Thunder Bay, city (1991 pop. 113,946), SW Ont., Canada, on Thunder Bay inlet of Lake Superior. The city was created in 1970 by the amalgamation of the twin cities of Fort William and Port Arthur and two adjoining townships. It is one of Canada's major ports, shipping wheat, lumber, coal, and iron ore. The city has shipyards, grain elevators, lumber and pulp and paper mills, breweries, and an oil refinery. Manufactures include structural steel, buses, trucks, aircraft, and chemical products. Port Arthur, originally a military post, was founded in the late 19th cent. Fort William was built by the North West Company in 1801 to serve as its western headquarters. It was the site of a fur-trading post built in 1679 and of Fort Kaministikwia, built by the French in 1717 and later abandoned. Kakabeka Falls, nearby, is a source of water power. Lakehead Univ. is located there.