Lake Mendota’s spring thaw and why it matters

Post by Sam Zipper

This past weekend, Madison finally made it to the point we’ve been waiting for all winter. No, I’m not talking about the UW-Madison Badgers’ victory over Kentucky.

Friday, April 3rd was the unofficial “end of winter” with Lake Mendota’s official ice-off.

While it may seem like Mendota has been melting for a while, determining the official ice-off date is both an art and science. Ice-off is defined as the date when one can row a boat from Picnic Point to Maple Bluff. For Lakes Monona and Wingra, which are smaller, scientists follow a general rule of “50 percent covered,” as can be observed (albeit subjectively) from certain vantage points.

Lake Mendota's ice-off date is considered when one can row from Picnic Point to Maple Bluff (route in red).

Lake Mendota’s ice-off date is considered when one can row from Picnic Point to Maple Bluff (route in red). Image credit: UW-Madison Center for Limnology

This year’s total ice cover was 91 days, about 2 weeks shorter than last year’s. While this may be a boon for the winter weary, fewer icy lake days has its drawbacks.

Over the past 150 years, the average annual ice duration on Lake Mendota has decreased by 29-35 days–a trend that provides evidence of climate change. This decreasing trend is true for not just Lake Mendota, but also many lakes and rivers in the Northern Hemisphere.

Children play on Lake Mendota's icy-but-thawing shores. Photo by Gregg Sanford

Children play on thawing Lake Mendota’s icy shores. Photo by Gregg Sanford

These shifts in ice duration can have important impacts on our lake ecosystems. For example, fewer days of ice cover can lead to warmer summer lake temperatures, which can increase the likelihood of algal blooms and change the thermal habitat, meaning water temperatures may no longer be suitable for native fish to survive.

The Wisconsin State Climatology Office, which maintains long-term historical records of the ice cover for Lakes Mendota, Monona, and Wingra, determines the official ice-on and ice-off dates.


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