Stream Temperature Modeling of Miller Creek, Duluth, Minnesota

This report summarizes a modeling study of heat loading and stream temperature in Miller Creek in support of the MPCA Miler Creek temperature TMDL. The MINUHET surface runoff modeling tool was used to characterize runoff temperatures for typical residential and commercial watersheds for the continuous period June 15 to September 15, 2008. These results were then generalized to the entire Miller Creek watershed using runoff volumes from a SWMM model developed at SAFL. These simulated runoff temperatures and volumes were then used to estimate point source heat loadings to Miller Creek for the same time period. Separate models for wet detention ponds, infiltration basins, and underground stormwater vaults were used to estimate possible reductions in heat loading from surface runoff. Standard wet ponds were found to increase overall heat inputs, but reduce peak heat loading rates and maximum stream temperature increases due to stormwater. The use of wet ponds with bottom outlet structures and underground vaults gave some reductions in effluent temperature for smaller rainfall events (< 1 cm), but were of little benefit for larger events. Infiltration practices give the greatest benefit in reducing temperature impacts of runoff, by direct reduction in runoff volume, however, widespread implementation of infiltration practices in the Miller Creek watershed may be difficult. In tandem with the runoff models, several stream temperature models for Miller Creek were developed based on the USGS SNTEMP modeling package. The stream temperature models were used to characterize atmospheric (non-point source) heat inputs to Miller Creek for current riparian shading conditions and for several mitigation scenarios with increased shading. Reductions of up to 1 ºC in maximum daily stream temperature were predicted for increasing shading levels in impacted areas from the wetland upstream of Kohl’s to Miller Hill Mall. Stream temperatures in Miller Creek were modeled with a focus on low flow (baseflow) conditions when trout habitat becomes critical. Both the stream temperature models used in this study and previous studies of the relationship of stream temperature to stream flow suggest, however, that increasing baseflow, by itself, will not necessarily lead to reductions in stream temperature. The temperature of Miller Creek was found to be relatively sensitive to air temperature, i.e. a 1 ºC increase in air temperature led to a 0.6 ºC increase in stream temperature. This sensitivity is likely due to low groundwater inputs, which tend to buffer diurnal and seasonal changes in air temperature. This suggests that Miller Creek, and perhaps North Shore streams in general, may be particularly sensitive to climate change.