The Great Lakes are five large interconnected freshwater lakes in North America: Lake Superior, Lake Michigan, Lake Huron, Lake Erie and Lake Ontario. They form the largest group of freshwater lakes on Earth by total surface area and contain a very large proportion of the world's surface freshwater. The lakes are located along the United States Canada border, with Lake Michigan entirely within the United States and the other four shared by the two countries.
Together they form a connected hydrological system through which water flows generally from Lake Superior through Lakes Michigan and Huron, then Lake Erie, Lake Ontario and finally the St Lawrence River toward the Atlantic Ocean. Their enormous size, depth, ecological diversity, economic importance and international character make them one of the world's most significant freshwater systems.
Lake Superior is the largest of the five Great Lakes by surface area and is also the deepest and contains the greatest volume of water. It lies between Ontario, Minnesota, Wisconsin and Michigan. Lake Michigan is the second largest by surface area and is the only Great Lake located entirely within the United States, bordered by Michigan, Wisconsin, Illinois and Indiana. Lake Huron lies between Michigan and Ontario and is connected to Lake Michigan through the Straits of Mackinac.
Because the water levels of Lakes Michigan and Huron are effectively connected, they are sometimes treated hydrologically as one lake even though they are geographically named separately. Lake Erie lies farther east and is shallower than the upper Great Lakes, while Lake Ontario is the smallest by surface area but considerably deeper than Lake Erie.
The Great Lakes were formed primarily through the action of continental glaciers during the Pleistocene Ice Age. Repeated glacial advances eroded existing valleys and bedrock depressions and reshaped the landscape. As glaciers retreated, meltwater filled the excavated basins and established the interconnected drainage system that exists today. The geological history is complex because the basins were influenced by preexisting geological structures, glacial erosion, isostatic adjustment and changing drainage routes.
The lakes have continued to change since the last major glacial retreat because sediment accumulates, shorelines erode, land continues to adjust following the removal of glacial ice, and water levels vary with climate and hydrology. Their modern form is therefore the product of both ancient geology and ongoing physical processes.
The five lakes differ substantially in size, depth, volume, shoreline configuration and ecological conditions. Lake Superior has the greatest volume and depth, while Lake Erie is much shallower and responds more quickly to seasonal temperature changes. Lake Ontario is smaller in surface area than Erie but considerably deeper. Lake Michigan and Lake Huron are connected through the Straits of Mackinac and have very similar water levels because of their hydrological connection.
Water residence time also varies among the lakes, meaning that water remains in some lakes longer than in others before flowing downstream. These differences influence temperature, circulation, nutrient dynamics, ice formation, chemical conditions, fish habitats and responses to pollution. Treating all five lakes as physically identical would therefore obscure important differences between them.
The Great Lakes are connected by rivers, straits and channels. Water leaves Lake Superior through the St Marys River and enters Lake Huron. Lakes Michigan and Huron are connected through the Straits of Mackinac. Water then moves from Lake Huron toward Lake Erie through the St Clair River, Lake St Clair and the Detroit River. Lake Erie drains through the Niagara River into Lake Ontario, where the famous Niagara Falls occur along part of that connection.
Lake Ontario then drains through the St Lawrence River toward the Atlantic Ocean. This sequence creates a large integrated drainage system. Although precipitation, evaporation, runoff, groundwater and human withdrawals affect individual lakes differently, water moves through the system over time from the upper lakes toward the Atlantic.
The lakes have enormous economic importance. They provide drinking water to millions of people, support commercial and recreational fishing, facilitate shipping, provide opportunities for tourism and recreation, and support industries and agriculture in surrounding areas. The Great Lakes St Lawrence Seaway allows ships to transport bulk commodities between inland ports and the Atlantic Ocean. Iron ore, grain, limestone and other materials have historically been major cargoes.
Shoreline cities developed around ports and industrial facilities, contributing to the rise of manufacturing centres in the American Midwest and southern Ontario. The lakes also support beaches, boating, angling, tourism and other recreational activities. Their economic importance therefore extends beyond direct water supply to transportation, industrial production, regional development and services.
The Great Lakes are major ecological systems containing diverse fish, invertebrate, plant and bird communities. Native species include lake trout and other coldwater and warmwater fish, while invasive organisms have substantially altered some food webs. Sea lamprey contributed to severe declines in native fish populations before control programmes reduced their impact. Invasive zebra and quagga mussels have transformed nutrient cycling and filtering processes, while invasive plants and other organisms have also affected habitats. Nutrient pollution can produce harmful algal blooms, particularly in Lake Erie, where phosphorus loading has been a major environmental management issue. Restoration programmes address invasive species, habitat degradation, toxic contaminants, nutrient pollution, wetlands, fisheries and water quality.
The Great Lakes face continuing environmental pressures from climate change, invasive species, nutrient pollution, industrial contaminants, habitat loss, urban runoff, shoreline development and changing hydrological conditions. Climate change can increase water temperatures, alter ice cover, affect evaporation and precipitation, and influence lake levels and ecosystems. Water levels naturally fluctuate over time, and extreme high or low levels can damage shorelines, infrastructure, wetlands and navigation facilities. Binational governance is therefore essential.
The Great Lakes Water Quality Agreement between the United States and Canada provides a framework for addressing shared environmental issues, while other institutions coordinate fisheries, navigation, pollution control and ecosystem management. The Great Lakes are thus simultaneously geological formations, freshwater reservoirs, ecological systems, economic resources and internationally shared environments.
