Effects of wildfire on streambed sediment in the Cascades and Klamath regions of the Pacific Northwest

Abstract Background Wildfire can influence erosion regimes and alter sediment delivery to river systems, which can change the sediment size composition of rivers. Understanding how wildfire impacts streambed grain size distributions is essential in understanding broader wildfire impacts on aquatic habitat and downstream water resources. In Western Washington, Oregon, and Northern California, the Aquatic and Riparian Effectiveness Monitoring Program (AREMP) has been collecting ongoing monitoring data since 2002. Since the beginning of the monitoring program, approximately 37% of AREMP monitored sites have experienced one or more wildfires within their contributing watersheds. In this study, we used machine learning models to analyze AREMP sites within the Cascades and Klamath regions to assess (1) how the presence and severity of wildfire explain streambed sediment size in western Pacific Northwest (PNW) watersheds, and (2) how watershed characteristics influence the effects of wildfire on streambed sediment across two distinct regions in the PNW. Results We found distinct patterns in how fire may be influencing instream sediment in the Cascades province compared to the Klamath province, which could be explained by differences in fire regime and land management legacies between the two regions. We also found that how streams respond to wildfire in the Cascades could therefore be controlled by the factors that influence hillslope sediment production and sediment transport dynamics. Conclusions Our findings offer insight into which landscape controls influence whether aquatic habitat might be vulnerable to or resilient to change post-fire, across different regions in the PNW.