Heterogeneity in post-fire thermal responses across Pacific Northwest streams: A multi-site study

Area of Science:

• Environmental Science
• Ecology
• Hydrology

Background:

• Pacific Northwest (PNW) stream temperatures are rising, threatening endangered salmonid populations.
• Wildfires exacerbate stream warming by removing forest shade, but post-fire thermal impacts are not well understood across diverse streams.

Purpose of the Study:

• To assess the impact of wildfires on daily summer water temperatures across 31 PNW stream sites with varying riparian burn extents (10-100%).
• To identify factors driving the heterogeneity in post-fire thermal responses in PNW streams.

Main Methods:

• Analyzed daily summer water temperatures across 31 PNW stream sites impacted by wildfires.
• Quantified fire effects using multiple approaches, considering riparian burn severity and watershed characteristics.

Main Results:

• Wildfires increased average daily summer water temperatures by 0.3-1°C for three years post-fire across the studied sites.
• Post-fire warming was more pronounced in streams with higher riparian burn severity.
• Watershed features (basin area, post-fire weather, bedrock permeability, pre-fire forest cover, snowpack depth) predicted thermal responses.

Conclusions:

• Wildfires significantly alter PNW stream temperatures, with impacts varying based on burn severity and landscape characteristics.
• Findings provide a multi-site perspective crucial for freshwater management and conservation of salmonid habitats in the PNW.