Final answer:
During the Quaternary Ice Age, Lake Agassiz formed over a large area in the northern United States and southern Canada, becoming the largest of the proglacial lakes associated with the melt of the Wisconsin glaciation.
Step-by-step explanation:
The lake that formed over an immense area of the northern United States and southern Canada during the Quaternary Ice Age is Lake Agassiz. This lake was the largest of the proglacial lakes associated with the melting of the Wisconsin glaciation and, at its peak, covered much of Manitoba, northwestern Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. Eventually, Lake Agassiz drained, as the ice sheet retreated and opened new drainage pathways, contributing to the formation of the current rivers and lakes in these regions, including the Great Lakes.