Lake Agassiz

Agassiz, Lake
Lake Agassiz was the largest glacial lake in North America. It was formed 11 500 years ago in front of the northeastwardly retreating Laurentide Ice Sheet, which acted as a dam. The lake covered much of Manitoba, northwestern Ontario, parts of eastern Saskatchewan and North Dakota, and northwestern Minnesota. At its largest, Lake Agassiz was about 1500 km long, over 1100 km wide and about 210 m deep. Its history is complicated by the behaviour of the ice sheet, which at times re-advanced and affected lake levels and drainage systems. Several outlet channels and about 35 beach ridges at different elevations have been identified. Some of these, such as the prominent Campbell Beach Ridge, in Manitoba, can be traced for hundreds of kilometres.

For the first 500 years, drainage was southward through the Minnesota River Valley. As glacial retreat continued into northwestern Ontario, the lake lowered and drainage shifted eastward into Lake Superior via various channels in the Lake Nipigon basin area. There may have also been a northwestward drainage into the Arctic Ocean via the Mackenzie River.

About 9900 years ago, a re-advance blocked the eastern outlet, raising the lake to earlier levels. Between 9500 and 9200 years ago, the ice sheet rapidly disintegrated and eastern outlets were once again opened. As deglaciation continued, lower lake levels were established and the overflow drainage shifted along a route across northern Ontario into the St Lawrence River Valley. The final drainage of the lake occurred about 7700 years ago north into Hudson Bay. Only remnants, eg, Lake WINNIPEG, remain today. The former lake basin and sediments have provided valuable agricultural land.

See also GLACIER; GLACIATION.

Author N.W. RUTTER

Suggested Reading
J.T. Teller and Lee Clayton, eds, Glacial Lake Agassiz (1983).

Links to Other Sites
Life of a Rock Star
This site tells the story of an extraordinary group of scientists who tramped, paddled and rolled across Canada in the nineteenth century to study the geology of Canada's varied terrain.

Investigating the impact of Lake Agassiz drainage routes on the 8.2 ka cold event
This scientific paper examines the 8.2 ka cold event, the largest abrupt climate change over the past 10 000 years. From the website "Climate of the Past."

Glacial Lake Agassiz
A brief geological and human history of the Glacial Lake Agassiz region. From the website for the Manitoba Heritage Council Commemorative Plaques.

Glacial Lake Agassiz
An article about the environmental impact of historical changes in the structure of Glacial Lake Agassiz. From the website for the Manitoba Historical Society.

EOS: Glacial Lake Agassiz
View a map that depicts the drainage pattern of water from Glacial Lake Agassiz into the Atlantic Ocean. From Earth and Ocean Sciences at the University of British Columbia.

Glacial Lake Agassiz Project
The website for the Glacial Lake Agassiz Project, which examines how changes in late-glacial environments affected human populations in central North America. From the website for Lakehead University.

Surface Geology of the Red River and Assiniboine Valleys
The text of an 1891 lecture delivered by a Rev. Dr. Bryce on the topic of "Old Lake Agassiz," and the surface geology of the Red River and Assiniboine valleys. From the website for the Manitoba Historical Society.