LAUSR

Abstract Road crossings at rivers or streams are known to create barriers to fish movement when they are improperly designed or constructed. Over the next 50–100 years climate change is projected to cause major changes in hydrology and channel morphology across Washington State. If culverts built today cannot accommodate future channel conditions, then climate change could indirectly create barriers to fish movement and consequent loss of fish habitats. To inform managers and engineers about the potential impacts of climate change on fish passage through culverts, we determined changes in bankfull width from projected changes in bankfull discharge. These changes were estimated from an ensemble of 10 statistically downscaled global climate models. The spatial resolution of these regional projections was 1/16° grid cells (≈33 km 2 ), and the spatial extent was Washington State. Projections of bankfull width for two future time periods, 2040s and 2080s, were compared to bankfull width estimated from simulations of historical bankfull discharge. Our projections indicate that in both time periods bankfull discharge and bankfull width will increase in about 80% of Washington State, although the magnitude of change varies by ecoregion. In the Pacific Maritime Mountain Ecoregion Division, for instance, the mean across all grid cells of the mean projected percent change in bankfull width was 12.1% in the 2080s but near zero (0.2%) in the Columbia Basin for the same time period. Because statistically valid ways to express uncertainty associated with climate change projections are currently lacking, our metric of uncertainty was the number of models that agree on the sign of change in bankfull width. In the 2080s, the percentage of grid cells with a majority of models projecting an increase in bankfull width was 69%, and 26% of grid cells had majority of models projecting a decrease. We also present some graphical depictions of our results to help managers and engineers understand climate-related uncertainty and risk in culvert design. We explain how our results can be used in culvert design through a case study.