Sediment transport and channel form in gravel-bed river meanders

Spatial and temporal variations in sediment transport in gravel-bed streams are investigated herein for three stream bends of the upper Colorado River in Rocky Mountain National Park. High-resolution field data collected over three years is used to characterize how gravel-bed rivers transport mixed sediment sizes through bends with differing radii of curvature. More than 60 co-located bed load, surface, and subsurface sediment samples were taken from the bends. The median grain size, D50, of the surface and subsurface sediments increases with distance towards the channel's outer bank. As bend curvature increases, there is a corresponding increase in the over-representation of coarse grains on the channel bed relative to their abundance in the substrate, leading to an increase in local armoring in the pool of sharp bends. The distribution of fluid forces through the bends was approximated with a two-dimensional flow model. With increasing flow, the zone of maximum Shields stress shifts inwards and narrows, and the area of the channel experiencing excess stress increases. Spatial variations in the distribution of Shields stress lead to the differential routing and deposition of fine and coarse fractions of the bed load. Fine grains are swept inward by secondary currents, while coarse grains are routed outwards by the lateral topography of the point bar. This pattern decreases with increasing discharge, indicated by a shift in the locus of coarse particle transport from the outside of the bend inward to the channel center at discharges greater than bankfull. Moreover, the size of various grain size percentiles of the bed load increases with increasing shear stress. While fine sediments are fully mobile throughout the bends, partial mobility of the coarse fractions of the bed load transitions to full mobility towards the outer region of the bend. These variations in transport intensity promote the equivalent amounts of bed load transport through stream reaches with contrasting morphologies. Finally, results suggest that meandering gravel channels may adjust to provide temporally-constant reach-average Shields stress.