DRAINAGE OF THE LAST GLACIAL LAKE LONDON, ONTARIO: MAPPING AND HYDRAULIC MODELING
The resulting flood that drained the lake is also reconstructed using two different approaches: 1) applying a variety of published parametric breaching equations to find the peak discharge (13,000-241,000 m3/s) based on breach geometric characteristics and reconstructed maximum lake volume (9.1 km3); and 2) field mapping of a series of unusual geomorphic features found downstream of the Lake London outlet and attributed to the dam-break flood. These features are used for HEC-RAS step-backwater modeling of water surface slope profiles to calculate peak discharge. Reconstructed peak flood discharges (13,400-75,600 m3/s) are in a similar range as other dam-break floods from relatively small ice-marginal lakes of the Laurentide Ice Sheet and 20 montane moraine-dammed lake outbursts. Our results confirm that contemporary hydrology could not have formed the Thames River valley downstream of the glacial lake outlet and that its formation required extreme discharges one or two orders of magnitude greater than historical maximum gauged floods (~1,500 m3/s).
The study shows that outburst floods and resulting spillway channels originating from moraine-dammed lakes are not only typical of ice-marginal lakes in steep montane environments, or proglacial lakes in front of a glacier, but also occur in subdued deglaciating terrains of a receding ice sheet.