RECONSTRUCTING THE CATASTROPHIC FLOODING FROM GLACIAL LAKE WISCONSIN

Glacial Lake Wisconsin 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 were only recently evaluated. Estimates based on step-backwater modeling indicate that peak discharge was between 3.6 and 5.3 x 10⁴ m³/s in the lower Wisconsin River. From a digital elevation model altered to incorporate isostatic depression, I estimated the lake volume to be 87 km³ just prior to dam breach, which, using a previously derived empirical relationship between lake volume and peak discharge for dam-break events, suggests that the flooding magnitude was as high as 1.5 x 10⁵ m³/s at the outlet. Adjusting these results for downstream flood wave attenuation gives a discharge of around 4.4 x 10⁴ m³/s in the lower reach, which closely matches the results of the step-backwater modeling.