A flow and temperature model for the Vermillion River, Part II: Response to surface runoff inputs

Stream temperature and stream flow are crucial physical parameters for aquatic habitat preservation in rivers and streams. Water temperature is particularly important in coldwater stream systems that support trout. Summer base (low) flow conditions with high water temperatures can be very detrimental to trout habitat. Surface runoff from rainfall events can lead to increases in stream temperature, particularly in developed (urban) watersheds. To better understand the interactions between stream temperature, land use, and climate, a stream thermal impact model has been developed for the Vermillion River, Minnesota. The model includes an unsteady streamflow and a water temperature model for the main stem of the Vermillion from Dodd Avenue to Goodwin Avenue and a number of tributaries, including South Branch, South Creek, North Creek, and Middle Creek. The EPD-riv1 package was used to simulate stream flow, including distributed groundwater inputs. A stream temperature model has been assembled based on previous work at SAFL. The stream temperature model uses flow and flow area from the flow solver, along with observed climate data to calculate surface heat transfer. The assembled flow and temperature model for the Vermillion River has been calibrated for baseflow conditions. Surface runoff inputs to the Vermillion River were simulated using a GIS-based land heat contribution model, which was developed and run by Applied Ecological Services. Surface runoff volume and temperature time series were simulated for a ½” rainfall event in 35 subwatersheds. Simulated runoff volumes and temperatures from the 35 sub-watersheds were used as input to the stream flow and temperature model, to simulate the hydraulic and thermal response of the Vermillion to runoff from the ½” rainfall event. Stream temperature increases due to surface runoff were found to be highest (1-4ºC) in smaller, upstream tributaries of South Creek and North Creek, and lowest in lower portions of the main stem and South Branch (< 1 ºC). Overall, the stream temperature response to multiple surface inflows was found to be quite complex. The coupled surface runoff and stream temperature model was used to examine several future urban development scenarios for the South Creek watershed and several possible strategies for mitigation of thermal impact downstream from the development. The model was able to resolve the stream temperature impact of a single 200 acre development. Full development in the upper South Creek watershed gave stream temperature increases over present conditions ranging from 3.8ºC in small tributaries to South Creek to less than 0.1ºC at the main stem of the Vermillion River near Empire. Downstream mitigation of thermal impacts from surface runoff was found to be ineffective, because the downstream watersheds were relatively undeveloped, and much of the thermal impact from upstream was lost by atmospheric heat transfer and dilution by the time the flow reached the sub-watersheds downstream. Adding channel shading to downstream stream reaches did not reduce the magnitude of the thermal impacts from upstream surface runoff, but did reduce maximum stream temperatures during dry weather periods, as would be expected.