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Effects of Hypolimnetic Releases on Two Impoundments and Their Receiving Streams in Southwest Wisconsin

Abstract The effects of bottom water withdrawals were evaluated within and below two southwestern Wisconsin impoundments. Like many man-made lakes in the unglaciated area of Wisconsin, Twin Valley Lake and White Mound Lake were constructed during the late 1960s for flood control and recreation. Both impoundments, which are located in large agricultural watersheds, were originally designed to release cold bottom water with the intended goal of managing trout below the dams. However, recent water quality monitoring results revealed that the streams became degraded due to frequent dissolved oxygen criterion violations and excessive filamentous bacteria growths. Organic loading from the bottom discharges is the likely reason that trout stream habitat was not successfully created below the dams as originally intended. Despite the accelerated phosphorus removal from the long-term withdrawals, blue-green algal blooms continued to be a problem in both impoundments. While maximizing total phosphorus export can improve lake water quality conditions, discharge rates from these impoundments were found to be excessively high, resulting in disturbance of their thermo-structure and entrainment of nutrients into the surface waters. In 2005, we blocked the bottom gate at Twin Valley Lake and monitored water quality and thermal responses. As a result, lake and stream water quality improved significantly while the downstream fish community structure did not change. The impoundment thermo-structure was restored with well-defined hypolimnion and epilimnion. We conclude that managing impoundments is often a balancing act between seemingly disparate goals of achieving optimum conditions above or below a dam, with undesirable consequences often occurring if the focus is disproportionately on a single goal.