The five Great Lakes of North America — Superior, Michigan, Huron, Erie, and Ontario — together hold approximately 21 percent of the world's surface fresh water by volume, the largest connected freshwater system on the planet. The lakes occupy a total area of approximately 244,000 square kilometres along the border between the United States and Canada, drain a basin of approximately 766,000 square kilometres, and ultimately discharge through the St. Lawrence River to the Atlantic Ocean. Eight US states and one Canadian province border the lakes; about 35 million people live in the basin; cities including Chicago, Toronto, Cleveland, Milwaukee, Detroit, Buffalo, and Hamilton sit on their shores. The lakes were formed during the retreat of the Laurentide Ice Sheet at the end of the last glacial period, between approximately 14,000 and 10,000 years ago. They are geologically young, ecologically complex, economically central to a region that contains some of North America's largest industrial cities, and at the centre of one of the more ambitious cross-border environmental management regimes in the world.
Why There Are Five
The Great Lakes occupy a series of basins carved by repeated cycles of continental glaciation over the past 2 million years. The bedrock geology — softer sedimentary rocks within a basin of older harder Precambrian rock — provided pre-existing topographic depressions that the advancing ice sheets enlarged through scouring. As the Laurentide Ice Sheet retreated at the end of the last glacial maximum, meltwater filled the depressions. The configuration of the final lake system was determined by the interaction of glacial isostatic rebound (the land rising as the ice weight was removed), the drainage outlets that opened and closed as ice dams retreated, and the position of the final ice retreat. Lake Superior is the largest by area; Lake Michigan and Lake Huron are technically a single hydrological lake (they have the same elevation and exchange water freely through the Straits of Mackinac) and are sometimes treated as one body called "Michigan-Huron"; Lake Erie is the shallowest and southernmost; Lake Ontario is the smallest by area but second-deepest. The lakes drain in sequence from Superior down through the others to Ontario, then out via the St. Lawrence.
The Volume Question
The 21 percent of world surface fresh water figure surprises many people. The lakes are not the deepest in the world (Lake Baikal in Siberia is much deeper and holds more water by volume) but they are vast in area. Lake Superior alone is the largest freshwater lake in the world by surface area at approximately 82,100 square kilometres. The combined volume of the five lakes is approximately 22,800 cubic kilometres. The water residence time — how long a typical water molecule stays in the lake before discharging downstream — is approximately 191 years for Superior, 99 years for Michigan, 22 years for Huron, 2.6 years for Erie, and 6 years for Ontario. Because of the very long residence times in the upper lakes, pollutants and other introductions to those lakes persist for human-relevant timescales.
The Industrial History
The Great Lakes basin became the industrial heartland of North America in the late 19th and early 20th centuries. The combination of accessible iron ore (from the Mesabi and other Lake Superior ranges), coal (from Appalachian and Illinois basins, reachable by lake and rail transport), and water transportation produced the steel industries of Pittsburgh-Cleveland-Detroit-Hamilton-Buffalo and the heavy manufacturing of Chicago-Milwaukee-Toledo. The opening of the Erie Canal in 1825 connected the lakes to the Atlantic via New York, and the opening of the Welland Canal around Niagara Falls in 1829 and the St. Lawrence Seaway in 1959 connected them more directly to ocean shipping. The "Rust Belt" transition of the late 20th century — the decline of the heavy industries that had built the basin's economy — has been one of the major economic and political stories of the region. The lakes themselves were heavily polluted during the industrial peak; Lake Erie was infamously described as "dying" in the 1960s and 1970s, with the 1969 Cuyahoga River fire in Cleveland (the river was so polluted it caught fire) becoming a symbol of the broader environmental decline.
The Cleanup
The 1972 Great Lakes Water Quality Agreement between the US and Canada was the foundational document of the modern cleanup. Subsequent amendments and renewals (1978, 1987, 2012) have substantially upgraded the agreement's scope and ambition. The Great Lakes have substantially recovered from the industrial-era pollution peak. Lake Erie's dissolved oxygen and phosphorus levels improved dramatically from the 1970s through the 1990s. The lakes' fisheries have partially recovered, though they remain different from their pre-industrial baselines. The Areas of Concern programme has identified and progressively cleaned up 43 specific contaminated areas (toxic sediments, urban industrial sites, and similar legacy issues), with about half now formally delisted. The cleanup has been incomplete — phosphorus loading from agricultural runoff is producing renewed algal bloom problems in Lake Erie in particular, and microplastic pollution is an emerging concern across the system — but the overall trajectory has been substantially positive.
