Case Study: Vermillion River
Much of the work under this project focused on the Vermillion River, a designated trout stream flowing through Lakeville and Farmington, MN. The Vermillion River is a good example of a trout stream with a rapidly developing watershed. Stormwater discharges from developed areas of the watershed are a concern for the Vermillion. The graph below shows an increase in stream temperature of about 3 °C (5 °F) due to stormwater runoff just downstream of Farmington. Notice, however, that the stream was nearly as warm on the previous day (July 24, which was sunny and dry), suggesting that solar heating also causes high temperatures in the Vermillion.
Stream temperature and precipitation in the Vermillion River, downstream of Farmington, MN, in July 2005. The total precipitation on July 25 was 0.9 cm (0.35 in).
We have also studied groundwater recharge and baseflow in the Vermillion River. Although we would expect groundwater recharge and baseflow to decrease as the watershed is developed, we found no significant trend in baseflow over the past 20 years. We believe that reductions in natural recharge of the shallow groundwater aquifer were offset by increases in artificial recharge from sources such as lawn watering and leaky water supply and sewer pipes.
The Vermillion River, just upstream of the measurement site.
We also modeled increases in shallow groundwater temperature due to paved surfaces. Using two dimensional heat transfer models, we found that paved areas can create a plume of warmer groundwater several degrees warmer than the ambient groundwater temperature. Results of a temperature simulation are shown on the right. This plume persists over a distance of approximately three times the width of the pavement.
Case Study: Miller Creek
The Duluth, MN area contains numerous trout streams with varying degrees of watershed development. Miller Creek originates near Duluth International Airport and through Hermantown and Duluth to St. Louis Bay in Lake Superior. Miller Creek has a naturally reproducing Brook Trout fishery, and has been placed on the list of impaired waters by the Minnesota Pollution Control Agency for temperature impairment. Our work on Miller Creek included computer models for surface runoff, stream flow, and stream temperature in support of the temperature TMDL (total maximum daily load). Development in the Miller Creek watershed has increased impervious area to about 23%, and reduced the extent of wetlands in the upper part of the watershed. As a result, Miller Creek is "flashy", with high peakflow and low baseflow (2008 discharge data shown below).
Using the MINUHET tool, we modeled stormwater runoff from typical commercial and residential developments in the Miller Creek watershed. The figure below shows the simulated runoff temperature from a commercial site and the corresponding observed storm sewer discharge temperature into Miller Creek in 2008. However, based on both observed stream temperatures and results from the stream temperature models, we found that low riparian shading is the main cause of high stream temperatures in Miller Creek.