56 PHYSIOGRAPHY 



for particles of such material are easily loosened and moved along 

 in the current. The sediment moved by a stream, whether in 

 suspension or at its bottom, is load. 



A stream is not a single, straightforward current. When water 

 runs through an open ditch or gutter, some of it may be seen to move 

 from sides to center, and some from center to sides, while eddies 

 are common. These lesser currents in the main current are espe- 

 cially distinct where the stream is swift. A swift river, too, " boils" 

 and eddies in a striking manner. In the swift Columbia, for ex- 

 ample, eddies are often so strong that it is difficult to row through 

 them. In such a current, objects are often " sucked under" and 



Fig. 44. Diagram to illustrate the effect of irregularities, a and 6, in a stream's 

 bed, on the current striking them. 



brought up again. There are similar movements, though less 

 readily seen, in slower streams. 



All these phenomena show that there are numerous subordinate 

 currents in the main current of a river, and that they move in 

 various directions. Many of them are caused by the unevenness 

 of the bed of the stream (Fig. 44). The subordinate upward cur- 

 rents frequently carry sediment up from the bottom of the stream, 

 bringing it into suspension. 



It might seem that swift streams should always be muddy and 

 slow ones always clear, for the minor currents are much stronger in 

 swift streams than in slow ones; yet many swift streams, especially 

 in the mountains, are remarkably clear. This is the case (1) if 

 immediate run-off and tributaries bring the stream no sediment, 

 and (2) if the materials of its bed are so coarse that it cannot pick 

 them up. The clearness of many swift mountain streams is due to 

 the fact that there is no fine material of any sort in their beds or 

 banks, while the muddiness of some sluggish streams, such as the 

 Lower Missouri and the Platte, is due, at least in part, to the fact 



