Glacial action shaped Minnesota's four hydrologic regions, determining the direction water flows towards the receiving waters of today.
Glacial Lake Agassiz never covered all of this area at one time, but this composite shows the total extent of its boundary over time. ( Based upon information in Minnesota's Geology, Ojakangas and Matsch, 1982).
Considered in geologic time, Minnesota’s landscapes are dynamic and constantly changing. Long before historic human occupation, drastic changes occurred when massive sheets of ice pushed across the state. As these sheets of ice inched southward, growing as snow accumulated, they shaped Minnesota’s four hydrologic regions.
When the glacial lobes began their retreat around 14,000 years ago, the resulting meltwater formed enormous rivers and lakes. The largest of these, Glacial Lake Agassiz, with a basin of almost 600,000 square miles, covered all of northwestern Minnesota at one time and was the largest glacial lake in North America.
Lake Agassiz began forming in the southern Red River valley 11,700 years ago and finally disappeared from the state around 9,000 years ago. During much of this period, the lake’s northern outlets were barricaded by ice. Thus, its only outlet was the Glacial River Warren, which drained to the south and whose river corridor is visible today as the broad Minnesota River valley. As the ice continued to retreat, previously blocked northern drainage outlets gradually opened, and Lake Agassiz began to drain northward, as the Red River does today (MN DNR Tomorrow's Habitat, 58).
The Red Lakes and Lake of the Woods in Minnesota, as well as some large lakes in south central Canada, are remnants of the much larger Glacial Lake Agassiz. This geologic history influenced the landscape in ways that are still very evident. The slightly elevated sand and gravel beach ridges in east central Minnesota, and the flat plain of silt and clay soils through which the Red River now runs, are landscape inscriptions from an ancient lakebed.