Watershed-Scale Stream Restoration Planning and Sediment Budgeting With A Modified Sediment Impact Assessment Methodology

Salmon recovery initiatives in the Pacific Northwest increasingly emphasize process-based restoration approaches implemented at a watershed scale. To date, effective implementation of these approaches has been limited by our understanding of the watershed-scale sediment processes to inform the planning and project implementation of salmon habitat restoration. In this study, we developed a planning-level sediment budget to evaluate sediment sources, transport, and storage by modifying and applying the Sediment Impact Assessment Methodology (SIAM) framework previously developed by Wallerstein et al. (2006). We applied the approach along a 143-mile stream network draining the 525 mi2 Upper Grande Ronde River watershed in northeastern Oregon. This modeling-based framework, built on one-dimensional HEC-RAS hydraulic models, allows for direct assessment of restoration strategies and designs on reach to watershed scales. The approach also proves a framework to help answer key questions about recovery timescales with and without restoration, the longitudinal connections (upstream and downstream effects) between these projects, and where and how to maximize the benefits of generally limited coarse (bed material) sediments in the watershed. This paper will discuss the improvements made to the original SIAM framework using an add-on developed with Python programming language that address limitations around bed erosion and bed material sediment transfer. We will also present the key assumptions and application of the model for restoration planning, including highlights about our approach that increased the cost-effectiveness of the improved model framework for watershed-scale applications.