Officials warn McMahon Lake’s low levels and heat-linked fish kills call for operational changes

Agency official (S1) said McMahon Lake is “about 2 feet below the OHW” and traced the lake’s recent extremes back to a 2020 high that put the lake roughly two feet above the ordinary high water mark. He told the board that a prolonged heat spell this summer coincided with reported fish kills — “We got probably about 30 of them, from different residents and concerns” — and that the kills were notable because they included cold-water species such as northern pike, which struggle once water temperatures rise into the mid-80s.

S1 recommended prioritizing shoreline restoration and promoting overhanging and in-lake vegetation to increase shade and lower summer surface temperatures. He also suggested revising the outlet drawdown trigger: while the existing practice was to begin pumping when water reached about a foot above OHW, S1 said the county should consider a two-foot trigger to create a larger buffer that reduces extreme highs and lows. S1 noted that the DNR must set minimum drawdown limits but that the county can establish the upper threshold and that the operations plan for the outlet could be updated and incorporated into the DNR permit (S1: “The DNR must approve, when you can draw it down.”).