EFFECTS OF WATER MANAGEMENT ON CHANNEL AND VEGETATION CHANGES ALONG THE MIDDLE GREEN RIVER, UTAH

The reach of the Green River from Jensen to Ouray, Utah has a wide, mobile, and complex channel that provides important habitat for threatened and endangered fish. Flow regulation, drought, and climate change are decreasing flows along the Green River allowing proliferation of riparian vegetation and resulting in channel narrowing and simplification. We analyzed 4 years of annual data on plant species occurrence, plot elevation, and plot inundation duration to relate channel and vegetation change to recent changes in river flow along the reach. From 2020 to 2023 we related flow to vegetation establishment and channel narrowing using annual data at 3 spatial scales: (1) local site surveys of approximately 2,000 1m² plots along 42 transects with elevation, vegetation, and inundation data, (2) high resolution UAS imagery of two 11km sub-reaches, and (3) Worldview satellite imagery of the full 80.5km reach from Jensen to Ouray. Peak flows at Jensen, Utah (USGS Gage 9261000) were 18,300cfs in 2020, 9,830cfs in 2021, 17,000cfs in 2022, and 20,300cfs in 2023; in comparison, the median peak flow from 1992 to 2019 was 19,150cfs. The mean percent cover of vegetation in all plots across our study reach increased strongly from 2020 to 2021 as a result of the lack of scouring flows, and less strongly from 2021-2022 in response to average flows. In the moderately wet year, 2022-2023, the vegetation cover decreased slightly but did not return to the levels observed in 2020. Abundance of the two dominant woody plant species, coyote willow and tamarisk, also increased from 2020-2022 and decreased slightly from 2022-2023. Similarly, active channel areas delineated from drone imagery showed a moderate decrease in channel area from 2020-2022 with only a minor increase from 2022-2023. Elevation change over time was more positive in densely vegetated than in sparsely vegetated plots, demonstrating that vegetation established in dry years is trapping sediment and retarding erosion in subsequent wetter years, leading to floodplain construction and channel simplification. Maintenance of a wide, complex channel requires flow peaks with high magnitude and duration. Any opportunity to increase the magnitude or duration of peak flows will likely slow vegetation establishment, channel narrowing and channel simplification.