Lake Agassiz was the largest of the glacial lakes in the last two phases of the Wisconsin Glacial Age. It covered a part of North Dakota, South Dakota, and Minnesota in the United States and Manitoba, Ontario, and Saskatchewan in Canada. It was created when the water flow from the glacier was blocked from it's natural flow to the Hudson Bay.

The lake was 700 miles long and 200 miles wide. Finally as the ice retreated, the water of the lake drained through a channel of what is now the Nelson River in Canada, leaving parts of Lake Manitoba and Lake of the Woods as reminders of this once great body of water.

The fine clay-like silt left by the lake is responsible for the fertility of the Red River Valley. It also explains the flooding that periodically inundates the area, usually in the spring. The heavy clay soil does not allow the rapidly melting water to soak in at a rate that would prevent flooding.

The Lake was named for the Swiss born geologist and naturalist Louis Agassiz.