Paleohydraulics of the last outburst flood from glacial Lake Agassiz and the 8200 BP cold event

Abstract During the last deglaciation of North America, huge proglacial lakes formed along the southern margin of the Laurentide Ice Sheet. The largest of these was glacial Lake Agassiz, which formed about 11.7 14 C kyr and drained into Hudson Bay about 7.7 14 C kyr (8.45 cal kyr ) . Overflow from these lakes was variably directed to the Mississippi, St. Lawrence and Mackenzie drainage systems and it is thought that switches in routing were accompanied by a response in ocean circulation that produced abrupt climate events. When the ice dam across Hudson Bay finally was breached, a massive flood drained Lake Agassiz, which was routed through Hudson Strait to the Labrador Sea. In terms of stored water volume the largest reservoir was associated with the Kinojevis level of Lake Agassiz. For this maximum filling, the impounded water volume available to produce floods is estimated as 40,000– 151,000 km 3 , depending on the location of the ice margin and route used. The timing of this rapid release of stored freshwater just precedes the early Holocene cooling event at 8.2 cal kyr BP . We use the Spring–Hutter theory to simulate flood hydrographs for floods that originate in subglacial drainage conduits and find that flood magnitude and duration are ∼5 Sv and ∼0.5 yr . Multiple fillings and floods are possible as are single floods having a complex multipulse structure. Modelling results suggest that the outburst flood from Lake Agassiz may have terminated before the lake surface elevation dropped to sea level and that the flood ended when a stable drainage channel was established, connecting Lake Agassiz to the Tyrrell Sea.

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