Temperatures of urban streams: Impervious surface cover, runoff, and the importance of spatial and temporal variations

Urbanization effects on stream temperatures were evaluated in the Gwynns Falls watershed, Baltimore, Maryland, from September 2004 to October 2005. Continuous stream water temperature data were collected at 2-minute intervals at eight sites along an urban-rural gradient in the Gwynns Falls watershed and two reference sites representing various mixtures of forest and impervious surface cover (ISC) as part of the Baltimore Ecosystem Study Long Term Ecological Research project. Air temperature, precipitation, and streamflow data were obtained. Fish data for the Gwynns Falls watershed were gathered from the Maryland Biological Stream Survey (MBSS). Carroll Park, the most downgradient site, showed 4.9°C higher summer mean water temperatures and 5.7°C higher maximum summer temperatures than one of the forested reference sites. A 1% increase in ISC was associated with a 0.07°C rise in mean temperature and 0.34°C rise in maximum summer temperatures, respectively. Urban buried streams in high ISC areas showed elevated baseline temperatures that were well above all the other monitoring sites, with extremely high maximum summer temperatures (>40°C).;Runoff-induced temperature spikes occurred at downstream sites as well as headwater streams. The spikes at the downstream receiving waters (Carroll Park) were greater than other sites during daytime storm events in June. Stream temperature at peak flow (TQmax) had a positive linear relationship (R2=0.85) with ISC in the warm season. Generally, MBSS fish data indicated that species number and the number of individuals decreased from upstream to downstream following the trend of stream temperature warming. In particular, the number of coldwater species dramatically decreased from upstream to downstream in the Gwynns Falls watershed. About 40% of all fish are threatened by high summer water temperatures at the two urban sites (Carroll Park and Dead Run). Temporal cumulative temperature frequency showed that the maximum temperature tolerance for fish was exceeded for a longer time in more urbanized streams than in forested ones. There was a positive correlation between percent exceedance of maximum temperature tolerance for longnose dace and ISC (R2=0.45, p<0.03) during the summer. The maximum temperature tolerated by longnose dace (coolwater fish) was exceeded for longer periods of time in stream reaches associated with runoff from higher ISC. These observations suggest that urban landscapes exhibit transient spikes of elevated runoff temperature representing thermal disturbances that can have detrimental effects on aquatic biota.