The lake level remained at 640' above sea level, but the ice margin was a third of the way north, opening a channel across Michigan, draining the Lake Saginaw and Lake Whittlesey proglacial lakes in the Lake Huron and Lake Erie basins.
At 640' above sea level, the lake's water level remained constant, but the advancing ice front split Michigan in two, diverting water from the Lake Saginaw and Lake Whittlesey proglacial lakes into the Lake Huron and Lake Erie basins.