Landscape Impacts

How do environmental and hydrologic variables, such as stream order, slope, drainage density, and land cover, influence summer stream temperature variability in the Tualatin River Basin, and what are the implications for thermal restoration efforts?

• Rationale: This question seeks to understand the relationship between stream morphology and drainage characteristics (e.g., slope and drainage density) and their effects on stream temperature variability. The answer can guide effective thermal restoration strategies tailored to specific stream segments based on their order and sensitivity to environmental influences.

This part of the project aimed to understand how different environmental and hydrologic variables influence summer stream temperatures across the basin, utilizing GIS and multivariate analysis. By analyzing stream temperature data alongside factors such as stream order, slope, drainage density, and land cover, our findings reveal that stream temperature variability aligns most closely with changes in stream slope and upstream drainage density. Specifically, as slope increases, stream temperatures tend to decrease, and as drainage density increases, temperatures tend to rise. Our analysis further shows that temperature variability diminishes with increasing stream order, meaning larger streams exhibit more stable temperatures, while smaller, headwater streams are more sensitive to various environmental influences. These insights emphasize the role of stream morphology and drainage characteristics in shaping thermal regimes, which is critical for guiding effective thermal restoration efforts within the basin.

Using non-metric multidimensional scaling (NMDS), we identified clusters of temperature variability that correspond with stream order. For example, large-order streams (mainstems) primarily varied by flow characteristics and channel width, showing minimal temperature variability, while mid- and small-order streams exhibited greater diversity in temperature regimes due to factors such as air temperature, slope, and forest cover. The limited clustering of sites based on temperature within the NMDS plots suggests that additional samples or specific variables (e.g., localized air temperature data or more precise land-use classifications) may help to further clarify temperature patterns in the basin. Overall, the findings provide actionable insights for watershed managers and highlight the importance of tailoring restoration efforts to the specific thermal needs of stream segments based on their order and morphology. Future steps could include more extensive sampling and spatially focused analyses to support predictive modeling and resource allocation for thermal restoration in similar urbanized basins. Thank you for your support of this valuable project.