Modeling the Effects of Reservoir Releases on the Bed Material Sediment Flux of the Colorado River in western Colorado and eastern Utah

Warm-water reaches of the upper Colorado River have historically provided important habitat for several endangered fish species. Over time, however, these habitats have been lost because of reductions in peak flows and sediment loads caused by reservoirs. In an effort to reverse these trends, controlled reservoir releases are now used to enhance sediment transport and restore channel complexity. In this presentation, we discuss the development a sediment routing model designed to assess how changes in water and sediment supply can affect the mass balance of sediment. The model is formulated for ten reaches of the Colorado River spanning 250 km where values of bankfull discharge, width, and reach-average slope have been measured. Bed surface grain size distributions (GSDs) have also been measured at 78 locations throughout the study area; these distributions are used as a test of the model, not as input, except at the upstream boundary. In modeling transport, we assume that the bed load transport capacity is determined by local hydraulic conditions and bed surface grain sizes. Estimates of the bankfull bed load transport capacity of each reach are computed for 14 size fractions of the surface bed material, and the fractional transport rates are summed to get the total transport capacity. In the adjacent reach, fluxes of each size fraction from upstream are used to determine the mean grain size, and fractional transport capacity of that reach. Calculations proceed downstream and illustrate how linked changes in shear stress and mean grain size affect (1) the total bed load transport capacity, and (2) the size distribution of the bed surface sediment. The results show that model-derived GSDs match measured GSDs very closely, except for two reaches in the lower part of the study area where slope is affected by uplift associated with buoyant salt bodies; here the model significantly overestimates the transport capacity in relation to the supply. Except for these two reaches, the modeled bed load fluxes seem quite reasonable (0.5-1.0 kg/m/s at bankfull flow), and exhibit downstream trends that are consistent with trends reported in two previous studies. Finally, model simulations show that if reservoir releases fall short of target flows (e.g. bankfull) this can have a disproportionate negative effect on the mass balance of sediment.