Assessing Geomorphic and Vegetative Responses to Environmental Flows in the Willamette River Basin

Abstract:

On regulated rivers, restoring flow regimes is a process-based restoration approach that may strongly affect downstream ecosystems. Developing realistic flow targets with meaningful geomorphic and ecological benefits, however, is challenging. For instance, hydraulic, geomorphic and biological processes are affected by more than manipulating water release—sediment supply and transport conditions also require consideration. Also, funding and programmatic directives rarely require the monitoring necessary to adaptively manage environmental flow programs. Recent research in the Willamette River basin in support of the Sustainable Rivers Project (SRP) demonstrates how such a monitoring program can be implemented. At the reach scale, initial efforts have assessed geomorphic and vegetative changes in alluvial sections of the Middle Fork Willamette and McKenzie Rivers using repeat mapping from aerial photographs and flow analyses. Overall, both rivers are largely stable because of reduced discharge, bed-material supply and local revetments, but some reaches of the McKenzie River are more dynamic, perhaps reflecting greater inputs of sediment from unregulated tributaries and higher magnitude peak flows. Repeat, reach-scale mapping on the Middle Fork Willamette River shows that frequent bankfull flows are able to scour minimally vegetated gravel bars and sustain a patchwork of actively shifting bed-material sediment. Repeat mapping on the McKenzie River in summer 2015 will reveal insights about the geomorphic effectiveness of bankfull flows. At the site scale, monitoring at two bars in summer 2015 is linking streamflow with the establishment of black cottonwood. Lastly, a review of hydrographs from 2000-2015 and retrospectively applying stakeholder-defined flow targets showed substantial variability in meeting objectives for the timing and types of flows under traditional regulated conditions and the SRP. Altogether, these related efforts help link streamflow, geomorphic processes, and black cottonwood establishment over three decades, and help refine ongoing and future environmental flow releases to achieve the goals of the SRP.