Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study
Over the past century, water temperatures of many headwater streams in the Pacific Northwest (PNW) have steadily risen, shrinking endangered salmonid habitats. Warming of stream reaches in the PNW can further accelerate due to wildfires burning forest stands that provide stream shade. However, past findings on wildfire effects on stream water temperatures have often been based on individual streams or burn events, limiting our understanding of post-fire stream water temperature responses at broader spatial scales. To address this knowledge gap, we conducted a study that examined the initial impact of wildfire burns on daily summer water temperatures of 31 downstream sites across the PNW. Also, to ensure robustness of our results, we employed multiple approaches to evaluate fire effects on the post-fire stream water temperature changes at local and regional scales. Results indicated that overall wildfires in the PNW corresponded to the warming of daily summer stream water temperatures for the first three years at a regional scale. Nevertheless, post-fire summer water temperature responses were highly variable across individual sites and daily water temperature indices. The likelihood of a post-fire summer water temperature warming increased with larger wildfire burns in watershed and riparian areas. Moreover, watershed elevation, hydrogeology, post-fire weather effects and land cover were correlated with the wildfire effects on summer water temperatures across our sites. Finally, post-fire summer thermal responses were higher for daily mean and maximum water temperatures than daily minimum water temperatures. Our study provides a regional perspective on the impact of wildfires on stream water temperatures in the PNW, which can aid in freshwater management efforts beyond individual streams and watersheds.