Several other minor lakes and rivers are also included in the Great Lakes region including the Niagra River, Detroit River, St. Lawrence River, St. Marys River, and the Georgian Bay. There are 35,000 islands estimated to be located on the Great Lakes, created by years of glacial activity.
Interestingly, Lake Michigan and Lake Huron are connected by the Straits of Mackinac, and can be technically considered a single lake.
The Formation of the Great LakesThe Great Lakes Basin (the Great Lakes and the surrounding area) began to form about two billion years ago – almost two-thirds the age of the earth. During this period, major volcanic activity and geologic stresses formed the mountain systems of North America, and after significant erosion, several depressions in the ground were carved. Some two billion years later the surrounding seas continuously flooded the area, further eroding the landscape and leaving a lot of water behind as they went away.
More recently, about two million years ago, it was glaciers that advanced over and back across the land. The glaciers were upwards of 6,500 feet thick and further depressed the Great Lakes Basin. When the glaciers finally retreated and melted approximately 15,000 years ago, massive quantities of water were left behind. It is these glacier waters that form the Great Lake today.
Many glacial features are still visible on the Great Lakes Basin today in the form of "glacial drift," groups of sand, silt, clay and other unorganized debris deposited by a glacier. Moraines, till plains, drumlins, and eskers are some of the most common features that remain.
The Industrial Great LakesThe shorelines of the Great Lakes stretch a little over 10,000 miles (16,000 km), touching eight states in the U.S. and Ontario in Canada, and make an excellent site for the transportation of goods. It was the primary route used by early explorers of North America, and was a major reason for the great industrial growth of the Midwest throughout the 19th and 20th century.
Today, 200 million tons a year are transported using this waterway. Major cargoes include iron ore (and other mine products), iron and steel, agriculture, and manufactured goods. The Great Lakes Basin also is home to 25%, and 7% of Canadian and U.S. agricultural production, respectively.
Cargo ships are aided by the system of canals and locks built on and between the lakes and rivers of the Great Lakes Basin. The two major sets of locks and canals are:
1) The Great Lakes Seaway, consisting of the Welland Canal and the Soo Locks, allowing ships to pass by the Niagra Falls and the rapids of the St. Marys River.
2) The St. Lawrence Seaway, extending from Montreal to Lake Erie, connecting the Great Lakes to the Atlantic Ocean.
Altogether this transportation network makes it possible for ships to travel a total distance of 2,340 miles (2765 km), all the way from Duluth, Minnesota to the Gulf of St. Lawrence.
In order to avoid collisions when traveling on the rivers connecting the Great Lakes, ships travel "upbound" (west) and "downbound" (east) in shipping lanes. There are around 65 ports located on Great Lakes-St. Lawrence Seaway system. 15 are international, and include: Burns Harbor at Portage, Detroit, Duluth-Superior, Hamilton, Lorain, Milwaukee, Montreal, Ogdensburg, Oswego, Quebec, Sept-Iles, Thunder Bay, Toldeo, Toronto, Valleyfiled, and Port Windsor.
Great Lakes RecreationAbout 70 million people visit these the Great Lakes every year to enjoy their water and beaches. Sandstone cliffs, high dunes, extensive trails, campgrounds, and diverse wildlife are just some of the many attractions of the Great Lakes. It is estimated that $15 billion is spent every year for leisure activities every year.
Sport fishing is a very common activity, partly because of the Great Lakes' size, and also because the lakes are stocked year after year. Some of the fish include bass, bluegill, crappie, perch, pike, trout, and walleye. Some non-native species such as salmon and hybrid breeds have been introduced, but have generally not succeeded. Chartered fishing tours are a major part of the Great Lakes tourism industry.
Spas and clinics are popular tourist attractions also, and couple well with some of the serene waters of the Great Lakes. Pleasure-boating is another common activity, and is more successful than ever as more and more canals are built to connect the lakes and surrounding rivers.
Great Lakes Pollution and Invasive SpeciesUnfortunately, there have been concerns about the quality of water of the Great Lakes. Industrial waste and sewage were the primary culprits, specifically phosphorus, fertilizer, and toxic chemicals. In order to control this issue, the governments of Canada and the United States joined to sign the Great Lakes Water Quality Agreement in 1972. Such measures have drastically improved the quality of water, though pollution still finds it way in to the waters, primarily through agricultural runoff.
Another major concern in the Great Lakes is non-native invasive species. Unanticipated introduction of such species can drastically alter evolved food chains and destroy local ecosystems. The end result of this is a loss in biodiversity. Well known invasive species include the zebra mussel, Pacific salmon, carp, lamprey, and alewife.