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An Ancient Lake’s Legacy at the Leopold Shack

The Leopold Shack and Farm is nestled in a landscape unlike any other. Just eight miles to the west begins the Driftless region, an area of distinct rocky formations and abundant cold streams. To the east is the most recently glaciated section of Wisconsin, characterized by lakes, wetlands, and individual hills. And here at the Leopold-Pines Memorial Reserve (LPMR), in the Baraboo, Dells, and Central Plain region, there are moraines, exposed cliffs of quartzite, sandy soil, and swathes of marsh. To the uncurious eye, the land around the LPMR is simply another gem in a state full of natural beauty. But just a peek into the natural history of central Wisconsin reveals a story so immense, it seems fit for a blockbuster film.

The Aldo Leopold Foundation is located in south-central Wisconsin on the edge of the Driftless and the most recently glaciated region of the state. Note the visible difference in topography. (Adapted from Google Maps)

My first exposure to this historical epic came while standing atop Pasque Flower Ridge in the LPMR. From this high point, the other Future Leaders Fellows and I looked down upon the Long Marsh, a treeless green expanse cutting through a sea of woodland canopy. This 180-acre wetland sits kitty-corner to the Leopold Shack and is a key conservation site for our present-day stewards. It is easy to overlook this innocuous stretch of sedge as you drive towards the Aldo Leopold Legacy Center in search of towering pines and colorful prairies, yet its existence is a consequence of an event of Biblical proportions:

The emptying of Glacial Lake Wisconsin.

Glacial Lake Wisconsin formed approximately 18,000 years ago when the Green Bay Lobe of the Laurentide Ice Sheet – the massive glacier that stretched into Wisconsin during an ice age period 30,000 years ago – began to melt. As the lobe retreated, it left behind ridges of rock and sand called moraines. These large hills created two main basins and several smaller valleys that filled up with glacial meltwater and formed the massive lake. At its peak, Glacial Lake Wisconsin covered at least 1,825 square miles and was 150 feet deep.

Map of the estimated boundaries of Glacial Lake Wisconsin approx. 14-15,000 years ago with modern-day sites for reference. (Modified from Clayton and Attig, 1989 in “Roadside Geology of Wisconsin” by Dott Jr. and Attig).

The smaller of the two main basins emptied first when the ice sheet retreated enough to let it drain into the Wisconsin River. The water in the largest basin was held in by the Johnstown moraine, a mass of sandy sediment supported by weak sandstone. This dam held for several thousand years, but it eventually succumbed to the forces of weather and time; the dam broke, and the water of Glacial Lake Wisconsin rushed out in a flood so massive, it profoundly altered the landscape of the lower Wisconsin Riverway. It carved out the gorge that became the Wisconsin Dells; it created a new course for the Wisconsin River, flowing east around the Baraboo Hills; and it cut through what would be the LPMR, leaving behind the scar that would become the Long Marsh.

The view of the Long Marsh from the top of Pasque Flower Ridge. The Long Marsh is connected to the Great Marsh, which Aldo Leopold mentions in A Sand County Almanac. (Leah Bieniak, 2022)

As the other fellows and I took in the impressive view from Pasque Flower Ridge, Steve Swenson, Leopold Foundation Program Director and our natural history guide for the day, asked us to guess the flow rate of the Wisconsin River during the flood. He gave us a point of reference: the average flow today is about 7,000 cubic feet per second, increasing or decreasing at times due to heavy rains or drought. We each took a guess: 100,000 cubic feet per second, 500,000, a million, two million… Steve gave us the additional fact that the Mississippi River’s largest flood in recorded history was in 1927, when the flow reached 2.47 million cubic feet per second.

When the dam broke on Glacial Lake Wisconsin 14,000 years ago, the flow is estimated to have been 3.6 million cubic feet per second. The 4,000-year-old lake drained in less than a week.

It is mind-boggling to think of something so old and mighty disappearing in the same amount of time it takes for an Amazon package to arrive. We – and the Leopold family – owe so much of central Wisconsin’s landscape to this cataclysmic event. From the characteristic soil of the Sand Counties that made farming poor but Leopold pines thrive; from the Wisconsin River running eastward far enough to one day be the “Leopold Lumber Yard;” from the Dells to state parks to the view from Pasque Flower Ridge; and to the Long Marsh, providing habitat for wetland species and inspiration for Aldo Leopold’s writing. The emptying of Glacial Lake Wisconsin may have been just a single week in the vastness of geologic history, but its story is written on this land with a legacy that carries us into the future.

Learn more about the Leopold-Pines Memorial Reserve ecosystem and stewardship efforts