Where a bowlder occurs in the bed of a stream (Fig. 101) the water which strikes it is in part forced up over it. If there be many bowlders the process is frequently repeated, and the number of upward currents 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 boiling and eddying readily seen in many streams. It is, of course, true that the sum of the upward currents is always less than the sum of the downward, so 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 constantly tending to fall, and frequently falling; but before they reach the bottom many of them are seized and carried upward by the subordinate 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. [Illustration: +Fig.+ 101.—Diagram to illustrate the effect of bowlders, _a_ and _b_, in a stream’s bed on the currents of water impinging against them.] Independently of the subordinate currents, the different velocities of the different parts of a stream tend to keep materials in suspension by exerting different pressures on the different sides of suspended particles.[46]