Analysis of Stream Temperature Data from Miller Creek, Duluth, MN

This report summarizes an analysis of stream temperature and associated climate data for Miller Creek, a trout stream in Duluth, MN. The study was undertaken in support of an MPCA-mandated temperature TMDL. The main goals of the analysis were 1) to characterize the spatial and temporal variations of stream temperature and 2) to determine the main drivers of stream temperature exceedances in Miller Creek. Stream temperature and flow data from 1997-98, 2003-05, and 2007-08 were analyzed at hourly to annual time scales. Included were water temperature data from the main stem of Miller Creek, its tributaries, and from storm sewer outlets to Miller Creek. Stream temperature in Miller Creek was found to be highly correlated to air temperature from the Duluth Airport at daily to annual time scales. Temperature exceedances (T > 20 ºC) were found to be caused mainly by strong atmospheric heat transfer to the stream due to low channel shading in the middle reaches of Miller Creek. Only 5 to 10% of all temperature exceedances appear to be associated with surface runoff from rainfall events, and even fewer are associated solely with surface runoff. Little evidence was found that lower stream flow leads to increased stream temperature and more frequent temperature exceedances. In mid summer tributaries of Miller Creek are typically at a lower temperature than the main stem of Miller Creek. The tributary at Chambersburg Ave. appears to measurably lower the temperature of the main stem, up to several degrees Celsius. The roles of groundwater and wetlands in the water (flow) and heat budgets of Miller Creek can not be quantified based on the available stream temperature records.