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Formation of the Great Lakes

One Billion Years Ago

About a billion years ago, a fracture in the earth running from what is now Oklahoma to Lake Superior generated volcanic activity that almost split North America. Over a period of 20 million years, lava intermittently flowed from the fracture creating mountains covering the regions now known as northern Wisconsin and Minnesota, and eastern Canada. Over time these mountains eroded, while occasional volcanic activity continued. Molten magma below the highlands of what is now Lake Superior spewed out to its sides, causing the highlands to sink and form a mammoth rock basin that would one day hold Lake Superior. Eventually the fracture stabilized and, over time, the rock tilted down from north to south.

5 Million Years Ago

The region went from fire to ice with the arrival of the glaciers, which advanced and retreated many times over the last 5 million years. During the periods of glaciation, giant sheets of ice flowed across the land, leveling mountains and carving out massive valleys. Where the glaciers encountered more resistant bedrock in the north, only the overlying layers were removed. To the south, the softer sandstones and shales were more affected. As the glaciers melted and began receding, their leading edges left behind high ridges, some of which can be seen today in the cliffs of Door County, Wisconsin, and the Bruce Peninsula in Ontario.

Huge lakes formed between these ridges from the retreating ice fronts, and continually changed over time as the ice sheet moved northward. Early drainage from these lakes flowed southward through the present Illinois River Valley toward the Mississippi River, through the Trent River Valley between present lakes Huron and Erie and through the Lake Nippissing-Ottawa River Valley from Georgian Bay on Lake Huron downstream to Montreal, Quebec.

As the ice retreated about 7,000 years ago, the Saint Lawrence Seaway established itself as the outlet to the Atlantic Ocean

About 4000 years ago lake levels dropped to current levels and present day river and stream inlets and outlets developed.

One Continuous Channel Way

Four of the five Great Lakes are at different elevations, leading like a series of steps toward the Atlantic Ocean. The five individual lakes are connected to each other through channel ways, forming one system. Water continually flows from the headwaters of the Lake Superior basin through the remainder of the system.

The International Joint Commission, a binational agency established under the Boundary Waters Treaty of 1909 between Canada and the U.S., has the responsibility for regulation of flows on the St. Mary’s and the St. Lawrence Rivers. These channels have been altered by enlargement and placement of control works associated with deep-draft shipping. Agreements between the U.S. and Canada govern the flow through the control works on these connecting channels.