CHAPTER V 



THE WORK OF STREAMS 



The run-off. It is estimated that the total amount of 

 rain which falls yearly on all the land is equal to about 

 35,000 cubic miles of water, enough to cover all New 

 England more than half a mile deep. As indicated on page 

 107, a portion of this rain water sinks into the ground, 

 a portion runs off over the surface, and a third part is evapo- 

 rated. The immediate run-off, reenforced by the overflow 

 of lakes, by contributions from springs and seepage and from 

 melting snows, flows always from higher to lower levels. 

 All that is not lost by evaporation or by sinking beneath 

 the surface therefore runs ultimately to the sea. Streams 

 are estimated to carry 6500 cubic miles of water (enough 

 to cover Connecticut and Rhode Island more than a 

 mile deep) to the sea each year. This water descends on 

 the average nearly half a mile before reaching the ocean. 

 During the descent a large but unknown amount of energy 

 is used up as internal friction in flowing on low slopes, and 

 as friction on the channels of the streams and on the sedi- 

 ment which the streams carry. In addition, an enormous 

 amount of energy is exerted in geological work. The nature 

 of this work and the results are discussed below. 



THE PROCESSES OF EROSION 



The general term erosion covers those processes by which 

 rock surfaces are worn and broken up, and the loosened 

 material removed. It therefore includes weathering, trans- 

 portation, and common. By the last is meant the mechani- 

 cal wearing of rocks, particularly by running water. 



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