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Staff links low lake levels and heat to fish kills; TMDL update suggests possible alum treatment for Cedar Lake
Summary
Staff reported low water levels at McMahon Lake (about two feet below ordinary high water), recent statewide fish‑kill reports tied to warm temperatures, and a TMDL update for Cedar Lake that recommends continued curly‑leaf control, increased carp reduction and that the lake may be ready for a phosphorus inactivation (alum) treatment, which would likely exceed $1 million.
County staff and SWCD representatives described recent aquatic monitoring results, low water levels, and lake management options. Melissa reported McMahon Lake's water levels were "probably close to 2 feet under the ordinary high water level" and noted that sustained high air temperatures reduce dissolved oxygen and contribute to fish kills. She also referenced recent DNR reports of hundreds of fish‑kill reports statewide.
On Cedar Lake, staff summarized a total maximum daily load (TMDL) update that recommended continuing curly‑leaf pondweed treatments, reducing carp populations to address internal phosphorus loading, and indicated the lake may now be a candidate for a phosphorus inactivation (alum) treatment. Melissa said the recommended alum treatment is costly — staff noted alum projects can exceed $1,000,000 — and would likely require grant funding and careful condition assessment before any treatment proceeds.
Staff cautioned that treatments have tradeoffs: the DNR had previously restricted lake‑wide herbicide treatment in cases where native plants were depressed, and staff urged that any control strategy be coordinated with DNR guidance and resident outreach. Committee members were forewarned that residents may contact commissioners about treatment decisions.
