118 GEOLOGY, 



is great. Any roughness will serve the same purpose, and every stream's 

 bed is rough to a greater or less extent. Where there are roughnesses 

 at the sides of a channel, currents are started which flow from them 

 toward the center. The varying velocities of the different parts of a 

 stream serve a similar purpose. The curves in a river tend to give 

 the water a rotatory movement. A river is therefore to be looked upon 

 not as a single straightforward current, but as a multitude of currents, 

 some rising from the bottom toward the top, some descending from 

 top to bottom, some diverging from the center toward the sides, and 

 some converging from the sides toward the center. The existence of 

 these subordinate currents is often evident from the boihng and eddying 

 readily seen in man}^ streams. It is, of course, true that the sum of 

 the upward currents is always less than the sum of the downward, so 



Fig. 101. — Diagram to illustrate the effect of bowlders, a and h, in a stream's bed 

 on the currents of water impinging against them. 



that the aggregate motion of the water is down slope; but it is also 

 true that minor upward currents are common. Sediment in suspension 

 is held up chiefly by such currents, which, locally and temporarily, 

 overcome the effect of gravity. The particles in suspension are con- 

 stantly tending to fall, and frequently faUing; but before they reach 

 the bottom many of them are seized and carried upward by the subor- 

 dinate currents, only to sink and be carried up again. Even if they 

 reach the bottom, as they frequently do, they may be picked up again. 

 It is probable that every particle of sediment of such size that it would 

 sink readily in still water is dropped and picked up many times in the 

 course of any long river journey, and its periods of rest often exceed its 

 periods of movement. 



Independently of the subordinate currents, the different veloci- 

 ties of the different parts of a stream tend to keep materials in sus- 

 pension by exerting different pressures on the different sides of sus- 

 pended particles.^ 



River ice sometiir^s facihtates the transportation of debris which 

 the water alone could uot carry. The ice freezes to bowlders in the 

 ^ Russell. Rivers of North America, p. 17. 



