116 



GEOLOGY. 



The velocity of a stream depends chiefly on three elements — its 

 gradient, its volume, and its load, (i.e., the sediment it is moving). 

 The higher the gradient the greater the volume, and the less the 

 load the greater the velocity. The relation between gradient and 

 velocity is evident; that between volume and velocity is illustrated 



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b 



c 



d 



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b 



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Figs. 97-99. — Diagrammatic representation of the effect of increased velocity on 



transporting power. 



by every stream in time of flood, when its rate of flow is greatly in- 

 creased. The relation between velocity and load is less obvious, but 

 none the less definite. Every particle of sediment carried by a stream 

 makes a draught on its energy, and energy expended in this way re- 

 duces the velocity. The draught on a stream's energy of a particle 

 carried in suspension is measured by its mass into the distance it would 

 fall in a unit of time in still water. It follows that a large particle 

 makes a stronger draught on a stream's energy than the same amount 

 of material in smaller pieces. It follows also that the comminution 

 of sediment facilitates transportation in much more than a simple 

 ratio, for not only can a given amount of energy carry more fine ma- 

 terial than coarse, but a larger proportion of a stream's energy can 

 be utihzed in the transportation of the fine. 



How sediment is carried. — Coarse materials, such as gravel stones, 

 are rolled along the bottoms of the swift streams which carry them. 

 Their movement is effected by the impact of water. The same is true 

 to a large extent of sand grains, especially if the}^ be coarse. So far 

 as concerns the material rolled along the bottom it is to be noted that 

 a stream's transporting power is dependent on the velocity of the 

 water at its bottom. This is much less than the surface, or even the 

 average velocity. The particles of fine sediments, such as silt and 

 mud, are frequently carried by streams quite above their bottoms, 

 as shown by the roihness of many streams. A particle of mud is usually 



