Equilibrium or indeterminate? Where sediment budgets fail: Sediment mass balance and adjustment of channel form, Green River downstream from Flaming Gorge Dam, Utah and Colorado

These findings demonstrate that sediment budgets that show a balance between inputs and outputs cannot necessarily be interpreted to indicate channel equilibrium. A sediment mass balance for 150-km reach between the dam and the first long-term gage indicates approximate balance of inputs and outputs for the pre- and post-dam periods. When uncertainty in budget components is considered, the mass balance is indeterminate. Although the Green River may have been in approximate equilibrium in the pre-dam period, we have shown that channel width is decreasing in the post-dam period. The post-dam deposits constitute a small but a significant component of the sediment budget upstream from the first major tributary. Sediment is supplied to this reach by small tributaries and, to a lesser extent, erosion of pre-dam alluvium. Downstream from the study area, the volume of the post-dam deposits is tiny relative to the volume of sediment input from the first major tributary.