Reach‐scale stream temperature response to large wood placement

Abstract

Climate change models indicate that stream temperatures within much of the Columbia River Basin will increase significantly in the next 50 years leading to inhospitable conditions for salmonid fishes in several stream reaches. Restoration measures such as riparian planting, maintaining instream flows, and large wood (LW) placement are thought to ameliorate the impacts of climate change on stream temperatures. While LW placement is known to improve habitat complexity, it is unclear whether it will also help reduce summer stream temperatures. To help understand the influence of LW placement on stream temperatures, we monitored nine interior Columbia River Basin LW projects using an extensive posttreatment study design. We mapped stream bottom temperatures, topography, and physical habitat in paired treatment (LW placement) and control reaches to compare water temperatures and cool‐water patches between reaches. We found no difference in the mean or the variance in overall reach temperatures or thalweg temperatures between treatment and control reaches. Mapping found variation in water temperatures of less than 0.5°C within reaches, making mapping of cool‐water patches impractical and further emphasized the lack of temperature differences. Our results indicate that LW placement projects in the interior Columbia River Basin do not appear to be decreasing reach‐scale summer water temperatures or creating cool water refugia for fishes. Moreover, the current configuration and implementation of LW placement projects, which typically focus on placing wood on banks and channel margins, are unlikely to ameliorate the effects of climate change on stream temperatures.