Megaflooding associated with glacial Lake Agassiz

Significant baseline and episodic megaflooding from glacial Lake Agassiz was routed to the south, east and north coasts of North America over the lake's nearly 6000 year history. The five phases of lake-level change were controlled by which outlet was active, which in turn was controlled by ice margin position and glacioisostatic adjustment. The southern outlet is the oldest and best understood outlet, while successively younger outlets are progressively less understood. Eastern drainage synchronous with the Younger Dryas chronozone had been assumed by most studies, but a spillway to accommodate the similar to 90 m drop in lake level is yet to be described. With the south and east outlets unable to accommodate the necessary lake level drawdown, the northwest outlet has become the default outlet with Arctic Ocean Core data supporting it. However, extant terrestrial data from the continent only provides data for a large flood from the Fort McMurray area closer to the end of the Younger Dryas, coinciding with the timing of the Preboreal Oscillation. The challenge remains to find agreement between the marine and terrestrial records. Uncontroversial is the geomorphic and sedimentologic evidence for flooding into and out of the lake consisting of large spillways, large boulders, and in places giant current ripples composed of boulders. Flood discharge estimates into and out of the lake range from 0.04 to similar to 1 Sv, with final subglacial drainage into the Tyrell Sea estimated at similar to 5 Sv, sufficient to raise global sea level by 0.18 m and initiate the 8.2 ka stadial.