Why Are There So Many Lakes Around Here?

The short answer

Most of the lakes around here were made by ice. During the last Ice Age, a giant sheet of ice slid down over most of Wisconsin. When it melted, it left behind holes and dams that filled up with water ^{1 2}.

The giant ice

Imagine ice so thick it would bury your house, your school, and the tallest building downtown. In the middle, the ice sheet was thousands of feet thick. At its edge it was a steep slope of ice, maybe several hundred feet high ¹. Scientists call this time the Wisconsin Glaciation, named after our state ¹.

Around Madison and Stoughton, the ice arrived about 25,000 years ago and pushed as far as it would go about 20,000 years ago. It started melting back about 15,000 years ago ². While it was here, woolly mammoths walked on the cold, treeless land in front of it ¹.

Kettle lakes

As the ice melted back, big chunks broke off and got buried in the sand, gravel, and mud the glacier was dropping. Buried ice melts very slowly. When those chunks finally melted, the ground on top sank into bowl-shaped holes, some 10 to 50 feet deep. Scientists call them kettles, and many filled with water to become lakes and ponds ².

Fish Lake, Crystal Lake, and Indian Lake, all near Madison, sit in kettles ².

Madison’s big lakes

Lake Mendota, Lake Monona, Lake Waubesa, and Lake Kegonsa, the lakes along the Yahara River near Stoughton, are a little different. Before the Ice Age, a river had carved a very deep valley here. The glacier filled most of it with more than 300 feet of sand and gravel, but not all of it. The lakes sit in the low spots that never got filled up ². Long ago, when the ice was melting, one big lake called Glacial Lake Yahara covered them all, and the hill where the State Capitol stands was an island ².

The place the ice missed

Drive west toward the Mississippi and the land changes: steep hills, deep valleys, and winding streams. That’s the Driftless Area. “Drift” is what geologists call the sand and rocks a glacier leaves behind, and this area doesn’t have any, because the ice sheet never covered it ³. With no buried ice chunks and no glacier dams, and with streams wearing away any natural dams over a very long time, it has almost no natural lakes ².

So many lakes

Wisconsin has about 15,000 lakes ⁴. The glacier made our state one of the thickest clusters of lakes anywhere in the world. Most of them are up north, where the ice left even more holes and dams ⁵.

So

There are so many lakes around here because a giant sheet of ice covered this land in the last Ice Age. When it melted, buried chunks of ice left holes called kettles that filled with water, and the ice left a deep old valley only partly filled, where Madison’s big lakes are now. The corner of Wisconsin the glacier missed has almost no natural lakes at all.