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Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study
Citation:
Beyene, M. AND S. Leibowitz. Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study. Journal of Hydrology X. Elsevier B.V., Amsterdam, Netherlands, 23(1):100173, (2024). https://doi.org/10.1016/j.hydroa.2024.100173
Impact/Purpose:
Over the past century, large, high severity wildfires have become more pervasive and the water temperature of many headwater streams in the Pacific Northwest (PNW) have steadily risen. This warming trend poses a threat to thermally sensitive and economically important cold-water fishes like trout and salmon in the PNW. The objective of this study was to assess initial effects of riparian fires on summer thermal response of streams in the PNW. Specifically, we address: (a) how do wildfires in the PNW affect summer water temperatures across streams and daily water temperature indices? And (b) what are the factors that influence the thermal response of streams to wildfires in the PNW? Our study found that, overall, PNW wildfires corresponded to warmer daily summer water temperatures at a regional scale. However, the effect of wildfire-related water temperature changes on coldwater fishes in the western US is still unclear because (a) the opportunistic nature of most studies severely limits their implications both temporally and spatially, (b) findings vary widely across locations, and (c) wildfires can impact salmonid habitats through multiple pathways. Our study should be of interest to EPA regional staff and state aquatic managers in areas impacted by wildfire as well as scientists research wildfire and water quality.
Description:
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.
URLs/Downloads:
DOI: Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study