Lake Agassiz was an immense glacial lake located in the middle of the northern part of North America. Fed by glacial meltwater at the end of the last glacial period, its area was larger than all of the modern Great Lakes combined, and at times it held more water than contained by all lakes in the world today.
During the last Ice Age, northern North America was covered by a glacier, which alternately advanced and deteriorated with variations in the climate. This continental ice sheet formed during the period now known as the Wisconsin glaciation, and covered much of central North America between 30,000 and 10,000 years ago. As the ice sheet disintegrated, it created at its front an immense proglacial lake, formed from its meltwaters, as the retreat of glacial margins is not caused by a reversal of the glacier's flow, but rather from melting of the ice sheet.
Around 13,000 years ago, the lake came to cover much of Manitoba, northwestern Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. At its greatest extent, it may have covered as much as 440,000 km2 (170,000 sq mi), larger than any currently existing lake in the world (including the Caspian Sea) and approximately the size of the Black Sea.
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), east through Lake Kelvin (modern Lake Nipigon) to what is now Lake Superior, or west via the Mackenzie River through the Northwest Territories. Geologists have found evidence that a major outbreak of Lake Agassiz, about 13,000 years ago, drained north through the Mackenzie River into the Arctic Ocean. A return of the ice for some time offered a reprieve, but after retreating north of the Canada–United States border around 10,000 years ago, Lake Agassiz refilled. The last major shift in drainage occurred around 8,200 years ago. The melting of remaining Hudson Bay ice caused Lake Agassiz to drain nearly completely. This final drainage of Lake Agassiz is associated with an estimated 0.8 to 2.8 m (2.6 to 9.2 ft) rise in global sea levels.
Lake Agassiz' major drainage reorganization events were of such magnitudes that they had significant impact on climate, sea level and possibly early human civilization. Major freshwater release into the Arctic Ocean is considered to disrupt oceanic circulation and cause temporary cooling. The draining of 13,000 years ago may be the cause of the Younger Dryas stadial. The draining at 9,900–10,000 years ago may be the cause of the 8,200 yr climate event. A recent study by Turney and Brown links the 8,500 years ago drainage to the expansion of agriculture from east to west across Europe; he suggests that this may also account for various flood myths of prehistoric cultures, including the Biblical flood myth.
Remnants and effects
Lake Winnipeg, Lake Winnipegosis, Lake Manitoba, and Lake of the Woods, among others, are relics of the ancient lake. 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. While the Red River gradually descends from south to north, these old strandlines ascend as one goes north, due to isostatic rebound since glaciation. Several modern river valleys, including those of 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, now drained by the Red River of the North, are formed from lacustrine deposits of silt from Lake Agassiz.
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