The Great Lakes, a massive system of interconnected bodies of water on the border of the United States and Canada, are impressive in their sheer scale. These five lakes—Superior, Michigan, Huron, Erie, and Ontario—collectively hold a significant portion of the world’s surface freshwater. Given their immense size, it is common to wonder why these waters are classified as lakes rather than seas. Understanding the fundamental differences between lakes and seas clarifies this distinction.
Understanding What Makes a Sea
A sea is defined as a large body of saltwater that is either partially or fully enclosed by land, and has a direct or indirect connection to the global ocean system. The defining characteristic of a sea is its high salinity, meaning it contains a significant concentration of dissolved salts, primarily sodium chloride. This salt content is a result of rivers carrying minerals into the oceans, where they accumulate as water evaporates. The continuous exchange of water with the broader ocean maintains this saline composition.
The Great Lakes’ Freshwater Nature
In contrast to a sea, the Great Lakes are vast bodies of freshwater. They contain about 20% of the world’s surface freshwater and approximately 95% of North America’s surface freshwater. The average salinity of Lake Michigan, for instance, is very low. These lakes are inland, and while they connect to the Atlantic Ocean via the St. Lawrence River and Seaway, this connection is not a direct part of the global ocean system.
How Lakes and Seas Are Formed
The origins of lakes and seas also highlight their distinct classifications. The Great Lakes primarily formed from glacial activity at the end of the last Ice Age. Massive ice sheets scoured the landscape, carving out deep basins, and as the climate warmed, meltwater filled these depressions. Seas, in contrast, form through large-scale geological processes such as tectonic plate movements, or the flooding of coastal lowlands.
Distinct Features of Lakes and Seas
Beyond salinity and formation, other features differentiate lakes from seas. Seas experience significant tides, the regular rise and fall of water levels caused by the gravitational pull of the moon and sun. While the Great Lakes experience extremely minor true tides, less than five centimeters, these are largely imperceptible. Instead, the Great Lakes are known for seiches, oscillating waves caused by strong winds and changes in atmospheric pressure, creating a sloshing effect. The differing water compositions also lead to vastly different ecosystems, with freshwater lakes supporting species adapted to low salinity, while marine ecosystems in seas host diverse saltwater life forms.