THE LIFE HISTORIES OF RIVERS 



163 



The cycles of stream meanders. The annual flooding with 

 water and simultaneous deposition of silt is not, however, the 

 only grading process which is in operation upon the flood plain. 

 It is characteristic of swift currents that their course is main- 

 tained in relatively straight lines because of the inertia of the 

 rapidly moving water. In proportion as their currents become 

 sluggish, rivers are turned aside by the smallest of obstructions; 

 and once diverted from their straight course, a law of nature 

 becomes operative which increases the curvature of the stream 

 at an accelerated rate up to a critical point, when by a change, 

 sudden and catastrophic, a new and direct course is taken, to be 

 in its turn carried through a similar cycle of changes. This 

 so-called meandering of a stream is accompanied by a transfer of 

 sediment from one bend or meander of the river to those below 

 and from one bank to the other. Inasmuch as the later meanders 

 cross the earlier ones and in time occupy all portions of the plain 

 to the same average extent, a process of rough grading is accom- 

 plished to which the annual overflow deposit is supplementary. 



The course of the current in consecutive meanders and the 

 cross sections of the channel which result directly from the mean- 

 dering process will be made clear from examination of Fig. 168. 

 So soon as diverted from its direct course, the current, by its 

 inertia of motion, is 

 thrown against the 

 outer or convex side 

 so as to scour or 

 corrade that bank. 

 Upon the concave 

 or inner side of the 

 curve there is in con- 

 sequence an area of 

 slack water, and here 



the silt scoured from higher meanders is deposited. The scouring 

 of the current upon the outer bank and the filling upon the inner 

 thus gives to the cross section of the stream a generally unsym- 

 metrical character (Fig. 168 a&). Between meanders near the 

 point of inflection of the curve, and there only, the current is cen- 

 tered in the middle of the channel and the cross section is sym- 

 metrical (Fig. 168 cd). 



FIG. 168. Map and sections of a stream meander. 

 The course of the main current is indicated by the 

 dashed line. 



