Lake Agassiz

Lake Agassiz, immense glacial lake that formerly covered much of present-day northwestern Minnesota, eastern North Dakota, southwestern Ontario, and southern Manitoba. The lake was formed about 12,000 years ago at the end of the Pleistocene Epoch. The historical existence of Lake Agassiz is indicated by deltas, created where former rivers flowed into the lake, and by well-marked shorelines that can be traced for long distances. Lake Agassiz discharged only relatively small amounts of water because its natural drainage toward the north was held back by the great Laurentide Ice Sheet. The southerly outflow, an ancient river known as the River Warren, went through a channel 80 km (50 mi) long from the southern end of the lake to the Minnesota River and on to the Mississippi River, carving a valley now occupied by both rivers. Because it retained so much water, the lake became very large; it reached its greatest extent in about 7900 bc, when it was about 1,130 km (700 mi) long and covered 284,900 sq km (110,000 sq mi). Lake Agassiz contracted drastically about 400 years later, when the ice sheet melted sufficiently for an outlet to flow north via a channel (now the Nelson River) to Hudson Bay. Lake of the Woods, Lake Winnipeg, Lake Winnipegosis, and Lake Manitoba remain as remnants of Lake Agassiz. Most of the former bed of Lake Agassiz is now a fertile plain where much grain is produced. The lake was named in 1879 for Louis Agassiz, a Swiss-American naturalist and geologist who studied glacial movement.