The Great Lakes represent the world’s largest system of surface fresh water, holding approximately 21% of the global supply. This immense volume of water must eventually find its way to the sea, following the natural decline in elevation across the North American continent. The lakes’ drainage basin extends across the Laurentian Divide, a continental boundary that separates waters flowing toward the Atlantic Ocean from those flowing into the Gulf of Mexico. This geographical context sets the stage for a cascading journey from the highest lake to the final oceanic outlet.
The Cascading Flow of the System
The Great Lakes system moves water sequentially from west to east, driven by elevation drops between the basins. Lake Superior, the largest and deepest, sits at the highest elevation, approximately 600 feet above sea level. Water exits Superior through the St. Marys River, flowing 74.5 miles into Lake Huron and dropping roughly 23 feet at the St. Marys Rapids.
Lake Huron and Lake Michigan are hydrologically considered a single body of water, connected by the Straits of Mackinac, and share the same surface elevation. This combined body drains southward into the St. Clair River, which flows into Lake St. Clair. The Detroit River then carries this outflow into Lake Erie.
Lake Erie, the shallowest, sits about 8 feet lower than Lakes Michigan and Huron. Water flows from Lake Erie via the Niagara River toward Lake Ontario. This 35-mile river accounts for a drop of about 326 feet, with Niagara Falls representing the most dramatic descent. Lake Ontario, the last in the chain, is the lowest, positioned approximately 245 feet above sea level.
The Primary Outlet to the Sea
The Great Lakes ultimately drain into the Atlantic Ocean, a journey that begins with the St. Lawrence River flowing out of Lake Ontario. This river serves as the natural outlet for the entire Great Lakes basin. Outflow is regulated by control structures that manage water levels for navigation and hydropower generation.
The St. Lawrence River begins near Kingston, Ontario, and flows northeasterly for 744 miles. It forms a portion of the international boundary before entering entirely into Ontario and Quebec, passing major cities like Montreal and Quebec City.
The river’s journey involves a drop of 226 feet between Lake Ontario and Montreal, necessitating a series of locks for deep-draft vessels. This entire system, the St. Lawrence Seaway, allows ocean-going ships to travel deep into the North American interior.
The river widens dramatically after Quebec City, transitioning into a vast estuary. This estuary eventually empties into the Gulf of St. Lawrence, an arm of the Atlantic Ocean that receives nearly all of the Great Lakes’ water.
Key Diversions and Alternative Paths
While the St. Lawrence River is the primary drainage route, a small percentage of Great Lakes water is intentionally redirected through human-made channels. The most significant is the Chicago Sanitary and Ship Canal, an engineering project completed in 1900 that created a non-natural outflow from Lake Michigan. The canal reversed the flow of the Chicago River to carry the city’s sewage away from its drinking water source.
The canal diverts water from Lake Michigan into the Des Plaines River, crossing the continental divide. This connects the Great Lakes watershed to the Mississippi River basin. This diverted water travels down the Mississippi River system, eventually draining into the Gulf of Mexico, an entirely separate ocean basin.
Other diversions also influence the water budget. The Long Lake and Ogoki diversions in northern Ontario, Canada, bring water into the Great Lakes system, rerouting water from the Hudson Bay watershed into Lake Superior. These Canadian diversions were constructed to generate hydroelectric power and help balance the water lost through the Chicago diversion. The net effect of these engineered changes is a complex, managed flow that alters the natural water levels and pathways.