The Great Lakes and St. Lawrence River

Forming part of the U.S. Canadian border, the Great Lakes (244,100 square kilometers) consist of Lakes Superior, Michigan, Huron, Erie, and Ontario. They contain 18 percent of the world's supply of freshwater and are the largest system of fresh surface water on Earth. Historically, these lakes have provided water for consumption, transportation, fishing, recreation, power, and industry.

A dominant factor in industry and agriculture, by the 1950s the lakes were polluted and eutrophic (high nutrient concentrations) from municipal wastes, industrial pollution, and agricultural runoff, making them unfit in many areas for fish populations and human use. In an unprecedented move of international cooperation the United States and Canada are cleaning up and reviving this shared resource and cultural treasure.

Created by erosion and deposition during the glacial movements of the Pleistocene epoch (1.6 million years ago), the Great Lakes are interconnected by rivers, straits, and canals. The St. Lawrence Seaway (1959) links them with the Atlantic Ocean via the St. Lawrence River, the primary outlet of the ecosystem. Except during icy months, these passages serve as one of the world's busiest shipping areas, with over 50 million metric tons of freight each year.

The Great Lakes region was home to numerous Native American groups who fished and traded. When Europeans arrived in the 1500s, the lake waters were cool and clear. However, early settlers (1700s) deforested the basins for agriculture and timber, leading to erosion, sedimentation, and high nutrient loads.

Eventually more than two-thirds of the wetlands were drained. By the late 1800s unregulated industries and municipalities were using the lakes as dumping grounds for untreated wastes. Because outflows from the Great Lakes are small, these pollutants were retained in the lakes and became more concentrated with time. Additionally, the lakes' large surface area made them vulnerable to increasing atmospheric pollutants of the 1900s.

Industrial pollutants (polychlorinated biphenyls [PCBs], and pesticides (dichlorodiphenyl-trichloroeth- ane [DDT], added to water quality problems. Consequently, many beaches were closed, and water became nondrinkable, raising public concern. Without their natural pollution sinks (bodies that act as a storage device or disposal mechanism, such as forests and wetlands) and with increasing pollution loads, the Great Lakes became warmer and eutrophic. In the 1950s Lake Erie was declared "dead."

Fish populations also changed dramatically in the twentieth century, first through overfishing and then through the introduction of exotic species such as the parasitic sea lamprey, which virtually eliminated lake trout in Lakes Huron and Michigan. In 1986 zebra mussels, which filter and clean water by consuming algae, were introduced from Europe. However, the mussels also absorb toxic substances, causing ongoing toxic buildup higher in the food chain.

In response, the Great Lakes Water Quality Agreements of 1972 and 1978, expressing the commitment of the United States and Canada to maintain the chemical, physical, and biological integrity of the Great Lakes, replaced the original Boundary Waters Treaty of 1909. Although problems still exist, since the agreements, beaches have reopened, water is drinkable and clear, most fish are edible, Lake Erie has thriving fish populations again, and most point source (from a single place) pollution has been controlled.