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.