The Great Lakes represent the largest system of fresh surface water on Earth, shared between the United States and Canada. Currently, the basin’s outflow is directed toward the Atlantic Ocean. The shift of this vast water supply to the Mississippi River system is predicted by ongoing, measurable changes in the Earth’s crust. This future redirection will be caused by a slow but relentless geological process fundamentally changing the shape of the North American continent.
The Current Drainage Route and Basin Divide
Today, the Great Lakes water flows eastward, following a defined path that eventually empties into the Atlantic Ocean. Water travels from the uppermost lakes—Superior, Michigan, and Huron—down through a chain of connecting rivers and straits. This flow continues through Lake Erie, over Niagara Falls, into Lake Ontario, and finally exits the system via the St. Lawrence River.
This easterly flow is dictated by the current relative elevations of the landscape, specifically the Great Lakes-Mississippi River watershed boundary. This subtle continental divide is a ridge of higher ground that separates the Great Lakes basin from the adjacent Mississippi River basin. Near the southwestern edge of Lake Michigan, this divide is particularly low and close to the lake.
The relative elevation of this divide ensures that all natural drainage from the Great Lakes is channeled toward the St. Lawrence River. This seemingly stable geography is in the process of being slowly but permanently altered by deep-seated geological forces.
The Mechanism of Uplift: Post-Glacial Rebound
The driving force behind the predicted drainage shift is post-glacial rebound. Thousands of years ago, the North American continent was buried under the immense weight of the Laurentide Ice Sheet, which was miles thick. This colossal mass of ice physically depressed the Earth’s crust into the planet’s semi-fluid mantle, much like a bowling ball sinking into a mattress.
When the ice sheet melted away, the removal of this weight triggered a slow, ongoing recovery of the crust. The land is gradually rising back toward its original elevation, a process that continues today because of the extreme viscosity of the mantle. The rebound, however, is not uniform across the entire Great Lakes region.
The uplift is happening much faster in the northern and northeastern parts of the basin, where the ice mass was thickest. Measurements show that the land near the center of the former ice sheet is rising at rates of up to 1 centimeter per year. Conversely, the southern and southwestern edges of the basin, which were at the periphery of the ice sheet, are rebounding much slower or are experiencing slight subsidence (sinking). This differential uplift is the mechanism responsible for reshaping the lakes’ basin.
Predicting the Future Drainage Pathway
The consequence of this differential rebound is that the entire Great Lakes basin is slowly tilting. The eastern and northern outlets, including the land around the St. Lawrence River, are rising faster than the southern and western shores of the lakes. As the St. Lawrence exit is continuously lifted higher, the water surface must rise relative to the rising outlet, effectively raising the system’s “spillway.”
This gradual tilting means that the lowest point of the basin will eventually no longer be the St. Lawrence outlet. Instead, the water will be forced to seek the next available low-elevation escape route. This future tipping point is predicted to occur at the southwestern edge of Lake Michigan, where the watershed divide is already very low.
The natural flow of water would reverse, channeling Lake Michigan’s water toward the Illinois River system, a tributary that flows directly into the Mississippi River. This eventual shift is not without precedent; at various times during the last glacial period, the ancestral Great Lakes did drain through the Mississippi system.
A modern, man-made connection already exists near this geological low point: the Chicago Sanitary and Ship Canal. This engineered waterway was built in the late 19th and early 20th centuries to reverse the flow of the Chicago River, redirecting water away from Lake Michigan toward the Mississippi watershed for sanitation purposes. While the canal currently manages a small diversion, it sits precisely at the location of the future natural drainage path. The time required for the land to tilt enough to fully redirect the Great Lakes is immense, spanning thousands of years, but the geological forces driving the change are already in motion.