THE INFLUENCE OF BASEFLOW ON STREAM TEMPERATURES IN AN URBANIZED STREAM

Boone Creek is a mountainous headwater stream that lies within an urbanized environment in northwestern North Carolina. The primary source of thermal pollution in Boone Creek is the urban infrastructure, which affects stream temperatures through (1) heated runoff, which creates temperature surges, (2) barriers that limit fish migration, and (3) the elimination of groundwater-surface-water interactions. In this study, we use a Monte Carlo thermal mixing model to predict the thermal impact of removing a 700-meter-long culvert. Our thermal mixing model balances stream discharge and temperatures with surface heat exchange parameters and restored baseflow. We calculate the daily-average groundwater discharge velocity at 15 locations in the stream using signal decay in streambed temperatures, and utilize a Monte Carlo implementation of the heterogeneous baseflow field in the thermal mixing model. We also calculate surface heat exchange per unit area for conditions upstream and downstream of the existing culvert and utilize those values in the thermal mixing model. Our modeled temperatures suggest a decrease in summer stream temperatures downstream of the culvert that average 1.35 °C and 1.17 °C for upstream and downstream surface heat exchange conditions, respectively. Stream temperatures are a vital habitat component for many freshwater fauna. Boone Creek currently experiences daily-average summer temperatures that are at the stress threshold for trout, which are indigenous to the stream. The results of our study have implications for the balance between baseflow and the urban infrastructure in any high-gradient urban stream that experiences similar thermal effects.