Oxbow lakes are filled in very slowly by deposition of fine sediment during overbank flows on the floodplain. The ends of an abandoned meander loop soon become plugged by fine sediment to form an oxbow lake (Figure 5-40). One of the best kinds of evidence that the bend shifts in position as it grows is the truncation of earlier meander scrolls by the present channel. These curving ridges are called meander scrolls or meander scars. The earlier positions of the top of the point bar are marked on the floodplain on the inner side of the meander bend by low and curving ridges arranged congruently within the meander bend (Figure 5-39). Point bars tend to be built episodically, during floods. Flow-transverse section through a meander bend, showing isovels. Cutoffs of that kind are called chute cutoffs (because the river comes to occupy a chute between the two sides of the bend). Sometimes there’s a less catastrophic variety of meander cutoff, whereby the river occupies an old slough that marks an earlier position of the bend then the amplitude decreases, but not nearly to zero. The river thereby straightens itself spectacularly in quantum jumps to offset the gradual amplification of the meander bends. Such an event is called meander cutoff, or neck cutoff. Eventually, the river jumps across the narrow neck during a flood. Meander bends generally have a tendency to change with time toward larger amplitudes and narrower necks. It’s this secondary circulation that causes the accretion on the point-bar surface. This spiraling secondary circulation is readily explained by the differences in centrifugal force between the high-speed regions of the flow above and the low- speed regions of flow below. There’s also a characteristic helical secondary circulation within the bend, such that the flow near the bed has a component inward toward the point bar, and the flow near the surface has a component outward toward the outer bank. Figure 5-38 is a cartoon of the flow-transverse profile, showing the characteristic pattern of isovels (lines of equal velocity).
GEOLOGY ABANDONED MEANDER FREE
The velocity of flow is highest in the region near the free surface and near the outer bank. The flow is deepest near the foot of the eroding outer bank, and it shoals gradually up the point-bar surface. The flow-transverse profile of a meandering river is highly asymmetrical. The bank is eroded by various processes, the most important of which is undercutting low on the bank and collapse of large masses into the channel. On the outer, or concave, side of the meander bend is a steep bank or cliff, often called a cut bank, of consolidated or semiconsolidated sediment that is eroded during strong flows. Sediment is gradually accreted to the point-bar surface, causing the point bar to shift laterally. This sand body, called a point bar, lies everywhere below bank-full stage and is inundated by floods. On the inner, or convex, side of the meander bend is a fairly smooth and largely non-vegetated body of sand, sloping gently downward toward the center of the channel. Elements of the plan-view features of a meandering river
The narrow part of the meander bend, between adjacent reaches of the channel above and below the bend, is called the meander neck. One can also think about a radius of curvature in the bend, but that’s likely to vary from point to point in the bend. The meander bends or meander loops are never perfectly regular, but they are often described as if they were (Figure 5-37), in terms of wavelength and amplitude.
Figure 5-36 shows most of the elements of a meandering river system. First of all, keep in mind that the tendencies toward meandering and braiding are complementary rather than mutually exclusive, in the sense that many rivers show elements of both at the same time and within the same reach.įirst, some descriptive stuff on the geometry of meandering rivers. The two most characteristic plan patterns assumed by rivers are meandering and braided.
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