THE CONTROL OF BASEFLOW ON STREAM TEMPERATURES IN AN URBANIZED, HIGH-GRADIENT STREAM

Boone Creek is a high-gradient headwaters stream that lies within an urbanized environment in northwestern North Carolina. Parameters including discharge, turbidity, electrical conductivity, and temperature have been monitored at ten locations within a 1.5 kilometer reach of the stream for over one year. The data, which were collected at fifteen minute intervals, suggest that stream temperature is the factor most influenced by urbanization.

The temperature data show an expected seasonality as well as increasing temperatures with distance downstream. Noting that the study reach is in an urbanized setting, these data suggest that either (1) baseflow decreases downstream, (2) the influence of urbanization increases downstream, (3) riparian vegetation decreases downstream, or (4) some combination of the above results in the higher stream temperatures observed with distance downstream.

The need to understand the significance of baseflow as a stream temperature regulator led to the installation of two piezometer nests where streambed temperatures could be monitored. Using streambed temperatures as a tracer, we utilized an analytical solution (Hatch et al., 2006, Water Resour. Res., 42, W10410, doi:10.1029/2005WR004787) to quantify baseflow velocities between dataloggers at multiple depths below the streambed. Our calculations suggest that baseflow velocities increase with distance downstream; thus, increasing urbanization with distance downstream is the likeliest cause of the warmer stream temperatures observed downstream.

We created a one dimensional stream temperature model to study the influence of baseflow and solar radiation on stream temperatures. Our model domain is a reach of Boone Creek that includes a 500m piped portion of the stream where neither solar radiation nor baseflow interacts with the stream. Stream temperature data immediately downstream of the piped section reflect temperature oscillations that have been damped due to insulation from external sources. After calibrating our model to existing stream conditions, we utilized the one-dimensional stream temperature model to perform a sensitivity analysis in order to understand the influence of solar radiation and baseflow on stream temperatures under day-lighted conditions.