The Great Lakes, a vast interconnected system of waterways, are freshwater. These immense bodies of water represent one of the planet’s most significant freshwater resources. Their sheer scale often leads to comparisons with oceans, yet their chemical composition places them in the category of freshwater lakes. This distinction is fundamental to understanding their origin, characteristics, and the diverse life they support.
Defining Freshwater
Water is classified as “fresh” based on its low concentration of dissolved salts, typically less than 1,000 milligrams per liter. The Great Lakes maintain this low salinity because they are continuously replenished by precipitation and numerous freshwater rivers, while also having a consistent outflow. Their formation began approximately 14,000 to 20,000 years ago, at the end of the last Ice Age. Massive continental glaciers carved deep basins into the Earth’s crust as they advanced and retreated. These depressions then filled with meltwater from the receding ice, which was fresh.
The hydrological cycle sustains their freshwater status through continuous inflow and outflow. Precipitation falls directly onto the lakes and their surrounding watershed, contributing to their volume. Numerous rivers also feed into the lakes, bringing additional freshwater. Water then flows eastward through the interconnected system, eventually draining into the Atlantic Ocean via the St. Lawrence River. This constant movement prevents the accumulation of dissolved minerals and salts that would otherwise occur in a closed basin, preserving their freshwater nature.
Unique Characteristics of the Great Lakes
The Great Lakes are the largest group of freshwater lakes by total surface area. They span approximately 94,250 square miles, an area comparable to the United Kingdom. This immense scale is matched by their significant volume, holding about 20% to 21% of the world’s surface freshwater. If this volume were spread evenly, it could cover the contiguous United States to a depth of about 9.5 feet.
Each of the five lakes—Superior, Michigan, Huron, Erie, and Ontario—contributes to this vast system, though they vary in size and depth. Lake Superior is the largest and deepest, reaching a maximum depth of 1,333 feet and an average depth of 480 feet. In contrast, Lake Erie is the shallowest, with an average depth of 62 feet.
The lakes are interconnected by a series of rivers and straits, forming a single navigable waterway. For example, Lake Superior drains into Lake Huron via the St. Marys River, and Lake Erie connects to Lake Ontario through the Niagara River. This connectivity allows water to flow through the entire system.
Ecological Significance
The freshwater environment of the Great Lakes supports diverse biological communities. Numerous fish species thrive in these waters, including commercially and recreationally important populations. Examples include lake trout, lake whitefish, yellow perch, walleye, and various types of salmon. The lakes are home to over 260 species of fish.
Aquatic plants are an important part of the Great Lakes ecosystem, providing shelter and food for aquatic animals. These plants, such as coontail, common waterweed, and pondweeds, help stabilize sediments and contribute to the food chain. The varied habitats, from shallow nearshore waters to deep, cold basins, accommodate a wide range of aquatic plants. This rich biodiversity extends to migratory birds and other wildlife, which rely on the freshwater conditions and associated plant and fish populations for sustenance.