POTENTIAL CLIMATE CHANGE IMPACTS TO GROUNDWATER FLOW AND STREAM TEMPERATURE IN THE MARENGO RIVER HEADWATERS, WISCONSIN

The Marengo River headwaters in the Chequamegon-Nicolet National Forest in northern Wisconsin have historically supported populations of brook trout and brown trout. Groundwater influx to the stream provides cool water refuges suitable for trout habitat. However, climate change forecasts make the future of the trout habitat uncertain. We use field measurements and models of groundwater flow and stream temperature to evaluate how climatic changes are likely to impact groundwater discharge and stream temperatures during summer low flow conditions in the Marengo headwaters.

Variations in groundwater discharge and stream temperature in the watershed correlate with the highly varied geology. Cool tributaries, with consistent flow around 10°C, correspond with likely groundwater discharge areas in glacial sand deposits and along bedrock faults and bedding planes. These stream segments also have physical characteristics that keep streams cool: narrow widths, shade, and high gradients. The warmest stream segments, common in flat wetland areas with shallow crystalline bedrock, reach temperatures upwards of 30°C, well above the lethal threshold for trout.

We are using groundwater flow and stream temperature models to evaluate the sensitivity of stream temperature to physical characteristics like shade and width, as well as climatic changes in baseflow and air temperature. Impacts to groundwater are simulated by modifying recharge in a steady-state groundwater flow model. Initial results suggest that the upper reaches of the watershed are most sensitive to changes in recharge. Simulated groundwater flows and climate change models of air temperature will be incorporated into a one-dimensional stream temperature network model to further evaluate sensitivity. An improved understanding of stream sensitivity will help the U.S. Forest Service manage the watershed for trout.