GLACIAL OUTBURST FLOODS FROM PROGLACIAL LAKES IN WISCONSIN

Large volumes of water contained in proglacial lakes along the southern margin of the Laurentide Ice Sheet occasionally drained catastrophically resulting in large outbursts of water to either the Mississippi or the St. Lawrence River. In Wisconsin, two such proglacial lakes are believed to have drained quickly as the Green Bay Lobe receded from its maximum extent ca. 21 ka ago.

One lake, glacial Lake Wisconsin, formed in central Wisconsin when the western margin of the Green Bay Lobe advanced onto the eastern part of the Baraboo Hills, damming the upper Wisconsin River. With recession of the ice margin 15 km from its maximum extent, the lake, containing more than 80 km3 of water, drained to the lower Wisconsin River Valley. This release of water from the lake cut through the sandstone bedrock, forming the Wisconsin Dells, and carried large boulders many miles downstream.

With continued recession of the Green Bay Lobe, glacial Lake Oshkosh formed in east-central Wisconsin along the ice margin at a lower elevation ca. 16.5 - 15.5 ka. The lake was approximately 5000 km2 in area and initially drained southward to the lower Wisconsin River Valley. With continued recession of the ice lobe, four lower outlets opened in succession to the northeast, diverting the drainage into the Michigan basin. The presence of deeply incised channels cut into bedrock below each of these outlets indicates that the lake must have drained quickly before reestablishing itself at a level that was on average 20 m lower. The volume of water that discharged quickly through each of these outlets is estimated between 18 and 57 km3. A series of rotosonic cores collected within the Lake Oshkosh basin commonly shows thick sequences of fine-grained laminated sediment capped by coarse-grained sediment. This coarsening-upward sequence indicates a rapid change in depositional environment that may be the result of a quickly falling lake level.

The discharge from these proglacial lakes may have been significantly less than the discharge from glacial Lake Agassiz located in Minnesota, North Dakota and Manitoba. However, the timing of the discharge events in Wisconsin may be correlated to earlier periods of incision recorded along the lower Mississippi River valley that preceded formation and subsequent discharges from Lake Agassiz.