Stage-dependent cross-section adjustments in a meandering reach of Fall River, Colorado

Abstract

Dam failure increased the bedload supply to Fall River, a sinuous, low gradient, snowmelt-fed stream, by a factor of about 1000. Because this sediment is mobile at nearly all discharges, channel topographys is able to change with stage. In a two-bend study reach with erosion-resistant banks, the relationships between flow and mobile bed material were studied by measuring bedload transport rates, bed and water surface topography, and longitudinal and transverse velocity patterns at 22 cross-sections. Measurements were made at different discharge levels through the 1986 hydrograph. At bankfull discharge, bend cross-sections showed maximum asymmetry. Point-bar platforms were built up to the water surface, and thalwegs were excavated to their greatest depth. Transverse flows included zones of high turbulence that contained bed- and bank-cells in the thalweg regions. Helical flow occurred over the point-bar slope, and outward-only flow occurred over the point-bar platforms. Bedload transport was greatest over the point-bar slope and almost zero through the thalweg. Bedload sorting was continuous through each bend. At intermediate flows, cross-section symmetry increased due to lateral erosion of the point-bar platform. Helical flows occupied a larger portion of individual cross-sections, including the thalweg. In planform, however, the zone of helical flow split into two smaller cells in each bend. The zone of maximum bedload transport had shifted outward from its high flow position due to point-bar progradation towards the concave bank, so that some sediment movement occurred in the thalweg. At low flows, characteristic of autumn and winter, cross-section symmetry was at a maximum. There was additional erosion of the point-bars, but the greatest change was due to thalweg filling. Transverse flows were weak and chaotic for the most part, and bedload transport was more uniformly distributed across the channel than at higher discharges. Sorting was discontinuous and confined to small pools in the otherwise flat-bed channel. This stage control of bed topography due to shifting velocity and bedload transport patterns is of significance to river engineering and management.