The Invasive Species Problem
One of the most consequential issues for the lakes has been the introduction of invasive species. Over 180 non-native species have been documented in the basin. Some of them — particularly zebra and quagga mussels, which arrived in the late 1980s in ballast water from European ships — have substantially altered the lake ecosystems. The mussels are extreme filter feeders; they clear the water column of plankton, which dramatically improves water clarity but starves the food chain that supports native fish. Sea lampreys, accidentally introduced after improvements to the Welland Canal allowed them to bypass Niagara Falls, devastated the lake trout populations of the upper lakes in the mid-20th century before the chemical lampricide TFM brought them under control. Asian carp — bighead and silver carp introduced to the lower Mississippi system for aquaculture — have been expanding northward and pose a serious threat to the lakes' fisheries if they cross from the Mississippi system into Lake Michigan via the Chicago canal system. Substantial effort has gone into preventing this crossing through a combination of electrical barriers and proposed physical separation.
The Niagara Connection
The lake system's most famous feature is Niagara Falls, where Lake Erie water discharges over a cliff face approximately 50 metres high on its way down to Lake Ontario. The falls are not the highest in the world (they don't even crack the global top hundred for height) but they are extraordinarily voluminous — the Horseshoe Falls section of Niagara carries about 2,400 cubic metres per second of water at average flow, one of the highest discharge rates of any waterfall on Earth. The falls have been a tourist attraction since the late 18th century and have become one of the most visited natural features in North America. They are also a substantial source of hydroelectric power; about half of the natural flow is diverted upstream of the falls to power stations on both the American and Canadian sides during high-demand periods, with the falls themselves looking diminished accordingly at those times.
The Indigenous History
Before European contact, the Great Lakes basin was home to numerous Indigenous nations: the Haudenosaunee (Iroquois Confederacy) east and south of Lake Ontario, the Anishinaabe (Ojibwe, Odawa, Potawatomi) around the upper lakes, the Wendat (Huron) and Petun around Lake Huron, the Erie around the lake bearing their name, and many others. The basin was a centre of pre-contact agriculture (corn, beans, and squash, particularly in the southern part of the basin) and extensive trade networks. The lakes were central to French and British colonial expansion in the 17th and 18th centuries; the fur trade depended heavily on the lake-and-river transport network. The dispossession of Indigenous populations through the 19th and early 20th centuries was substantial; treaty rights to fishing and other lake resources remain active legal and political issues, with several major court decisions in recent decades affirming Indigenous fishing rights against state and provincial restrictions.
The Climate Question
The lakes interact with regional climate in complex ways. They moderate temperatures in the immediate lakeshore areas (warmer winters, cooler summers compared to inland), producing distinctive lake-effect weather. The lake-effect snow downwind of the lakes — particularly in the snowbelt regions of upstate New York, northern Indiana, Michigan's Upper Peninsula, and southern Ontario — can produce enormous snowfall events when cold air crosses over still-unfrozen lake water in early winter. The lakes are warming under contemporary climate change; surface temperature increases of approximately 1°C since the late 20th century have been documented across the system. Winter ice cover has decreased substantially. The implications for fisheries, lake-effect weather patterns, and shoreline ecology are still emerging.
The Future
The Great Lakes are one of the most consequential freshwater systems in the world and one of the most actively managed. The combination of cross-border legal regimes, ongoing pollution challenges (particularly agricultural phosphorus loading), invasive species pressures, climate-driven changes in temperature and ice cover, and the urban water-supply needs of approximately 35 million people in the basin makes the management of the system one of the major environmental governance projects in North America. The basin's industrial trajectory is shifting; some of the heavy industry is gone, some of it has been replaced by knowledge and service economies, some of the urban regions are seeing renewed growth, others are continuing to decline. The lakes themselves will continue regardless of those human trajectories, though their condition will depend substantially on the choices made within the basin and beyond it over the coming decades.