The Relationship Between Atmospheric Heat Waves and Stream Temperatures in Streams of the Oregon Coast Range

Studies of streams across the Pacific Northwest have reported diverse stream temperature responses to increasing air temperatures. However, there is a knowledge gap in how short‐term extreme events, such as atmospheric heat waves (AHWs), impact stream temperatures, particularly for headwater streams that are important habitats for endangered salmon. This study investigates the response of four headwater streams in the Oregon Coast Range to AHWs. The average number of AHWs per year for the study period (2012–2022) at each site varied between 5 and 6. Four years (2014, 2015, 2016, and 2021) had more AHWs than average. Based on seasonal averages, 18 AHWs occurred in spring, 11 in summer, and 14 in both fall and winter. Average daily air temperature (mean for all sites by season) during AHWs was 14.8°C in fall, 19.4°C in spring, 22.4°C in summer, and 12.0°C in winter. Stream temperatures were significantly elevated (above 95% exceedance for the DOY) at all sites in all seasons during AHWs. However, stream temperatures in the post‐AHW period (defined as the period 8–14 days after an AHW event) were not significantly elevated except in the fall (October–December), when stream temperatures remained significantly elevated at all sites. Thermal sensitivity, defined as the slope of the linear regression between stream temperature and air temperature, was used to assess the relationship between stream temperature and air temperature. Across sites, thermal sensitivity ranged from 0.34°C/°C to 0.69°C/°C in spring, 0.34°C/°C to 0.59°C/°C in summer, 0.36°C/°C to 0.50°C/°C in fall, and 0.21°C/°C to 0.34°C/°C in winter. In all seasons, AHWs are associated with significant increases in stream temperature and may result in persistent warming during periods of low flow, with potential impacts to aquatic ecosystems.