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For other senses of watershed, see watershed (disambiguation).

A 'watershed' is a region of land where water flows into a specified body of water, such as a river, lake, sea, or ocean. In this sense it is also called water basin or catchment basin, and this is the sense intended in this article, unless otherwise indicated. The boundary of a watershed is called water divide or "watershed" (European terminology); the first term will be used, to avoid confusion.

Rain that falls anywhere within a given body of water's watershed or basin will eventually drain into that body of water. A map of the primary watersheds in the world can be found at [1]. A watershed that drains into a river is usually known as a river basin.

Each area of a drainage basin has its own drainage system.


Watersheds in ecology

Watersheds constitute a very important type of ecoregion. They do things such as provide habitats for animals, lessen flooding, and prevent erosion. 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-Western 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.

Analyzing watersheds

Rain gauge data is used to measure total precipitation over a watersheds, and there are different ways to interpret that data.

If the gauges are many and evenly distributed over an area of uniform precipitation, using the arithmetic mean method will give good results.

In the Thiessen polygon method, the watershed is divided into polygons with the rain gauge in the middle of each polygon assumed to be representative for the rainfall on the area of land included in its polygon. These polygons are made by drawing lines between gauges, then making perpendicular bisectors of those lines form the polygons.

The isohyetal method involves contours of equal precipitation are drawn over the gauges on a map. Calculating the area between these curves and adding up the volume of water is time consuming.

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, the western Mississippi River watershed; as well as the eastern half such as the Ohio River and its tributaries); 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 the Atlantic drains into the Mediterranean due to its higher evaporation.

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, there are numerous endorheic watersheds, 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.

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