An Ice Age-Old Tale. At 435 miles, the mighty Wisconsin River flows from edge to edge in the state. A tributary of the Mississippi, the Wisconsin flows out of the Lac Vieux Desert, a spring-fed lake in the north near the boarder with the UP. The river you see today was formed in several stages due to glaciation during the last ice age; the upper river was formed from glacial meltwater runoffs, which slowly carved out the river as the glaciers retreated northward. Retreating glaciers also left in their wake what was known as Glacial Lake Wisconsin: a prehistoric body of water held in place by