QUATERNARY REVERSAL OF FLOW ALONG THE LOWER WISCONSIN RIVER VALLEY

The lower Wisconsin River flows westward from the Baraboo Hills across the Driftless Area of southwest Wisconsin to its confluence with the Mississippi River near Prairie du Chien, WI. However, Knox and Attig (1988) documented sediments consistent with eastward flow of water along the valley. They attributed this to a temporary reversal of flow caused by pre-Illinoian ice blocking the mouth of the Wisconsin River prior to ~780 ka.

Coring on the Bridgeport terrace in the lower Wisconsin River valley documents that the bedrock strath that forms the terrace dips to the east—in the opposite direction of modern stream flow. The direct implication of this observation is that eastward flow of water was the prevailing condition through the late Cenozoic as the valley was incised to the level of the Bridgeport strath. Numerous geomorphic features in the lower Wisconsin River valley support this interpretation, including: (1) abundant ‘barbed’ tributary valleys to the lower Wisconsin River; (2) the smooth curved radius of the valley wall at the confluence of the Mississippi and Wisconsin Rivers, which is typical of the inside bend of a single river rather than the confluence of two rivers; (3) the narrowing of the Wisconsin River valley in the downstream direction, which is opposite of typical morphology; and (4) the reach of the Mississippi River immediately south of its confluence with the Wisconsin River that distinctly narrows and has only short, steep tributaries.

We propose that through the late Cenozoic a single river, which we refer to as the Wyalusing River, flowed along the course of the upper Mississippi River as far south as the modern confluence of the Mississippi and Wisconsin Rivers, and then made a tight left turn and flowed east along the valley now occupied by the lower Wisconsin River. Subsurface data indicates this river continued to the northeast and ultimately flowed to the Gulf of St. Lawrence. Blockage of the St. Lawrence valley by middle Quaternary (?) glacial ice impounded water in the valley until it spilled over the lowest drainage divide. The resulting stream piracy event caused a reversal of flow along the lower Wisconsin valley to its current westward flow as the drainage area upstream of the modern confluence of the Mississippi and Wisconsin Rivers was shifted from the St. Lawrence basin to the greater Mississippi basin.