Catastrophic flooding from Glacial Lake Wisconsin

Glacial Lake Wisconsin was a large proglacial lake that formed along the southern margin of the Laurentide Ice Sheet during the Wisconsin glaciation. It was formed when ice of the Green Bay Lobe came into contact with the Baraboo Hills in southwestern Wisconsin and blocked the south-flowing Wisconsin River. During early glacial recession, the ice dam failed catastrophically and the lake drained in about a week. Despite early recognition of the former lake and the likelihood that it failed catastrophically, outflow rates during the failure have not been previously evaluated. Estimates based on step-backwater modeling indicate that peak discharge was between 3.6 and 5.3 × 104 m3/s in the lower Wisconsin River. As an alternate method, we used a previously derived empirical relationship between lake volume and peak discharge for dam-break events. From a digital elevation model altered to incorporate isostatic depression, we estimated the lake volume to be 87 km3 just prior to dam breach, suggesting that the flooding magnitude was as high as 1.5 × 105 m3/s at the outlet. Adjusting these results for downstream flood wave attenuation gives a discharge of around 4.4 × 104 m3/s in the lower reach, which closely matches the results of the step-backwater modeling. These estimates of discharge from the catastrophic failure of ice-marginal lakes improve our understanding of the processes that have produced the morphology and behavior of present-day upper Midwest river systems.