Oral Presentation Society for Freshwater Science 2025 Annual Meeting

Effects of changing flow regime on riparian plant communities and channel simplification along the Green and Yampa Rivers in Colorado and Utah, USA (116973)

Catherine M. McClure 1 , Jonathan M. Friedman 2 , Elizabeth R. Skaggs 2 , Christopher L. Holmquist-Johnson 2 , Kyle D. Enns 2 , Dustin W. Perkins 3 , Luke Gommermann 3 , Ryan R. Morrison 1 , Catherine McClure 1
  1. Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado, USA
  2. U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colorado, USA
  3. National Park Service, Northern Colorado Plateau Network, Moab, Utah, USA

Flow regulation of major river systems has resulted in changes to channel geometry and riparian vegetation. Along the Green River decreases in peak flows have promoted proliferation of riparian vegetation on the channel bed resulting in sediment deposition, channel narrowing, and channel simplification. To better understand the relationship between changes in flow regime from water management and channel narrowing, we analyzed ten years of riparian plant data coupled with plot inundation-duration and elevation data along 19 sites on the Green River and three sites along the Yampa River. Sampling methods included local site surveys of 1-m2 plots measuring plot elevation, riparian plant percent cover (total cover and species composition), and inundation-duration. The sampling years cover a range in peak flows from 2015 through 2023. For the sites along the middle Green River, we observed significant increases in total percent vegetative cover during the low and average flow years (2020-2021 and 2021-2022), while in the wetter year (2022-2023) we observed a slight decrease in total percent vegetative cover. These results confirm that in low flow years reduced physical disturbance allows encroachment of riparian vegetation. Additionally, we observed an increase in elevation of more densely vegetated plots in comparison with sparsely vegetated plots in wetter years. These results suggest that vegetation establishment in low flow years is subsequently causing entrapment of sediment and reduced erosion in wetter years. Results along the upper Green River demonstrate increased site-by-site variation potentially related to whether they are upstream or downstream of the confluence with the Yampa River. These observed feedback loops between changes in flow regime and riparian vegetation encroachment result in channel narrowing and simplification within this typically complex channel system. Results from this study suggest that flow peaks with high magnitude and duration have the ability to promote wide, complex channels which ultimately improve habitat for threatened and endangered fishes.