Great Lakes

The Great Lakes lie in east-central North America. From west to east, the five principal lakes are Superior, Michigan, Huron, Erie and Ontario. Lake Michigan is entirely within the United States; the other four straddle the U.S.–Canada border. Their connecting rivers and channels make them a single hydrologic system, whose combined surface area is about 244,000 square kilometres. By surface area, it is the largest system of freshwater lakes in the world. The wider Great Lakes basin also includes streams, wetlands and land that drains into the lakes.

The region’s present landscape was strongly shaped by the advance and retreat of continental glaciers during the Pleistocene. The Laurentide Ice Sheet scoured and enlarged existing valleys; meltwater later filled the resulting basins as the ice retreated. The lakes are linked in a mainly eastward drainage route: Superior flows toward Huron, Huron toward Erie, Erie toward Ontario, and Ontario drains into the St. Lawrence River and ultimately the Atlantic Ocean. Michigan and Huron, joined by the Straits of Mackinac, have the same water level and are often treated as one lake hydrologically, though they remain distinct named lakes.

Together, the lakes hold roughly one-fifth of the world’s surface freshwater supply and about 84 percent of North America’s surface fresh water. These proportions refer to surface freshwater, not all water stored underground or in ice. The system is an important source of drinking water and supports fisheries, recreation, tourism, manufacturing and commercial navigation. NOAA reports that about 40 million people in the United States and Canada depend on the Great Lakes system for clean drinking water, and that more than 200 million tons of cargo move through its waters each year. The basin’s communities and economies are closely connected to the condition of the lakes and their tributaries.

The Great Lakes region contains aquatic and coastal habitats as well as forests, marshes, wetlands and dunes. Its ecological health is affected by conditions across the basin, not only by events on open water. NOAA identifies invasive species, climate change, pollution and habitat destruction among the threats to the region. Nutrient pollution can contribute to excessive algal growth, while shoreline development and other pressures can alter habitat and water quality. These issues vary across the lakes and require monitoring and management at both local and basin-wide scales.

Because the lakes and their tributaries cross an international boundary, their protection depends on cooperation between Canada and the United States. The two countries signed the Great Lakes Water Quality Agreement in 1972 and revised it in 1978, 1987 and 2012. The agreement commits them to cooperative action to restore and maintain the chemical, physical and biological integrity of Great Lakes waters. The International Joint Commission assesses progress, consults the public and advises governments on implementation. The agreement provides a framework for shared work; it does not eliminate the environmental challenges facing the basin.