Great Lakes ecoregion

This lake system contains the largest supply of freshwater in the world.

The Great Lakes system offsite link includes five large lakes, one small lake, four connecting channels, and the St. Lawrence Seaway. The large lakes are Superior, Michigan, Huron, Erie, and Ontario. They hold about 90% of the freshwater in the United States and approximately 20% of the world's freshwater supply. Forty million residents of the United States and Canada depend on this system for clean drinking water.

A satellite image of the great lakes with little cloud cover.

SeaWiFS satellite captured this view of the Great Lakes, including the turbid waters of Lake Erie, in this recent overpass. (Image credit: Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE)

How the Great Lakes formed

During the last ice age, the mile-thick Laurentide ice sheet covered most of Canada and the northern contiguous United States. The massive weight and movement of this glacier gouged out the earth to form the lake basins. About 20,000 years ago, the climate warmed and the ice sheet retreated. Water from the melting glacier filled the basins offsite link, forming the Great Lakes. Approximately 3,000 years ago, the Great Lakes reached their present shapes and sizes. Today, the Great Lakes ecoregion contains a variety of habitats, including aquatic, forest, marsh, wetland, and dune ecosystems. Widely varying climate, soils, and topography support more than 3,500 species of plants and animals.

Humans and the Great Lakes

Humans are also part of the Great Lakes system. Commercial and sport fishing, agriculture, recreation, tourism, manufacturing, and shipping are all important to the region. These activities create jobs and provide goods and services. The fishing industry extracts millions of pounds offsite link of fish per year from the lakes. Farmers within this watershed produce corn, soybeans, hay, milk, and other food products. The area is also known for its industry that produces steel, chemicals, and other products. The shipping opportunities in the Great Lakes played a critical role in settlement of the region and development of industry. Today more than 200 million tons of cargo pass through its waters each year.

Threats to the Great Lakes

Threats to the Great Lakes' ecosystems, include invasive species, climate change, pollution, and habitat destruction. Climate change affects water temperatures, weather patterns, and lake levels. Pollutants from residential, agricultural, and industrial areas reduce water quality. Land development decreases the amount of wildlife habitat. Fish populations have been declining offsite link in recent years as a result of these threats and increased fishing pressure.


The Great Lakes Literacy Principles offsite link provide a framework for educators teaching about the Great Lakes, helping teachers and students think about the Great Lakes as a system, rather than a set of unrelated parts. Thinking systemically can provide a greater understanding and help identify solutions to the issues threatening the region.