Climate change influences the footprint of anthropogenic habitat alteration: Implications for endangered species and regulatory reviews of riverbank stabilization projects

Background/Question/Methods

Our improved ability to model climate allows us to better estimate how climate change will alter freshwater habitat quality and availability. However, significant challenges remain, particularly in combining the effects of climate change and human habitat alteration. We examined these impacts on shallow water beach habitat (SWH) in the lower Willamette River, Oregon (U.S.A.). A proposed project to stabilize ~450 m (1500 ft) of shoreline with riprap would alter SWH and is subject to Endangered Species Act (ESA) Section 7 consultation. The lower river has been highly altered by urban and industrial development. Mainstem beach SWH is important to subyearling ESA-listed Upper Willamette River Chinook salmon (Oncorhynchus tshawytscha), and the amount of SWH fluctuates with river stage levels. Historically, the lower river was connected to its floodplain and river stages in the spring often significantly exceeded modern flood stages, providing additional but now lost opportunities for SWH formation. Using a geo-spatial representation of the present-day lower Willamette River channel system we explored how recent river stages, and future river stages driven by climate change effects, combined with the riprap project’s impacts altered SWH during the peak subyearling occupancy (April-June) period.

Results/Conclusions

Recent observations of river stages were near levels that maximized April-June SWH in the project reach area from the mouth to approximately river km 12 (8 mi); however, SWH was maximized at lower river stages upstream of the project reach. SWH decreased in the 2080s as a consequence of projected increases in April-June Columbia River flows. By the 2080s, this small riprap project could preclude >1.5% of SWH that could be beach in the area from the mouth to river km 12. The project’s footprint, estimated for 2080s flows, contained >15% beach SWH which would be displaced by riprap. SWH reductions restrict subyearling feeding and growth opportunities, resting and refuge from predators, and thereby inhibit expression of Chinook salmon diversity which limits hedging of survival bets across multiple life history strategies and has implications for species viability. While climate change analysis was informative for this project review, detailed analyses are prohibitive for the many small projects reviewed annually. Studies like this provide a wider context for reviewing small projects, incorporate potential interactions with climate, create a framework to analyze cumulative effects of similar actions, facilitate alignment of the duration of the actions’ effects with the time horizons in consultations, and allow for inclusion of long-term climate forecasts.