Physical features

The Great Lakes are a natural feature of the Earth. Their basins were probably scooped out by glaciers about 1 million years ago during the Ice Age. Most geologists believe that the lakes occupy old river valleys that were made larger by the movement of the glaciers.

The total area of all the lakes is about 94,850 square miles (245,660 square kilometers). Lake Michigan is the only one of the five that is completely within the United States. The remaining four form a natural border between Canada and the United States. Lake Superior is the deepest as well as the largest; Lake Ontario is the smallest; Lake Erie is the shallowest; and Lake Huron has the longest shoreline.

Both natural and man-made sources supply water to the Great Lakes. The single largest source of water is rainfall. The lakes are also fed from numerous streams, creeks, brooks, and small rivers, as well as from underground water that seeps into the lakes. The lakes form a continuous waterway and drainage system. The excess water from the higher lakes flows into the lower lakes in the chain. A small percentage of water is added artificially. In one such case, water is diverted from the Hudson Bay watershed into Lake Superior.

The level of water in the Great Lakes changes greatly from time to time. This happens with all natural lakes. This is due mainly to the amount of rainfall that the lakes receive during a particular season.

Climate

The lakes influence the climate of the surrounding regions. They absorb a large quantity of heat in the warmer months. The heat is lost to the atmosphere in the colder months. This leads to cooler summers and warmer winters in the areas closest to the lakes. Severe storms can occur over the lakes, particularly in late fall and early winter. The strong winds sometimes generate waves that reach 10 feet (3 meters) or higher. Large areas of the surface waters freeze during winter, though open patches usually remain in the centers of the lakes.

Animal life in the lakes

The Great Lakes have several forms of algae as well as microscopic crustaceans. The spiny water flea and the zebra mussel gained access to the lakes during the 1980s. Both have threatened the food web of native species. The zebra mussels became a nuisance by clogging water-intake pipes and covering the hulls of boats. Perch, walleye, bass, and catfish are abundant in the shallow, warmer waters. The fish community also includes lake trout, whitefish, and lake herring. Alewives entered the Great Lakes through the St. Lawrence Seaway. They multiplied so rapidly that they became a nuisance, threatening the native fishes by competing for the same food sources. Coho and king salmon were introduced to control the alewife population.

Herring and ring-billed gulls and terns are the most common birds. Ducks are known to spend winters on the lakes, and many shorebirds and songbirds migrate through the region.

Economy

The Great Lakes play a significant role in the economy of North America. Ships carry large quantities of iron ore, coal, grain, and manufactured goods annually between lake ports as well as to overseas ports through the St. Lawrence Seaway. The seaway was constructed in the 1950s to connect the lakes with the Atlantic Ocean. In addition to their role in shipping, the lakes also supply water to bordering industries and cities.

The Great Lakes are among the nation's most popular recreation areas. Power boating and sailing have become major recreational pastimes. There are many beaches that stretch along the lake shores. Hundreds of camping, picnic, and park areas add to the successful tourist industry.

Commercial fishing was once an important industry on the lakes, but populations of the more desirable species of fish declined in the 20th century because of pollution in the lakes. A limited amount of commercial fishing still takes place, especially for the popular whitefish.

Study and exploration

Various Native American tribes inhabited the Great Lakes region for many centuries. The first Europeans entered this region most likely in 1535, when the French explorer Jacques Cartier traveled up the St. Lawrence River to the site of modern Montreal. Another French explorer, Samuel de Champlain, reached Lake Huron in 1615. He is said to have been the first European to see the Great Lakes. Other French explorers made peace with the Indians and established early settlements. During the War of 1812, Lake Erie was the site of a major naval battle.

Since 1848 several scientific expeditions have been undertaken in this region. Studies have been made of fluctuating water levels, plants and animals, and currents. More recent studies have been carried out to understand the erosion of the shorelines and how the lakes might be affected by the problem known as global warming.

In the 20th century the water in the Great Lakes became polluted by chemicals and other products from the factories on the shores of the lakes and by sewage from the cities. This pollution harms the fish and wildlife in the area. To fight this problem, the governments of the United States and Canada have instituted programs for cleaning the lakes.