Great Lakes

The Great Lakes began to form about 14,000 years ago as the Laurentide Ice Sheet retreated, carving deep basins into the landscape. ¹ Lake Superior, the largest and deepest, holds more water than the other four combined. Lake Michigan is the only one entirely within the United States; the others are bisected by the international border. The lakes are connected by a system of straits, rivers, and canals, including the Mackinac Strait (between Michigan and Huron) and the Niagara River (between Erie and Ontario). The St. Lawrence Seaway allows ocean-going vessels to reach inland ports, making the system a vital trade corridor. The basin drains roughly 295,000 square miles and supports a diverse array of ecosystems, from coastal wetlands to boreal forests.

The Great Lakes ecosystem has faced significant pressures from invasive species, pollution, and climate change. ² Zebra and quagga mussels, introduced via ballast water in the 1980s, have altered nutrient cycling and fouled infrastructure. Lake Erie has experienced recurrent harmful algal blooms driven by agricultural runoff, leading to "dead zones" and drinking-water crises. The U.S. and Canadian governments have invested in restoration through the Great Lakes Restoration Initiative and the Great Lakes Water Quality Agreement. Despite these efforts, microplastics, toxic contaminants like PCBs, and warming water temperatures continue to threaten fish populations and water quality. The lakes also moderate local climate, producing lake-effect snow in downwind areas.

The Great Lakes region supports a $6 trillion economy, with sectors including manufacturing, shipping, tourism, and commercial fishing. ³ Major cities such as Chicago, Detroit, Cleveland, Toronto, and Buffalo grew around the lakes as hubs of trade and industry. The St. Lawrence Seaway, opened in 1959, connects the lakes to the Atlantic, handling cargo like iron ore, coal, grain, and petroleum. The lakes are also a major recreational destination, drawing millions of visitors for boating, fishing, and beach activities. Culturally, the region has inspired art, literature, and music, including Gordon Lightfoot's ballad "The Wreck of the Edmund Fitzgerald" and the works of the Group of Seven painters. Indigenous nations, including Ojibwe, Odawa, and Haudenosaunee, have lived along the shores for millennia and continue to assert treaty fishing rights.

Beyond the well-known facts, the Great Lakes harbor many surprising details. Lake Michigan has a dangerous "underwater rip current" system that causes dozens of drownings each year, though it is often overlooked as a risk. ⁴ Lake Superior's water is so clear that visibility can exceed 30 feet and its temperature rarely rises above 55°F even in summer. The lakes contain shipwrecks numbering in the thousands—the Thunder Bay National Marine Sanctuary alone protects over 200 wrecks. A little-known geological feature is the "Nipissing Great Lakes," a higher post-glacial water level that created relict shorelines visible today. The lakes also experience seiches—standing waves that can shift water levels by several feet in minutes. The Great Lakes circle tour, a 6,500-mile scenic drive, is one of the longest continuous road trips in the world.

Lake-effect snow

Heavy snowfall that occurs when cold air passes over the relatively warm lake waters, picking up moisture and depositing it downwind.

Seiche

A standing wave oscillating in a water body, often caused by wind or atmospheric pressure changes, that can cause rapid water level fluctuations.

St. Lawrence Seaway

A system of locks, canals, and channels that enables navigation from the Great Lakes to the Atlantic Ocean.

Zebra mussel

An invasive bivalve mollusk introduced to the Great Lakes in the 1980s that has disrupted ecosystems and infrastructure.

Nipissing Great Lakes

A higher post-glacial water level stage of the Great Lakes, which existed about 5,000 years ago and left behind elevated shorelines.