Summary
Megafloods from glacial lakes were common along the margin of the Laurentide Ice Sheet during the last deglaciation. These outbursts resulted in a complex network of spillways with characteristic erosional and depositional forms. Meltwater was impounded in front of and beneath the ice margin as the Late Wisconsin Laurentide Ice Sheet melted back from its maximum extent. Lakes that formed in this dynamic environment drained completely or partially. Various factors aided in the impoundment of meltwater lakes, including isostatic depression of the land surface in the vicinity of the retreating margin, topography that sloped toward the ice margin, glacial erosion of trough-like depressions by ice streaming in major outlet lobes, and moraine ridges formed when the ice was more extensive. Subaerial megafloods were triggered probably by the failure of ice-cored or sediment-cored moraine dams or rapid incision of outlets caused by incoming subaerial and/or subglacial megafloods. Complete drainage of lakes was likely where non-resistant glacial or bedrock materials made up the outlet region.
Proglacial megaflood discharges were typically 0.1–1.0 Sv, short in duration, and, in some places, achieved velocities in excess of 10 ms−1. Outburst flows were highly erosive and carved a suite of small-scale to large-scale erosional forms, including potholes, longitudinal grooves, streamlined hills, transverse bedforms, anastomosing channels and spillways. Not all these forms are present along all megaflood pathways, except for spillways, which are ubiquitous; they are trench-shaped, 1–4 km wide, and tens of metres deep.