Salmon Passage Through Culverts: Considerations for Design and Evaluation

Culverts can block salmon in many ways, primarily during upstream migratory movement and prevent salmon from accessing upstream habitats. Culverts can also impede the natural processes of the stream. Survival of juvenile salmon may depend on access through culverts to utilize potential rearing areas, and escape displacement from their territories during high flows.  Culverts can be built to achieve passable conditions for both juvenile and adult salmon if they are adequately sized to pass all individuals and mimic conditions found in the adjacent natural stream.  We compared data from culverts and the adjacent natural stream and found that culverts with widths large enough to maintain shallow, slow-velocity channel margin areas during a large range of flows offered more consistent fish passage conditions as similar to their adjacent natural stream counterparts. Margin areas are often lumped into cross-sectional averages by most hydraulic modeling techniques and thus not captured as a necessary feature of fish passage with culvert design. Maintenance and replication of these features will provide better certainty for salmon passage as well as help maintain natural stream processes. Restoring salmon access to their historic habitat range is perhaps one of the most beneficial and effective restoration tools for salmon production.