Modeling the Thermal Impact of Land Use Change in the Vermillion River Trout Stream Watershed, MN

Thermal impacts on streams due to changes in land use can have dramatic effects on aquatic life, especially on cold-water species like trout. Summer thunderstorms coupled with mid-summer low flow conditions and urbanization can raise temperatures and the risk of cold-water fish and macroinvertebrate mortality. A series of spatial models was developed in the Vermillion River trout stream watershed to understand the impacts of land use change on runoff and stream temperature. Simulated full development in upstream subwatersheds increased stream temperature slightly several miles downstream, but increased it 3.8°C in small tributaries. Downstream mitigation of thermal impacts was found to be ineffective, because much of the upstream thermal impact was already lost by atmospheric heat transfer and dilution by the time the flow entered the downstream reaches. In conclusion, thermal mitigation in the Vermillion trout stream watershed should take place near the impact source, and shading should be employed as a general trout stream management strategy.