As the last Pleistocene Ice Sheet retreated into Canada, 13,000 years ago, a large meltwater lake filled a broad, flat, glacial basin along the North Dakota-Minnesota border. Soon filled to the brim, the lake spilled southward over the glacier's terminal moraine, draining into the Minnesota River. Continually filling in behind the shrinking Ice Sheet, the lake spread northward across southern Manitoba and, by 11,000 years ago, began draining into the Great Lakes chain. At its peak, this meltwater lake covered 365,000 square miles, an area larger than the State of California, and is renowned as one of the largest freshwater lakes in the history of our planet.
Named for Louis Agassiz, a famous Swiss geologist, Lake Agassiz eventually covered most of Manitoba, eastern sections of Saskatchewan, western Ontario, eastern North Dakota and northwestern Minnesota. By 8000 years ago, the Wisconsin Glacier had retreated far enough to allow the lake to drain northward, into Hudson Bay. This drainage pattern continues today and Lake Winnipeg, Lake Manitoba, Lake Winnipegosis, Lake of the Woods and numerous smaller lakes persist as remnants of Lake Agassiz. The Red River now flows northward through the flat glacial valley along the North Dakota-Minnesota border and ridges of sand, remnant beach dunes of Lake Agassiz, are found throughout the region.
