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 maintained 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 accomplished to which the annual overflow deposit is supplementary.
Fig. 168.—Map and sections of a stream meander. The course of the main current is indicated by the dashed line.
The course of the current in consecutive meanders and the cross sections of the channel which result directly from the meandering 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 consequence 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 unsymmetrical character ([Fig. 168 ab]). Between meanders near the point of inflection of the curve, and there only, the current is centered in the middle of the channel and the cross section is symmetrical ([Fig. 168 cd]).
Fig. 169.—Tree in part undermined upon the outer bank of a meander.
The scour upon the convex side of a meander causes the river to swing ever farther in that direction, and through invasion of the silted flood plain to migrate across it. Trees which lie in its path are undermined and fall outward in the stream with tops directed with the current ([Fig. 169]). Whenever the flood plain is forested, the fallen trees may be so numerous as to lie in ranks along the shore, and at the time of the next flood they are carried downstream to jam in narrow places along the channel and give the erroneous impression that the flood has itself uprooted a section of forest (see [p. 418]).
