The Great Lakes, spanning the border of the United States and Canada, are definitively freshwater bodies. They represent the largest group of freshwater lakes on Earth by total surface area and the second-largest by total volume. Containing approximately 21% of the world’s surface fresh water, this immense volume (roughly 5,439 cubic miles) often leads people to question the water’s composition.
How Scientists Classify Water Types
Scientists categorize water based on its salinity, or the concentration of dissolved salt, measured in parts per thousand (PPT). Freshwater must contain a salinity level below 0.5 PPT. This low concentration means the water is suitable for drinking and supporting non-marine life. Water with a salinity level between 0.5 PPT and 30 PPT is classified as brackish, a mixture often found in estuaries. Saltwater, characteristic of oceans and seas, is defined by a salinity that is generally 30 PPT or higher.
The Great Lakes’ Self-Cleaning Drainage System
The primary mechanism that maintains the freshwater composition of the Great Lakes is continuous flushing and hydrological balance. Fresh water is constantly added through precipitation and runoff from the surrounding watershed, acting as the system’s inflow. The water then flows out through a singular main channel, the St. Lawrence River, which carries accumulated minerals and salts out to the Atlantic Ocean. This continuous movement prevents the concentration of minerals that would otherwise occur in a closed basin.
The speed at which the water is replaced, known as the residence time, varies significantly across the five lakes due to their differing sizes and depths. Lake Superior, the largest and deepest, has the longest residence time, requiring approximately 191 years for its water to be completely replaced. In contrast, the shallower lakes have much faster replacement rates. Lake Erie, the shallowest of the five, has the shortest residence time at only 2.6 years, while Lake Huron’s water is exchanged in about 22 years. This constant outflow ensures that the overall salinity remains well below the 0.5 PPT threshold.
Why the Great Lakes are Unique Among Freshwater Bodies
The sheer physical scale of the Great Lakes contributes to the common misconception that they must be salty. Lake Superior, for example, is the largest freshwater lake in the world by surface area, covering 31,700 square miles. The lakes exhibit characteristics associated with oceans, including powerful wave action, sustained winds, and deep basins that create distant horizons.
Individual lakes also possess tremendous depth. Lake Superior reaches a maximum depth of 1,333 feet, which is deeper than the average depth of the Atlantic Ocean. This vast size allows them to generate strong currents and experience significant seiches, which are standing waves that cause water levels to oscillate like tides. While they behave like inland seas in their physical dynamics, their connection to the St. Lawrence River system ensures they remain a dynamic, open system, distinguishing them from smaller freshwater lakes.