Abstract

Observations are described on four natural, gravel-bedded, meandering rivets at two scales: that of individual bends and that of relatively long reaches. A distinction is made between bends that are overwidened relative to intervening channel traverses and bends that are found in constant-width channels. Imbrication directions on bars in the former show no evidence of bed flow aligned toward the inner bank; bar gravel seems to accrete there because of general flow divergence. Even in constant-width channels, imbrication patterns in the main part of the bends indicate bed flow toward the outer bank, causing intense flow convergence and scour along that zone. This scour pattern has implications at the reach level; localized scour along the outer bank produces a gravel bar just downstream that, then, diverts flow against the upvalley bank. The result is a premature inflection (a reversal of normal meander asymmetry) and a frequent tendency for upvalley channel migration. Avulsion is common, eventually arresting such migration and acting to keep over-all sinuosity relatively low. The similarity between the behavior of these rivers, at the reach level, and late-stage pseudomeandering observed in flumes is noted.

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