city, seat of Thunder Bay district, west-central Ontario, Canada, on Lake Superior's Thunder Bay, at the mouth of the Kaministiquia River. Probably first occupied by French fur traders as early as 1678, its site was permanently settled only after the birth of the towns Port Arthur and Fort William in the 19th century. Fort William originated shortly after 1800, when the North West Company built a fur-trapping fort at the mouth of the Kaministiquia River. Port Arthur, the “Hill City,” developed in the 1850s as a silver-mining settlement on the hummocks a few miles to the north. Both communities prospered in the early 1870s from silver strikes in the vicinity and again with the arrival of the Canadian Pacific Railway in the 1870s and '80s. An intense rivalry that existed between the two for some time was resolved with the unification of their harbour facilities in 1906. Plans to amalgamate the twin cities began in the 1950s, and the merger created the city of Thunder Bay in 1970.

inlet of northwestern Lake Superior, indenting the coast of west-central Ontario, Canada. The bay is 35 miles (55 km) long and 15 miles (24 km) wide; it receives the Kaministiquia and Current rivers. Pie Island lies at the entrance to the bay, and Sibley Provincial Park, 94 square miles (243 square km), is located on the peninsula that forms the bay's eastern shore. The important port city of Thunder Bay is situated on the western shore.