The Great Lakes, a prominent feature of the North American landscape, are often mistaken for saltwater bodies due to their immense size. However, these five interconnected lakes—Superior, Michigan, Huron, Erie, and Ontario—are entirely freshwater. They collectively represent the largest system of freshwater lakes on Earth by total area and the second-largest by total volume, holding approximately 21% of the world’s surface freshwater supply.
Understanding Water Types
Water bodies are primarily distinguished by their salinity, which refers to the concentration of dissolved salts. Freshwater has a very low salt concentration, typically less than 1 part per thousand (ppt), or less than 0.05% salt. In contrast, saltwater, such as that found in oceans, has a much higher salt content, averaging around 35 ppt, or 3.5% dissolved salts. Freshwater contains similar elements but in much smaller quantities.
How the Great Lakes Stay Freshwater
The freshwater nature of the Great Lakes is a result of their geological history and continuous hydrological processes. These lakes began forming around 14,000 years ago at the end of the Last Glacial Period, as massive ice sheets retreated. The glaciers carved out large basins in the land, which then filled with meltwater, a naturally low-salinity source.
The primary source of water for the Great Lakes is precipitation, including rain and snow, along with runoff from the surrounding land. This incoming water is naturally low in dissolved salts. Runoff flows into streams and rivers, eventually reaching the lakes.
A significant factor in maintaining the lakes’ freshwater state is their continuous outflow. Water consistently flows through the interconnected lakes, eventually draining into the Atlantic Ocean via the St. Lawrence River. This constant flushing action prevents the accumulation of salts that are naturally leached from the surrounding bedrock and soils. Unlike closed basins, such as the Great Salt Lake, where water evaporates and leaves salts behind, the Great Lakes’ outflow ensures that dissolved minerals are carried away, preventing high salinity.
Why Freshwater Matters
The Great Lakes, as a vast freshwater resource, hold significance for both human populations and natural ecosystems. They serve as a drinking water source for over 40 million people in the U.S. and Canada. This freshwater supply also supports various economic activities, including agriculture, manufacturing, and shipping.
Beyond human use, the Great Lakes provide diverse habitats for a wide array of plant and animal species. Their unique freshwater ecosystems support numerous fish species and serve as important areas for migratory birds. The availability of this large volume of freshwater shapes the climate and supports the distinct ecological character of the region.