SEDIMENT SORTING IN MEANDERING GRAVEL BED RIVERS AND IMPLICATIONS FOR THE EVOLUTION OF REACH TOPOGRAPHY

Hydraulic, grain scale sorting of mixed bed sediment influences the mobility of grains in discrete areas of river channels; this effect is well-documented for straight river reaches. To assess the importance of sediment sorting in curved channels, local values of surface grain size sorting were compared with measurements of bed load at corresponding locations in a bend of the Colorado River in Rocky Mountain National Park (RMNP), and the distribution of local Shields stress through the reach was derived from a two-dimensional flow model. With decreasing degrees of local sorting, the relative mobility of the fine- and coarse-size fractions of the load appeared to decrease and increase, respectively. Furthermore, back-calculated critical Shields stress values for sediment entrainment decreased with values of local sorting, particularly for the upstream portion of the reach where particles were more poorly sorted and coarse grains had higher relative exposure. To evaluate the pervasiveness of these and other patterns of sorting in gravel rivers, detailed field measurements of channel topography and surface grain size (317 pebble counts, 15,850 total grains) were obtained for seven additional reaches of differing curvature (radius of curvature/width from 1 to 28) near the headwaters of the Colorado and Fall rivers in RMNP. Moderately curved and tight bends (radius of curvature/width less than or equal to 7) were significantly better sorted than comparatively straight reaches. Values of local sorting decreased with distance downstream for the majority of curved channels, reflecting a reduction in the standard deviation of surface grain sizes toward the lower end of the reach; this effect increased slightly with bend sharpness. Reach-scale variations in particle mobility (from the sorting effects outlined above) may influence the development of channel topography by generating areas of enhanced particle entrainment (for smaller-size grains) and inhibited regions (for coarse grains). These features have been observed in downstream portions of bends for the Colorado River and may contribute to the generation of persistent irregularities in reach topography, such as scour holes past the bend apex, channel widening at the bend exit, and local breaks in curvature.