Wisconsin River - Geology

Geology

The modern Wisconsin River was formed in several stages. The lower, westward-flowing portion of the river is located in the unglaciated Driftless Area, and this section of the river's course likely predates the rest by several million years. The lower reach of the river is narrower than its upstream valley, leading to the suggestion the upper portions of the ancestor of the river flowed east previous to the Pleistocene. The remaining length of the river was formed gradually as glaciers advanced and retreated over Wisconsin. The stretch of river from Stevens Point north to Merrill was a drainage route for meltwater flowing away from the glaciers which covered northern Wisconsin during the Wisconsin Glaciation. As the glaciers retreated further northward, the river also grew in that direction. South from Stevens Point, the meltwater would have flowed into Glacial Lake Wisconsin, a prehistoric proglacial lake that existed in the central part of the state. As temperatures warmed around 15,000 years ago, the ice dam holding the lake in place burst, unleashing a catastrophic flood that carved the Dells of the Wisconsin River and joined the upper stretches of the river with the pre-existing lower river valley that today flows from Portage to Prairie du Chien.