The Red River valley plain was formed as a result of glaciation, followed by sedimentation deposited by the glacial Lake Agassiz.
In the Pleistocene era, some two million years ago, Manitoba was almost completely covered by a glacier. It is estimated that about 16,000 years ago, the climate began to warm up and the glacier began to shrink northwards. Lake Agassiz was the largest lake formed in Manitoba by the meltwater from the glacier.
When it was first formed, Lake Agassiz flowed into the Mississippi River system, because the glacier prevented it from flowing north. As the glacier continued to melt, the lake first emptied eastwards towards the Great Lakes and then north-west towards the Mackenzie River system. Finally, it became possible for the waters to flow northwards along the Nelson River, as they do now. Lakes Winnipeg, Winnipegosis and Manitoba are what remains of Lake Agassiz. About 10,000 years ago, Lake Agassiz was over 200 metres deep at the place where Winnipeg is now. The thick, clayey deposits from Lake Agassiz now form the Red River valley.