Spatial and temporal variations in bed load transport intensity in a gravel bed river bend

Variability in bed load sediment transport is investigated for a bend of the Colorado River in Rocky Mountain National Park. Data from 190 bed load samples taken at 50 to 125% bankfull flow at a large number of locations throughout the reach indicate that the fine and coarse fractions of the load are differentially routed, such that fine grains are swept inward over the point bar, while coarse grains are routed outward toward the pool. At bankfull flow, all size fractions of the available sediment are transported throughout the bend, but the median grain size of the bed load increases with distance toward the outer bank. This differential routing decreases slightly with increasing discharge beyond bankfull, as indicated by a shift in the locus of coarse particle transport from the outside of the bend inward to the channel center. Furthermore, while fine sediments are fully mobile throughout the bend, partial mobility of the coarse fractions of the load transitions to full mobility toward the bend's outer region. These patterns are flow dependent; with increasing discharge the distribution of excess Shields stress becomes more uniform, resulting in reduced variability in bed load transport rates across the channel. These variations in transport intensity lead to a volumetric balance of transported sediment between the inner and outer regions of the channel, which thereby promotes the geomorphic stability of the bend.