Where the Stream Meets the Road: Prioritizing Culvert Replacement for Fish Passage

Streams across the Pacific Northwest are highly fragmented due to the presence of in-stream barriers (e.g., dams and stream-road crossings), many of which restrict or block fish passage. Retrofitting or replacement of these structures is a high priority for restoring habitat connectivity for native fish species and other aquatic organisms. The task of mitigating problematic stream-road crossings is daunting given the many thousands of barriers that are present and the massive level of investment required. Further, risks to road infrastructure from flooding, debris flows, and climate change will need to be addressed to ensure the best allocation of resources.

Our project takes a holistic, landscape-level approach to culvert replacement by including considerations of fish access, costs, and potential climate change impacts on both habitat suitability and culvert failure risk. Our presentation introduces an optimization model that can be used to help inform culvert replacement decisions. We illustrate our application and explore trade-offs in the Siuslaw National Forest, Oregon, a forest with hundreds of stream-road crossings where river habitat restoration is being actively carried out. A variety of fishes are considered, including salmon, trout, sculpin, and lamprey. We explore how priorities in management objectives might change decisions and on-the-ground actions.