Hudson Bay, inland sea of North America, c.475,000 sq mi (1,230,000 sq km), c.850 mi (1,370 km) long and c.650 mi (1,050 km) wide, E central Canada. Hudson Bay and James Bay (its southern extension) and all their islands border Nunavut Territory, Manitoba, Ontario, and Quebec. Hudson Strait (c.450 mi/720 km long) connects Hudson Bay with the Atlantic Ocean, and Foxe Channel leads to the Arctic Ocean. Mansel, Coats, and Southampton islands are at the northern end of the bay. Hudson Bay occupies the southernmost portion of the Hudson Bay Lowlands, a depression in the Canadian Shield formed during the Pleistocene epoch by the weight of the continental ice sheet. As the ice retreated, the region was flooded by the sea, and sediments were deposited in it. With the burden of ice removed, the floor of the lowlands has been slowly rising and the bay is gradually becoming shallower. The western shores are generally low and marshy and covered by tundra, while the east coast is barren and rocky, with the Ottawa and Belcher island groups offshore. Many rivers, including the Churchill and Nelson, drain into the bay. Hudson Bay moderates the local climate; it is ice-free and open to navigation from mid-July to October. The bay was explored and named (1610) by Henry Hudson in his search for the Northwest Passage. The surrounding region was a rich source of furs, and France and England struggled for its possession until 1713, when France ceded its claim by the Peace of Utrecht. Hudson's Bay Company set up many trading posts there, especially at river mouths; some of the posts have operated continuously since 1670. The Hudson Bay Railway (opened 1929) links the prairie provinces with Churchill, Man., a port for oceangoing freighters.

Inland sea, indenting east-central Canada. With an area of 480,000 sq mi (1,243,000 sq km), it is bounded by Nunavut, Manitoba, Ontario, and Quebec. It is connected with the Atlantic Ocean via the Hudson Strait and with the Arctic Ocean via the Foxe Channel. Named for Henry Hudson, who navigated its eastern coast in 1610, the bay and the surrounding area, known as Rupert's Land, were controlled by the Hudson's Bay Company (1821–69). Hudson Bay is shallow, with an average depth of 330 ft (100 m); the coast is mainly a marshy lowland. The islands it contains are administratively part of Nunavut. For conservation purposes, the Canadian government has designated the whole Hudson Bay basin a “mare clausum” (closed sea).

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