The broad valley along this stretch of the Wisconsin River was eroded over several million years, like the rest of the Driftless Area. However, the rapid drainage of glacial Lake Wisconsin (as explained on Central Sand Plains page) changed its shape. Huge volumes of glacial meltwater transported large amounts of sand and gravel and undercut the sandstone and dolomite adjacent to the river. This formed the steep bluffs of the lower Wisconsin River valley. Today, the steep bluffs, broad floodplain, and numerous sandbars make this stretch of the Wisconsin River a major attraction for recreation.
Early in the history of North American exploration, this section of the river was part of the water route for trade and exploration from the Great Lakes to the upper Mississippi River. People could travel by boat from Green Bay up the northeastward-flowing Fox River to the present site of the city of Portage. At this location, boats were portaged approximately 2 miles across the drainage divide into the Wisconsin River, which flows into the Mississippi River near the city of Prairie du Chien.
