Trout Stream Thermal Impact Assessment Study

Abstract

Mechanisms by which urbanization in Minnesota leads to temperature increases in coldwater streams and subsequent loss of trout habitat were investigated and quantified. Hydrologic and thermal computer models for surface and groundwater systems were developed, calibrated, and applied to the Vermillion River in Dakota County and to Miller Creek in Duluth. Surface runoff from impervious surfaces can have significant impact on stream temperatures in small tributary coldwater streams. Runoff from pavements and commercial rooftops creates more thermal pollution than runoff from residential rooftops. Runoff from afternoon or evening rainfall events with rapid onset tends to produce the most thermal pollution. Storms with high dewpoint temperatures can also produce high levels of thermal pollution from both pervious and impervious surfaces. Wet stormwater detention ponds tend to increase total heat input to receiving streams compared to untreated runoff, but do reduce the rate of heat release, and thus create smaller but longer-lasting temperature changes in streams. Pond designs with subsurface outlets reduce heat loadings for smaller rainfall events. Infiltration practices are generally a highly effective stormwater management practice for mitigation of thermal pollution. Loss of riparian shading causes more frequent high stream temperatures than does surface runoff. Efforts to mitigate temperature impacts of land development need to consider both mitigation of surface runoff and maintaining or restoration of riparian shading. Since tree growth is a slow process long-term watershed planning is important. Coldwater streams in Minnesota rely on inputs of cold water from groundwater or wetlands to maintain moderate summer temperatures. Urbanization has the potential to impact stream temperature by reducing groundwater recharge and stream baseflow. Shallow groundwater aquifers, e.g. in the Vermillion River, can be heated by paved surfaces leading to local stream temperature increases. Establishing buffer distances between pavements and stream channels is advisable. Although not a focus of this study, changing climate will be a significant cause of temperature changes in coldwater streams. Long term planning to maintain coldwater streams in Minnesota needs to consider both land development and climate change. Additional study is needed to assess agricultural impacts, particularly on coldwater streams in southeastern Minnesota. The results of this and previous work should be condensed into a management document, similar to the Minnesota Stormwater Manual.