GLACIAL LAKE AGASSIZ AND THE HYDRAULICS OF ICE-AGE MEGAFLOODS

During the deglaciation of North America (ca. 12,000-8,000 BP), huge proglacial lakes formed along the southern margin of the Laurentide Ice Sheet. Depending on the location of the ice margin, overflow from these lakes was variably directed to the Mississippi, Hudson and St. Lawrence drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events. On several occasions the ice dam formed by the Laurentide Ice Sheet was penetrated and massive outburst floods were routed to Hudson Bay and to the Arctic Ocean. In terms of released water volume the largest of these outburst floods was associated with the Kinojévis stage of Glacial Lake Agassiz. The impounded water volume has been estimated as 163,000 km³ and the timing of the outburst coincides with the early Holocene cooling event at 8,200 BP. Although the total water discharge is reasonably well constrained by glacial geological observations, the magnitude and duration of the flood, which have a potential influence on oceanic response, remain matters for speculation. To examine the range of possibilities we use the Spring-Hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and we develop new theory to treat the case of outburst floods that occur by downcutting of a supraglacial drainage channel. Representative values for flood magnitude and duration are 5 Sv and 1 yr.