Lake Agassiz was an immense glacial lake located in the center of North America. Fed by glacial runoff at the end of the last glacial period, its area was larger than all of the present-day Great Lakes combined.
Forming around 13,000 calendar years before present (almost 12,000 14C years before present), the lake came to cover much of Manitoba, western Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. At its greatest extent, it may have covered as much as 440,000 square kilometers, larger than any currently existing lake in the world (including the Caspian Sea). This is roughly the size of Iraq, the 58th largest country in the world, and larger than California, the third largest U.S. state, but smaller than the Yukon, the ninth largest Canadian territory or province.
The lake drained at various times south through the Traverse Gap into Glacial River Warren (parent to the Minnesota River, a tributary of the Mississippi River), into the Great Lakes, or west through the Yukon Territory and Alaska. Climatologists believe that a major outbreak of Lake Agassiz in about 11000 BC drained through the Great Lakes and Saint Lawrence River into the Atlantic Ocean. This may be the cause of the Younger Dryas stadial. A return of the ice for some time offered a reprieve, and after retreating north of the Canadian border about 9,900 years ago it refilled. These events had significant impact on climate, sea level and possible early human civilization.
Much of the final drainage of Lake Agassiz may have occurred in a very short time—perhaps as little as one year. A recent study by Turney and Brown links this rapid drainage and subsequent global sea level rise of about one meter to the expansion of agriculture in Europe; he suggests that this may also account for various flood myths of prehistoric cultures, including the biblical flood.
The last major shift in drainage occurred about 8,400 calendar years before present (about 7,700 14C years before present), when the lake took up its current watershed, draining into Hudson Bay. The lake drained nearly completely over the next 1,000 years or so, leaving behind Lake Winnipeg, Lake Winnipegosis, Lake Manitoba, and Lake of the Woods, among others. The outlines and volumes of these lakes are still slowly changing due to differential isostatic rebound.
Other geological and geomorphological evidence for Lake Agassiz can also be seen today. Raised beaches, many kilometers from any water, mark the former boundaries of the lake at various times. Several modern river valleys, including the Red River, the Assiniboine River and the Minnesota River, were originally cut by water entering or leaving the lake. The fertile soils of the Red River Valley agricultural region are Lake Agassiz silt.