Abstract

Meander bends in the lower Wabash River of Illinois and Indiana display coherent patterns of current velocity, bed topography, lower-regime bed forms, and bed-material size. The upstream reach of a bend is a zone of transition from the reversed hydraulic and sedimentologic conditions of the preceding bend. A downstream shift of the cross-sectional maximum velocity magnitude from the inner (point-bar side) bank to the outer bank defines a transitional zone for velocity magnitude in each bend; in this zone, current velocities are strongest and dunes and sand waves most prominently developed at near-bankfull and higher flows. Similar translations of the cross-sectional maxima of depth, mean size of bed material, and dune height and a reversal in the orientation of spiral flow define a transitional zone for each of these four other parameters in each bend. A given meander bend at any stream discharge shows the following sequence of increasing length of transition zones: spiral flow, depth, velocity magnitude, mean size of bed material, and dune height.

Downstream from each transitional zone in a bend, the normal asymmetrical cross-sectional distribution of each parameter defines a fully developed zone for that parameter. Only in the fully developed zone does each parameter increase from the inner bank to the outer bank, as implied in the standard facies models for meandering streams. In both sharply curved and gently curved bends, fully developed zones for bed-material size do not exist. Sharply curved bends do not contain fully developed zones for velocity magnitude, which in other bends show the strongest velocities and largest dunes at near-bankfull or lower flows, especially if the zone is bypassed by overland flow during higher stream discharges.

Available data from laboratory channels and from other meandering streams suggest that the above patterns of velocity magnitude, spiral motion, and bed-material size characterize freely meandering streams. The transitional zones of sedimentary and hydraulic parameters and the variations of these parameters through different flows and dissimilar bend curvatures introduce significant complications to present depositional models of meandering streams.

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