Hudson Bay is a large (1.23 million km²), relatively shallow body of water in northeastern Canada. It is approximately 850 miles long and 650 miles wide. It drains a very large area that includes parts of Ontario, Quebec, Saskatchewan, Alberta, most of Manitoba, parts of North Dakota, South Dakota, Minnesota, and Montana, and the southeastern area of Nunavut. A smaller offshoot of the bay, James Bay, lies to the south. The International Hydrographic Organization lists Hudson Bay as part of the Arctic Ocean. On the east it is connected with the Atlantic Ocean by Hudson Strait, and on the north with the rest of the Arctic Ocean by Foxe Basin (which is not considered part of the bay) and Fury and Hecla Strait. Geographic coordinates: 78° to 95° W, 51° to 70° N.
The Eastern Cree name for Hudson and James Bay is Wînipekw or Wînipâkw , meaning muddy or brackish water. Lake Winnipeg is similarly named by the local Cree, as is the location for the City of Winnipeg.
Hudson Bay was the growth centre for the main ice sheet that covered northern North America during the last Ice Age. The whole region has very low year round average temperatures. (The average annual temperature for Churchill at 59°N is -5°C; by comparison Arkhangelsk at 64°N in a similar cold continental position in northern Russia has an average of 2°C.) Water temperature peaks at 8°-9°C on the western side of the bay in late summer. It is largely frozen over from mid-December to mid-June when it usually clears from its eastern end westwards and southwards. A steady increase in regional temperatures over the last 100 years has been reflected in a lengthening of the ice-free period which was as short as four months in the late 17th century.
Hudson Bay has a salinity that is lower than the world ocean on average. This is caused mainly by the low rate of evaporation (the bay is ice-covered for much of the year), the large volume of terrestrial runoff entering the bay (about 700 km³ annually; the Hudson Bay watershed covers much of Canada, with many rivers and streams discharging into the bay), and the limited connection with the larger Atlantic Ocean (and its higher salinity). The annual freeze-up and thaw of sea ice significantly alters the salinity of the surface layer, representing roughly three years' worth of river inflow.
The western shores of the bay are a lowland known as the "Hudson Bay Lowlands" which covers 324,000 km². The area is drained by a large number of rivers and has formed a characteristic vegetation known as muskeg. Much of the landform has been shaped by the actions of glaciers and the shrinkage of the bay over long periods of time. Signs of numerous former beachfronts can be seen far inland from the current shore. A large portion of the lowlands in the province of Ontario is part of the Polar Bear Provincial Park, and a similar portion of the lowlands in Manitoba is contained in Wapusk National Park, the latter location being a significant Polar Bear maternity denning area.
In contrast, most of the eastern shores (the Quebec portion) form the western edge of the Canadian Shield in Quebec. The area is rocky and hilly. Its vegetation is typically boreal forest, and to the north, tundra.
Measured by shoreline, Hudson Bay is the largest bay in the world (the largest in area being the Bay of Bengal).
There are many islands in Hudson Bay, mostly near the eastern coast. All are part of the territory Nunavut. One group of islands, with a reputable name, is the Belcher Islands. Another group includes the Ottawa Islands.