Maher, S. and Veldhuisen, C., 2023. Stream Temperature Monitoring in Forested Tributaries of the Skagit River Basin – 15-year Update and Analysis. Skagit River System Cooperative, Burlington, WA. pp. 37.

Maximal summer stream temperatures are an important factor affecting juvenile salmon survival. In 2008, the Skagit River System Cooperative began collecting stream temperature data each summer season in lands managed for timber throughout the Skagit River basin. Using 15 years of data, our objectives for this report are to 1) investigate the effects of the extreme heatwave that occurred in 2021 on stream temperature maxima 2) determine if there is a trend present at the basin and/or site scale, and 3) explore the year-round data at four stations that have received multi-season monitoring since 2018. We used a combination of general linear models and univariate linear regression but did not identify a trend at the basin scale, though two sites (Hobbit and Savage Creeks) have significantly increasing maxima. A review of yearly maximum stream temperature shows that there was not an extreme impact on water temperature during the 2021 heatwave. Most sites’ highest recorded temperatures occurred in years other than 2021, suggesting the important role of snowpack and discharge in determining the impact of extreme heat events on aquatic habitats. We found that winter temperatures at four sites are within the optimum threshold for salmonid incubation and emergence using peer-reviewed optimal temperature ranges and calculations of Accumulated Thermal Units. This report provides data that will help identify summer-rearing stream habitat that would benefit most from protection and/or restoration. We emphasize the need to continue collecting temperature data and expand the spatial network to increase our understanding of temperature regimes throughout the basin.