The Great Lakes: Freshwater Seas
The Great Lakes of North America—Superior, Huron, Michigan, Erie, and Ontario—represent the world's largest freshwater system by surface area, containing approximately 21% of global freshwater supplies while supporting major industrial development, international shipping, and complex ecosystems that demonstrate both the benefits and challenges of managing shared freshwater resources between nations.
Lake Superior, the largest of the Great Lakes by both surface area (82,000 square kilometers) and volume, contains more water than all the other Great Lakes combined while maintaining remarkably clear, cold water that supports unique ecosystems including some of the world's most pristine freshwater environments. The lake's maximum depth of 406 meters and enormous volume create thermal inertia that moderates regional climate while preventing the lake from freezing completely even during severe winters.
The Great Lakes system functions as a single hydrological unit connected by rivers, straits, and human-made canals that enable water and shipping traffic to flow from the western end of Lake Superior to the Atlantic Ocean via the Saint Lawrence Seaway. This connection has made the Great Lakes region a major industrial and transportation hub, with cities such as Chicago, Detroit, Cleveland, and Toronto developing around ports that provide access to both the Great Lakes and global shipping networks.
Industrial development around the Great Lakes has created both economic prosperity and environmental challenges, with heavy industry, steel production, automobile manufacturing, and chemical processing contributing to water pollution, habitat destruction, and invasive species problems that require ongoing management efforts. The region's industrial legacy includes contaminated sediments, polluted harbors, and areas of environmental degradation that have taken decades to remediate through international cooperation and environmental regulations.
Invasive species represent one of the most significant ongoing threats to Great Lakes ecosystems, with over 180 non-native species established in the lakes through shipping ballast water, canal connections, and human introductions. The zebra mussel invasion beginning in the 1980s fundamentally altered lake ecosystems by filtering massive amounts of water and changing nutrient cycling, while more recent threats include Asian carp species that could disrupt entire food webs if they become established in the lakes.
The Great Lakes Water Quality Agreement between the United States and Canada represents one of the world's most comprehensive international environmental treaties, addressing pollution control, habitat restoration, and ecosystem management through binational cooperation that has achieved significant improvements in water quality while establishing frameworks for ongoing environmental protection and restoration efforts.
Climate change impacts on the Great Lakes include rising water temperatures, altered precipitation patterns, more frequent extreme weather events, and potential changes in lake levels that could affect shipping, coastal communities, and ecosystem functioning. Warmer temperatures extend the growing season for both beneficial and harmful organisms while potentially altering the timing of seasonal processes such as ice formation, spring turnover, and fish spawning.
The Great Lakes support major commercial and recreational fisheries worth hundreds of millions of dollars annually while providing recreational opportunities for millions of people. However, fish populations have been dramatically altered by overfishing, habitat loss, pollution, and invasive species, requiring ongoing management efforts including fish stocking programs, habitat restoration, and fishing regulations to maintain sustainable fisheries.
Water diversions and consumption from the Great Lakes create ongoing political and legal challenges, with proposals to export water to arid regions generating controversy over water rights, ecosystem protection, and international law. The Great Lakes Compact, signed by Great Lakes states and provinces, establishes legal frameworks for protecting Great Lakes water while allowing limited diversions under specific circumstances that prioritize ecosystem protection and sustainable use.