DYNAMICS OF EPISODIC TEMPERATURE VARIATIONS IN AN URBANIZED, HIGH-GRADIENT STREAM

The effect of urban environments on stream temperatures is an important problem in assessing the health of headwaters streams. Many fauna, such as trout, depend on the cooler temperatures typical of mountain streams for viable habitats. Urban infrastructure and minimal riparian buffers typical of urban environments may produce elevated stream temperatures that negatively affect many of these cool-water fauna. Stream temperature data are a common and inexpensive method of observing groundwater-surface water interactions along a stream. We collected stream temperature data between May and November 2006 at three gauging sites to monitor the effects of urbanization along trout streams in northwestern North Carolina. We monitored a two km reach of an urban catchment (Boone Creek) and a ten km reach of a rural control catchment (Howard Creek). Boone Creek lies adjacent to downtown Boone and the campus of Appalachian State University. Stream temperature data show (1) typical temperature seasonality, (2) temperature increases with distance downstream, and (3) many episodic temperature variations. The maximum difference in daily-averaged temperatures along Boone Creek approaches five degrees Celsius within the two km reach. The heating of urban environments by solar radiation has a dramatic influence on stream temperatures. Summer convective storms produce runoff that becomes heated as it flows through the urban infrastructure. Headwaters stream temperatures in Boone Creek rose at least one degree Celsius within 15 minutes during 46 separate episodes. These episodes correlate with large increases in stream discharge and were most pronounced at the headwaters gauging station due to the influence of baseflow and relatively cool stream temperatures. The effects were less pronounced downstream because of minimal baseflow and elevated stream temperatures throughout the summer months. Howard Creek did not show any episodic temperature variations, suggesting that they occur in response to urbanization.