The Great Lakes

The Great Lakes are more than just vast pools of water; they are the heartbeat of North America’s freshwater system, straddling the US-Canada border and influencing everything from weather patterns and shipping lanes to ecosystems and regional identities. Comprising Lakes Superior, Michigan, Huron, Erie, and Ontario, this interconnected system is the largest group of freshwater lakes by surface area in the world—holding nearly 21% of the planet’s fresh surface water.

Dine into the geology, hydrology, ecology, and human impact on the Great Lakes, unraveling the complex forces that shaped them and the urgent issues they face today.

More in The USThe 5 Largest US Lakes by Volume

A Geological Legacy: Born from Ice

Frozen Lake Superior shoreline. Apostle Islands National Lakeshore.

The story of the Great Lakes begins over a million years ago during the Pleistocene Epoch, a period dominated by immense glaciers. These ice sheets carved through ancient river valleys and lowlands, gouging out the lake basins we see today. When the glaciers finally retreated around 14,000 years ago, meltwater filled these enormous basins—creating the Great Lakes.

Each lake's age and water level stabilized at different times:

Lake Erie: ~10,000 years ago
Lake Ontario: ~7,000 years ago
Lakes Huron, Michigan, and Superior: ~3,000 years ago

During postglacial uplift—where the Earth’s crust slowly rebounded after being compressed by ice—the drainage patterns shifted dramatically. Early lakes like Lake Chicago and Lake Maumee eventually gave way to today’s five major lakes, all draining eastward through the St. Lawrence River into the Atlantic Ocean.

Mapping the Five Great Lakes

Each lake is unique in size, depth, and ecology, but together they span over 94,000 square miles—an area larger than the United Kingdom. Here's a snapshot of each:

Lake	Surface Area (sq mi)	Mean Depth (ft)	Notable Features
Lake Superior	31,700	483	Deepest and largest by volume; water retention of ~191 years
Lake Michigan	22,300	279	Only Great Lake entirely within the United States
Lake Huron	23,000	195	Home to the world’s largest freshwater island—Manitoulin Island
Lake Erie	9,910	62	Shallowest and warmest; most biologically productive
Lake Ontario	7,340	283	Final lake in the chain; drains into Atlantic via St. Lawrence River

Climate Engineers of the Midwest

Grand Haven Lighthouse - Grand Haven , Michigan.

The Great Lakes have an outsized influence on the climate of the surrounding regions. In summer, the lakes absorb heat, keeping coastal areas cooler. In winter, they release stored heat, moderating cold temperatures.

This dynamic effect leads to:

Lake-effect snow: Heavy snowfall downwind of lakes (example, Buffalo, NY)
Seasonal storm surges: Fall and winter gales can produce waves over 10 feet tall
Microclimates: Ideal for fruit orchards along the shores of Lake Michigan and Ontario

Lifeblood of Industry and Transportation

An aerial of a Lake Freighter sailing in the Welland Canal, Canada.

The Great Lakes form a vital transportation corridor, linking the industrial heartland of North America with global markets via the St. Lawrence Seaway. Ships navigate a complex network of locks, canals, and rivers to transport goods like iron ore, coal, grain, and automobiles.

Key connections include:

Chicago Sanitary and Ship Canal: Links Lake Michigan to the Mississippi River system
Welland Canal: Bypasses Niagara Falls, connecting Lakes Erie and Ontario

This inland maritime highway supports billions in trade annually—but also opens pathways for invasive species.

Pollution, Invasives, and Environmental Stress

Aerial view of oil contaminated water pollution being released into Lake Erie from the steel mills, seen at the beach at the shore of Port Dover, Ontario.

Industrialization brought prosperity to the Great Lakes but also severe pollution. By the 1960s, Lake Erie was so affected by agricultural runoff and sewage discharge that it was declared “dead” due to eutrophication. While cleanup efforts have improved conditions, major challenges remain.

Invasive species are a key issue. Zebra and quagga mussels clog water intakes and disrupt ecosystems. Sea lampreys, parasitic fish, decimated native lake trout, while spiny water fleas compete with native zooplankton, impacting the food web.

Nutrient runoff—particularly phosphorus and nitrogen from farms—continues to cause harmful algal blooms, especially in Lake Erie.

Climate change adds pressure by warming waters, reducing ice cover, and encouraging invasive species spread. It also brings more intense storms, leading to shoreline erosion and infrastructure damage.

Diverse Ecosystems, Rich Biodiversity

The great lake trout (salvelinus namaycush)otherwise known as the mackinaw trout or cristivomer or lake char, from north america.

Despite these threats, the Great Lakes remain ecologically rich. Each lake hosts a blend of cold and warm water species, with unique communities above and below the surface.

Notable native species in the Great Lakes include lake trout, whitefish, and sturgeon, which are found in deep, cold waters. Walleye, bass, and catfish thrive in shallower, warmer areas of the lakes. Plankton and algae also play a crucial role in the ecosystem. Diatoms, microscopic algae with silica shells, dominate the waters, while blue-green algae are common during summer months and often bloom near urban runoff zones.

In the deepwater ecosystems, rare species such as the deepwater sculpin and opossum shrimp are found, particularly in northern latitudes, where they are native to the colder, deeper regions of the lakes.

International Stewardship

A high angle of Great Sand Bay of Lake Superior covered with greenery on Keweenaw peninsula, Michigan.

Managing the Great Lakes requires ongoing cooperation between the United States and Canada. The Boundary Waters Treaty of 1909 laid the groundwork for peaceful joint management, while the International Joint Commission (IJC) continues to oversee water levels, pollution, and usage.

Today, efforts focus on:

Reducing agricultural runoff
Preventing new invasive species introductions
Restoring shoreline wetlands and habitats

Frequently Asked Questions About the Great Lakes

Grand View of Bruce Peninsula National Park — Grand View of Bruce Peninsula Nationsl Park

Which Great Lake is the largest?

Lake Superior is the largest by both surface area and volume, covering 31,700 square miles and holding more water than the other four lakes combined.

How were the Great Lakes formed?

The lakes were formed by glaciers during the Ice Age. As glaciers advanced and retreated, they carved out deep basins that later filled with meltwater.