CHANNEL GEOMETRY ADJUSTMENTS TO A MODIFIED STREAMFLOW / SEDIMENT FEED RATE REGIME IN A SAND-BEDDED REACH OF THE GREEN RIVER IN UTAH

Adjustments of river geometry to flow regulation are not always readily predictable due to the many variables that can adjust concurrently. On a portion of the Green River in Utah, downstream of the Flaming Gorge Dam and a large tributary (Yampa River) that contributes substantial flow and sediment, geomorphic changes in response to a modified discharge and sediment load regime involved modified sandbar dynamics and associated backwater habitat alterations and reduced channel widths. Narrowing occurred without apparent change in the longitudinal profile. Determining the end state (dynamic equilibrium), and whether the end state can be changed through dam operations, requires a thorough understanding of the processes with the ultimate goal of quantitatively modeling system evolution.

The Green River longitudinal profile transitions fairly abruptly from a steep course through the Uinta Mountains (the north perimeter of the Uinta basin) to a low-gradient reach on the plateau comprising the basin interior. The reach of interest is the upper ~100 km on the plateau (~50 km downstream of the Yampa River confluence); the reach contains no substantial tributary inputs of flow or sediment. The sand-sized sediment bypass condition (i.e. under transport capacity) in the steep reach, in conjunction with abundant USGS sand load data at that location, forms a highly useful upstream boundary condition. Previous analyses have attempted to predict the dynamic equilibrium condition by using the effective discharge concept along with an empirical hydraulic geometry relationship. In this analysis, a more mechanistic approach is applied. Simple relationships for hydrodynamics and sediment transport are used in conjunction with specified relationships for sediment load versus discharge applied at the upstream boundary. Channel geometry variable combinations are solved that satisfy water and sediment mass continuity constrained by the specified load-discharge relationship for a full range of flows. Pre-dam condition solutions are compared with solutions representing potential system changes: (a) flow duration curve modification alone; (b) load-discharge relationship modification alone; and (c) combinations of both. The results shed light on observed post-dam channel geometry adjustments.