Wisconsin is the home of over 10,000 lakes, nearly all of them owe their existence to glacial action. Some 15,000 years ago, Wisconsin was partially covered by a great glacier. As the glacier retreated, huge chunks of ice were left behind. Some of these ice chunks formed depressions in the landscape that then became our lakes.
Normally glaciers “glide” over the landscape. From time to time a portion of the ice stagnates and becomes frozen to the underlying surface. The frozen segments remain stationary (or dead) and do not move with the surrounding glacier.
As the glacier retreats, the dead ice becomes separated (the process is also known as calving) from the glacier. If it is not surrounded by till (rubble caused by the glacier) it will simply melt and have no long term impact on the landscape.
If the dead ice becomes surrounded by till, it will then be filling a void. When it eventually melts a kettle depression will form. If the underlying water table is high enough, the kettle is filled with water and becomes a lake.
As the glacier melted, it released torrents of meltwater that contained huge amount of rock debris. This till covers our post-glacier landscape and is most evident on the retreating side of terminal moraines. If the rubble surrounded dead ice, a kettle formed. Almost by definition, the depth of our kettle lakes can be no greater than the surrounding till. Typically the lake is not as deep as the surrounding till as glacial till tends to have over topped the dead ice and erosion subsequently accumulated on the lake bottom.
Many of today’s kettle lakes have no supply of surface water so the lake levels are dependent on the ground water. As ground water levels fluctuate, so do the water levels in the lake. Eventually erosion will fill the kettle leaving a meadow that overtime will become forested.