Agassiz

 

(ăg'əsē), glacial lake of the Pleistocene epoch, c.700 mi (1,130 km) long, 250 mi (400 km) wide, formed by the melting of the continental ice sheet beginning some 14,000 years ago; it eventually covered much of present-day NW Minnesota, NE North Dakota, S Manitoba, central E Saskatchewan, and SW Ontario. The lake was named in 1879 in memory of Louis Agassiz for his contributions to the theory of the glacial epoch. Lake Traverse, Big Stone Lake, and the Minnesota River are in the channel of prehistoric River Warren, Lake Agassiz's original outlet to the south. As the ice melted, the water drained E into Lake Superior, and after the ice disappeared, N into Hudson Bay. The lake's disappearance (c.8,400 years ago) left lakes Winnipeg, Manitoba, and Winnipegosis, Red Lake, Lake of the Woods, and other smaller lakes. The bed of the old lake, the Red River valley, has become an important crop-growing region due to its rich soil.

Lake Agassiz was an immense lake theorized to be in the center of North America. As such, it was fed by glacial runoff at the end of the last ice age which accumulated into a huge lake that was bigger than all of the present-day Great Lakes combined.

Conception

First postulated in 1823 by William Keating, it was named after Louis Agassiz in 1879 after he was the first to realize it was formed by glacial action.

Geological progression

Geologists have come to a consensus on the likely geological progression of Lake Agassiz.

The lake's modern-day remnants, the largest of which is Lake Winnipeg, dominate the geography of Manitoba. 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 lake currently in the world, or even the Caspian Sea. This is roughly the size of Iraq, the 58th largest country in the world, larger than California, the third largest U.S. state, and 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. 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 civilizations in the Younger Dryas event.

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, that of 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. Much of the final drainage of Lake Agassiz may have occurred in a very short time—perhaps as little as one year.

While mostly gone along with the ice sheet that fed it, Lake Agassiz left marks over a large geographic area. Apparent beaches, kilometers or miles from any water, can be found in many locations—these mark the former boundaries of the lake. Several modern river valleys, including the Red River, the Assiniboine River and the aforementioned Minnesota River, were originally cut by water entering or leaving the lake. The Red River Valley agricultural region also exists because of the silt that sank to the bottom of the lake.

See also

• Glacial River Warren
• Glacial history of Minnesota
• Proglacial lakes of Minnesota

References

Sources

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