Thermal Regimes of Beaver Impacted Stream Reaches in the Tualatin River Basin

Climate change and urbanization are expected to cause major changes to hydrologic regimes over the Pacific Northwest Region. The Tualatin River Basin, an area of the Pacific Northwest just west of Portland, OR has been subjected to considerable water-quality issues, including stream temperature. As ecosystem engineers, beavers have the potential to mitigate stream degradation and, consequently, there is increasing interest in beaver-related restoration. However, different conclusions have been found on beaver dam impacts to stream thermal regimes. These differences have been attributed to geographical differences in study location and variability in methods and investigated spatiotemporal scales. We investigated summer thermal regimes of three beaver impacted stream reaches of tributaries to the Tualatin River using fiber-optic distributed temperature sensing techniques. Our study approach allowed us to compare thermal regimes across multiple spatial scales. We found considerable variability in thermal regimes among beaver impacted reaches that were influenced by air temperatures, stream depths, surrounding vegetation, and location along study reaches. Thermal sensitivity of the three impacted stream reaches was significantly different (p <0.001). Daily maximum stream temperatures and daily stream temperature range were significantly different (p <0.05) depending on location within stream reach and proximity to beaver dams. Daily maximum stream temperatures were higher in areas of the study reach that had shallow stream depths, and lower in areas that had deeper stream depths. Comparatively, daily maximum temperatures were greater in both areas that had high and low percent canopy cover, indicating stream depth may be more important for influencing daily maximum stream temperatures. Additionally, temperature differences surrounding beaver dams were, on average, cooler downstream when stream depths were greater downstream compared to upstream of beaver dams and warmer, on average, when stream depths were shallower downstream compared to upstream of beaver dams. Our results indicate thermal regimes in beaver impacted reaches are influenced by local factors and vary across spatial scales. Fully understanding the role beaver dams play in influencing stream temperatures requires multi-scale study approaches.