Near the end of the Pleistocene, about 11,800 years ago, a massive lake formed across the Northwest Territories of Canada, extending southward into northeast Alberta and northwest Saskatchewan. Occupying a broad basin between the mountains to the west and the retreating Wisconsin Glacier to the east, its shape and size changed continuously, a balance between the inflow of meltwater and evolving drainage patterns due to glacial deposits, ice dams and periodic floods. The maximum extent of Lake McConnell, reached about 10,500 years ago, was 210,000 square kilometers (about 2/3 the size of glacial Lake Agassiz, to its southeast).
Inflow to Lake McConnell came primarily from the lobes of the continental ice sheet but also from mountain glaciers to the west and directly from Lake Agassiz via a corridor now occupied by the Clearwater and lower Athabasca Rivers; indeed, geologic evidence suggests that these large glacial lakes were intermittently connected and that water flowed in either direction, depending on the region's changing hydrology. At times, Lake McConnell drained southeastward into Lake Agassiz while, during other periods, the connected lakes drained northwestward, into the Arctic Ocean via the MacKenzie River. Eventually, as the ice retreated and the land rebounded, this connection was lost and both lakes began to diminish in size.
While Lake Winnipeg is the primary remnant of Lake Agassiz, a trio of Canadian lakes have been left in Lake McConnell's wake: Great Bear Lake, the largest in Canada, Great Slave Lake, the deepest in North America (2014 feet), and Lake Athabasca. All three of these postglacial lakes now drain to the Arctic Ocean via the MacKenzie River system.
