The Great Lakes form the world’s largest system of surface freshwater, covering a surface area of nearly 100,000 square miles. This immense scale often prompts the question of whether they should be classified as seas. The answer is definitively no; despite their size, the Great Lakes remain true lakes. Scientific classification relies on three distinct geographical and chemical criteria that separate a landlocked lake from a sea.
Defining the Key Distinctions
The primary geographical distinction between a sea and a lake is its connection to the global ocean system. A sea is a large body of saline water that is partially enclosed by land but maintains a direct, continuous, and often tidally-influenced connection to the world’s oceans. Conversely, a lake is an inland body of water that is wholly surrounded by land, making it completely landlocked.
The second major difference is the water’s chemical composition, or salinity. Seas are fundamentally saltwater bodies, containing dissolved salts and minerals. Lakes are overwhelmingly composed of freshwater, though a small number of exceptions, like the Great Salt Lake or the Caspian Sea, have become saline due to a lack of outflow and high evaporation rates.
Finally, the geological origin of the basin plays a role in the classification. Seas and ocean basins are primary features of the Earth’s hydrosphere, often formed by tectonic processes involving the movement and separation of continental plates. Lakes are often temporary on a geologic timescale and are formed through secondary processes like volcanic activity, tectonic uplift, or glacial carving and meltwater accumulation.
Applying the Classification Criteria
Applying these established criteria to the Great Lakes confirms their identity as a complex, interconnected lake system. Regarding water chemistry, the lakes hold approximately 21% of the world’s total surface freshwater, making their water virtually free of salt. This low salinity is maintained because the system has a continuous outflow through the St. Lawrence River.
This outflow ensures that dissolved minerals and salts carried in by tributary rivers are constantly flushed out of the system, preventing the accumulation that would lead to a saline environment. Despite their ultimate connection to the Atlantic Ocean via this river, the lakes are geographically landlocked. The St. Lawrence River provides a non-tidal, non-sea-level connection between Lake Ontario and the sea.
The lakes’ geological history also aligns with the definition of a lake. The basins were formed approximately 14,000 years ago during the last glacial period when the massive Laurentide Ice Sheet retreated. The sheer weight and scouring action of the ice carved out deep depressions in the bedrock, which then filled with meltwater as the glaciers receded.
Characteristics That Lead to the Confusion
The sheer scale of the Great Lakes is the most significant factor leading to the perception that they are seas. Standing on the shore of Lake Superior, the largest by surface area, one cannot see the opposite bank, creating an endless horizon that mimics the open ocean. Lake Superior also reaches a maximum depth comparable to the depths of some marginal seas.
The lakes are also known for generating powerful, unpredictable, and dangerous weather and water phenomena. Strong, sustained winds can create significant wave action, with waves that are often steeper and closer together than oceanic swells. These conditions necessitate that mariners treat the waters with the same respect given to a sea.
The navigation and commerce on the Great Lakes further enhance this confusion. The system supports a massive commercial shipping industry, with specialized vessels known as “Lakers” and ocean-going ships called “Salties” transporting millions of tons of cargo annually. This level of port activity and the presence of a dedicated U.S. Coast Guard district are characteristics typically associated with coastal or maritime operations, cementing the enduring and descriptive nickname of “inland seas.”