A Process-Based Assessment for Watershed Restoration Planning, Chehalis River Basin, USA

Three key questions in identifying and prioritizing river restoration are: (1) How have habitats changed?, (2) What are the causes of those habitat changes?, and (3) How of those changes affected the species of interest? To answer these questions and assist aquatic habitat restoration planning in the Chehalis River basin, USA, we quantified habitat changes across the river network from headwaters to the estuary. We estimated historical habitat capacity to support salmonids using a combination of historical assessments, reference sites, and models. We also estimated current capacity from recent or newly created data sets. We found that losses of floodplain habitats and beaver ponds were substantial, while the estuary was less modified. Both tributary and main channel habitats—while modified—did not show particularly large habitat changes. Assessments of key processes that form and sustain habitats indicate that riparian functions (shading and wood recruitment) have been significantly altered, although peak and low flows have also been altered in some locations. The next step is to link our habitat assessments to salmon life-cycle models to evaluate which life stages and habitat types currently constrain population sizes of spring and fall Chinook salmon, coho salmon, and steelhead. By comparing model runs that represent different components of habitat losses identified in the analysis above, life-cycle models help identify which habitat losses have most impacted each species and population. This assessment will indicate which habitat types provide the greatest restoration potential, and help define a guiding vision for restoration efforts. Future analyses may include development and evaluation of alternative restoration scenarios, including different climate change scenarios, to refine our understanding of which restoration actions provide the greatest benefit to a salmon population.