EFFECTS OF WATER MANAGEMENT ON CHANNEL AND VEGETATION CHANGES ALONG THE GREEN AND YAMPA RIVERS, UTAH

The ongoing drought in the Colorado River system is decreasing river flows, with cascading effects on channel geometry, riparian vegetation, and habitat for endangered fish. Discussions of future changes to flow releases from Flaming Gorge Dam on the Green River require a quantitative understanding of these effects. We are monitoring the effects of flow conditions on channel change and riparian vegetation by analyzing data from permanent 1-m² plots (established by 2016) at 22 sites along the Green and Yampa rivers in Utah and Colorado. We are combining up to 7 years of annual data on plant species occurrence, plot elevation, and time underwater to relate the history of channel and vegetation change to recent changes in river flow at 4 of our sentinel sites. These sites encompass changes that may be seen on the Green River from below Flaming Gorge Dam (Browns Park), the unregulated Yampa River (Deerlodge Park), changes just below the confluence of the Green and Yampa (Echo Park), and changes further downstream near the Ouray National Wildlife Refuge (Refuge Camp). We are also using drone and satellite imagery to scale up from plots to river reaches to assess how future changes in flow related to climate change and dam management might affect channel dimensions, vegetation, and fish habitat. This work requires integration of the disciplines of hydrology, fluvial geomorphology and plant and animal ecology. Availability of annually resolved data at many sites and multiple scales allows investigation of complex changes across the river network. Recent decreases in peak flows and base flows resulting from the ongoing drought have promoted vegetation encroachment and sediment deposition on sand bars above the inundation zone of summer base flows. For example, at Refuge Camp vegetation encroachment is occurring on sand bars that are inundated by flows between 5000-7000 ft³/s. We are quantifying the conditions where this process occurs and testing the hypothesis that this will lead to channel narrowing and simplification.