Lake Erie is the fourth largest lake of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also has the shortest average water residence time. At its deepest point Lake Erie is 210 feet (64 m) deep.
Situated on the International Boundary between Canada and the United States, Lake Erie's northern shore is the Canadian province of Ontario, specifically the Ontario Peninsula, with the U.S. states of Michigan, Ohio, Pennsylvania, and New York on its western, southern, and eastern shores. These jurisdictions divide the surface area of the lake with water boundaries. The largest city on the lake is Cleveland, anchoring the third largest U.S. metro area in the Great Lakes region, after Greater Chicago and Metro Detroit. Other major cities along the lake shore include Buffalo, New York; Erie, Pennsylvania; and Toledo, Ohio.
Situated below Lake Huron, Erie's primary inlet is the Detroit River. The main natural outflow from the lake is via the Niagara River, which provides hydroelectric power to Canada and the U.S. as it spins huge turbines near Niagara Falls at Lewiston, New York and Queenston, Ontario. Some outflow occurs via the Welland Canal, part of the Saint Lawrence Seaway, which diverts water for ship passages from Port Colborne, Ontario on Lake Erie, to St. Catharines on Lake Ontario, an elevation difference of 326 ft (99 m). Lake Erie's environmental health has been an ongoing concern for decades, with issues such as overfishing, pollution, algae blooms, and eutrophication generating headlines.
Lake Erie has a complex ecosystem with many species in interaction. Human activity, such as pollution and maritime ship traffic, can affect this environment in numerous ways. The interactions between new species can sometimes have beneficial effects, as well as harmful effects. Some introductions have been seen as beneficial such as the introduction of Pacific salmon. Occasionally there have been mass die-offs of certain species of fish, sometimes for reasons unknown, such as many numbers of rainbow smelt in May 2010.