CHAPTER IV 

 THE WORK OF RUNNING WATER 



Rivers are estimated to carry about 6,500 cubic miles of water 

 to the ocean annually. 1 Since the average height of land is nearly 

 half a mile, the waters which flow from it to the sea fall, on the 

 average, nearly half a mile in their flow. Their total energy is 

 therefore great, and they are the great carriers of sediment from 

 land to sea. The sediment which they carry is composed largely 



Fig. 36. Spokane River, 4 miles above Spokane, during flood. (Photo, by 

 Tolman.) 



of decayed rock, but undecayed rock is sometimes worn away, 

 especially where streams are very swift. 



Though the flow of some streams is so gentle that they do not 

 appear to work great changes in their valleys, others wear away 

 their banks so rapidly that the changes they produce may be seen 

 from year to year, or, when the stream is in flood (Fig. 36), from 

 day to day. Flooded streams occasionally sweep away dams, 

 bridges, and buildings on their banks. The strong rods and beams 

 of bridges and the steel rails of railways are bent almost as if they 

 were twigs by the force of the occasional flood (Fig. 37). 



1 Murray, Scot. Geog. Mag. Vol. III. p. 70. 



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