Geology
As recently as the end of the last glacial period eleven thousand years ago the majority of northern Canada was buried under the enormous continental Laurentide ice sheet. The tremendous erosive powers of the Laurentide and its predecessors, which at maximum extent completely buried the Mackenzie River valley under thousands of meters of ice and flattened the eastern portions of the Mackenzie watershed. When the ice sheet receded for the last time, it left a 1,100 km (680 mi)-long postglacial lake called Lake McConnell, of which Great Bear, Great Slave and Athabasca Lakes are remnants. Significant evidence exists that roughly 13,000 years ago, the channel of the Mackenzie was scoured by one or more massive glacial lake outburst floods unleashed from Lake Agassiz, formed by melting ice west of the present-day Great Lakes. At its peak, Agassiz had a greater volume than all present-day freshwater lakes combined. This is believed to have disrupted currents in the Arctic Ocean and led to an abrupt 1,300-year-long cold temperature shift called the Younger Dryas.
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