VERTICAL TEMPERATURE PROFILES AS AN INDICATOR OF GROUNDWATER RESPONSE TO HIGH- AND LOW-WATER EXTREMES IN WISCONSIN’S ROCK RIVER

The Rock River of the Upper Midwest meanders from its headwaters in Horicon Marsh (WI) through southern Wisconsin and northern Illinois, eventually flowing to the Mississippi River. A 52-mile stretch of the river tracks through mostly agricultural lowlands of southeastern Wisconsin and has been the subject of ten years of research due to anomalous flooding since the early 2000s. This segment experiences flooding inconsistent with the magnitude and timing of precipitation. Management between dams at either end of the stream segment is the likely cause, but no clear solution or expected changes are on the horizon. Farmers and residents in the floodplain struggle with the inconvenience and economic impacts of the flooding (and extreme water lows) and impacts to the natural system are not well understood. Using vertical temperature profiles, we highlight the influence of flooding extremes on groundwater/surface water exchange. A pilot project to measure temperature data from two stations within the township of Lebanon, WI was undertaken in September 2017. After one year, these vertical temperature profiles suggest that high-water periods (over 843 feet above datum) moderate stream temperatures and groundwater interaction is significant. However, low-water periods (below 843 ft.) create a disconnect between groundwater and surface water as the stream bed dries up and the water table falls. The temperature signal is especially strong in the summer and may have a negative impact on the river’s ecosystems, including increased nutrient loading during flooding and dangerously high water temperatures during low-water periods.