PROCESSES AND RECONSTRUCTION OF SOUTHERN LAURENTIDE ICE SHEET MELTWATER DISCHARGES: CONSTRAINTS AND UNCERTAINTIES

Although the retreating margins of the southern Laurentide Ice Sheet (LIS) produced huge amounts of meltwater, paleohydraulic reconstruction of the mechanisms, timining and magnitude of these large and often catastrophic flows is fraught with uncertainty. Discharge estimates rely on channel size as an indication of magnitude, using methods such as the step-backwater method developed for bedrock channels. Applications of these techniques assume a relatively stable channel cross section, with various types of high-water marks used as an indication of depth. If these assumptions are valid, accurate discharge magnitudes can be calculated. Even in these best-case scenarios, however, fluvial erosion of bedrock during a flood, and the resulting channel enlargement, is still a source of uncertainty. Paleohydraulic reconstructions become even more problematic along the southern margins of the LIS, in areas of non-resistant bedrock and/or glacial sediment. Here, proglacial lakes that failed and drained catastrophically could erode a large spillway during a single event. Where no previous valley or channel existed, broad, shallow scoured zones evolved to narrow, deep inner channels during the flood. Discharge estimates based on the shallow scoured zone, inner channel, or both differ greatly. When floods discharged into existing valleys, their impact is difficult to separate from earlier events. The most important indicators of high magnitude, short duration events include deeply scoured channels with little subsequent sedimentary fill, streamlined erosional residual landforms, and boulder fans or bars in spillway channels. Subglacial discharge events are even more difficult to evaluate because of a lack of understanding of mechanisms, sources of meltwater, and subsequent modification during ice sheet downwasting or retreat. Current debates regarding these events focus on whether drainage occurs as sheetfloods, channelized flows (tunnel valleys or channels), or both, and whether the events the events are catastrophic or repetitive events of smaller magnitude. Coring and detailed analysis of tunnel valley sedimentary fills, along with geophysical techniques such as SkyTEM, which can define sedimentary architecture, should eventually resolve some of these issues.