STREAM PIRACY AND THE GENESIS OF THE UPPER MISSISSIPPI DRAINAGE

The lower Wisconsin River flows westward from the Baraboo Hills to its confluence with the Mississippi River near Prairie du Chien, WI. Several remnant sections of a strath terrace, known as the Bridgeport terrace, are preserved along this deeply incised low-gradient river valley. Knox and Attig (1988) identified eastward-dipping foreset bedding in alluvial sediments deposited on the Bridgeport strath surface. They interpreted these deposits to be the result of a temporary reversal in flow direction of the lower Wisconsin River when pre-Illinoian ice flowing eastward from Minnesota and Iowa dammed the mouth of the Wisconsin River, forcing the river to flow eastward for a time.

Recent coring to characterize the nature of the Bridgeport strath surface indicates that rather than the pre-Illinoian event being a temporary reversal of flow, it represents the conditions under which the lower Wisconsin River valley was incised. Geoprobe coring to the Cambrian sandstone surface of the Bridgeport strath, combined with precise ground surface elevation control provided by high-resolution LiDAR data, reveals that the bedrock surface of the strath dips gently to the east. This implies that major incision of the lower Wisconsin River valley occurred while it was occupied by an eastward-flowing river, an interpretation supported by geomorphic features including the abundant ‘barbed’ tributaries found along the valley.

The most reasonable scenario is that a pre-Quaternary river that followed the course of the modern upper Mississippi River made a bend at Prairie du Chien, WI, and flowed eastward along the modern lower Wisconsin River valley. This river, which we here refer to as the Wyalusing River, likely flowed to the northeast along a now-buried bedrock valley in the Fox River lowlands of eastern Wisconsin as part of the headwaters of the modern Great Lakes (St. Lawrence) drainage basin. This northeast flow path would have been blocked at some point by a glacial advance from the northeast, resulting in ponding and spill-over just south of the modern confluence of the Mississippi and Wisconsin Rivers. Stream piracy at this location established the modern drainage pattern of the upper Mississippi River and westward-flowing lower Wisconsin River as headwaters of the greater Mississippi River basin.