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A watershed is the region of land that drains into a specified body of water, such as a river, lake, sea, or ocean. Rain that falls anywhere within a given body of water's watershed will eventually drain into that body of water.

The extent of a watershed is determined by topography, with mountain ridges dividing up watersheds.

Watersheds in ecology

Watersheds constitute a very important type of ecoregion. Pollution anywhere within the watershed can potentially affect life anywhere downstream from it.

Watersheds in politics

Watersheds have been important historically in determining boundaries, particularly in regions where trade by water has been important. For example, the English crown gave the Hudson's Bay Company a monopoly on the Indian trade[?] in the entire Hudson Bay watershed, an area called Rupert's Land. The company later acquired the North American watershed of the Arctic Ocean (the North-West Territory[?]). These lands later became part of Canada as the Northwest Territories, making up the majority of Canada's land area.

Today, bioregional democracy can include agreements of states in a particular watershed to defend it. These include the Great Lakes Commission, which deals with the largest fresh watershed in the world.

Ocean watersheds

One can divide up the world among the watersheds of the oceans and largest seas.

The Atlantic Ocean watershed consists of the Saint Lawrence River and Great Lakes watersheds, plus the Eastern Seaboard[?], Canadian Maritimes, Newfoundland and Labrador in North America; nearly all of South America (that portion east of the Andes); northern Europe; and the greatest portion of western Sub-Saharan Africa.

The Caribbean Sea watershed consists of all of the American interior (the Louisiana Purchase, which involved the watershed of the Mississippi River); eastern Central America; and far northern South America.

The Mediterranean Sea watershed consists of much of northeastern Africa, including Egypt, Libya, and Sudan (the Nile watershed), as well as southern and eastern Europe, Turkey, and the Levant.

Of course, the previous two can be considered part of the Atlantic watershed, since the Caribbean Sea is part of the Atlantic ocean and drains into the Mediterranean. (higher evapuration)

The Indian Ocean watershed consists of the eastern coast of Africa, the coasts of the Red Sea and the Persian Gulf, the Indian subcontinent, Burma, and most of Australia.

The Pacific Ocean watershed consists of much of China, southeastern Russia, Japan, Korea, most of Indonesia and Malaysia, the Philippines, the rest of the Pacific islands, and the northeast coast of Australia; as well as Alaska, British Columbia, the western United States and Central America, and the coast of South America (the smaller portion west of the Andes).

The Arctic Ocean watershed consists of the aforementioned Rupert's Land, and most of the territory of Russia.

In addition to the oceanic watersheds, a portion of the Earth's land surface consists of inland basins, which drain into no ocean. The largest of these consists of much of the interior of Asia, and drains into the Caspian Sea and the Aral Sea. Other basins include the Great Basin in the United States, much of the Sahara Desert, the watershed of the Okavango River[?], highlands near the African Great Lakes[?], the interiors of Australia and the Arabian Peninsula, and parts in Mexico and the Andes.

See hydrology.

Watershed is also a term used to describe a time in television schedules beyond which it is permissable to show a television programme which have an 'adult content', ie nudity, explicit sex, violence, strong language. In the United Kingdom and the Republic of Ireland the specified Watershed time in 9pm.

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