Investigating Water Temperature Dynamics and Hydrologic Interactions in an Urban Stream-Lake System near Portland OR

This study investigated the factors controlling stream temperature and thermal stratification in Rock Creek and Bethany Lake, an urbanized watershed featuring an in-line irrigation pond and buried wastewater infrastructure. Previous research suggested that subsurface water flow facilitated by buried infrastructure (urban karst) might drive temperature shifts in Rock Creek during summer periods when flow from Bethany Lake ceases. From 2023 to 2024, continuous temperature and stage monitoring, combined with statistical methods, were used to evaluate seasonal temperature dynamics and their controlling factors. Results confirmed that seasonal reversals in temperature trends occurred within Rock Creek, though these may have been strongly influenced by monitoring techniques and thermal stratification. During summer periods, Bethany Lake did not flow into Rock Creek, and streamflow within Rock Creek was negligible. Solar irradiance emerged as the most influential parameter for stream warming during these periods. Unexpectedly, lake elevation had a measurable influence on Upper Rock Creek stage, suggesting hydrologic interactions that are not yet fully understood. Piezometer data indicated that variability in subsurface water elevation was correlated with fluctuations in Bethany Lake elevation, but only within the middle of the study reach. Despite this, there was no measurable fluctuation of stream temperature in relation to subsurface water elevation. This study underscores the importance of understanding interactions among anthropogenic modifications, hydrologic connectivity, and environmental controls on stream temperature, with implications for urban stream restoration and management.