The Great Lakes, an interconnected system on the border of the United States and Canada, represent the largest surface area of freshwater globally. This chain of five lakes—Superior, Michigan, Huron, Erie, and Ontario—is so vast that it often prompts the question of whether they should be classified as inland seas. This article will explore the geographical and hydrological criteria used by scientists to define an inland sea, contrasting those elements with the specific characteristics of the Great Lakes.
Defining the Scientific Criteria for an Inland Sea
A body of water classified as an inland sea, also known scientifically as an epicontinental or epeiric sea, must meet specific geological and hydrological criteria. These seas are extensive, relatively shallow bodies of water that cover central areas of continents. They are typically connected to the open ocean through a restricted channel or narrow strait, which allows for water exchange. The formation of an inland sea usually involves marine transgression, which is the flooding of continental lowlands due to a rise in global sea levels. Salinity is a defining factor, as a true inland sea is generally saline or brackish due to its connection and historical exchange with the ocean.
These seas are distinct from enclosed freshwater lakes because their water chemistry and circulation patterns are dictated by their oceanic link. The presence of oceanic tides, though often muted, is a consequence of this connection. Therefore, the classification relies heavily on the water’s salt content, its connection to the global ocean, and the geological process that created the basin.
Why the Great Lakes Are Often Compared to Seas
The Great Lakes possess numerous physical attributes that make the comparison to a sea understandable. Their combined surface area is approximately 94,250 square miles, giving them a limitless horizon, much like the ocean. This immense scale generates powerful winds and strong currents, creating waves that can rival those found on smaller seas.
The dynamic water movements and great depths contribute to the perception of a marine environment. The deepest point in Lake Superior plunges to 1,333 feet, which is deeper than the average depth of the North Sea. The sheer volume of water, accounting for 21% of the world’s surface fresh water, profoundly affects regional weather, generating lake-effect snow.
The lakes also support a massive international shipping trade, with vessels navigating across thousands of miles of coastline. This level of commercial navigation and infrastructure is more typical of a marine environment than a standard inland lake. These similarities lead to the popular, though scientifically inaccurate, use of the term “inland sea.”
Key Differences and the Definitive Classification
Despite their sea-like qualities, the Great Lakes are definitively classified as a system of freshwater lakes, immediately disqualifying them from being true inland seas. The fundamental difference lies in their chemical composition: the water in all five lakes is fresh and bicarbonate-rich, contrasting sharply with the saline or brackish nature of a true epicontinental sea. The lakes hold approximately 84% of North America’s surface freshwater supply.
The geological origin is another defining contrast. The Great Lakes basins were formed approximately 14,000 to 20,000 years ago, not by tectonic activity or flooding from the ocean. They are proglacial lakes, created by the massive Laurentide Ice Sheet, which scoured and deepened existing river valleys during the last Ice Age. As the ice sheet retreated, the depressions filled with glacial meltwater.
The hydrology of the Great Lakes also lacks the necessary open connection to the global ocean. Their water flows sequentially, stepping down in elevation from Lake Superior to Lake Ontario, before draining into the Atlantic Ocean via the St. Lawrence River. This defined river channel outflow does not permit the free circulation and tidal action typical of a true inland sea, confirming the Great Lakes remain the world’s largest system of freshwater lakes by area.