought about between separated portions of the land areas that 

 makes it possible for the water which falls upon the higher portions 

 to descend by gravity to the lower. This natural " head " due to 

 differences of elevation is the motive power of the local streams, 

 and for each increase in elevation there is an immediate response 

 in renewed vigor of the streams. The elevated area off which the 

 rivers flow is here termed an upland. 



The velocity of a stream will be dependent not only upon the 

 difference in altitude between its source and its mouth, but upon 

 the distance which separates them, since this will determine the 

 grade. The level of the mouth being the lowest which the stream 



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