Drainage basin

From Free net encyclopedia

A drainage basin (also known in North America as a watershed) is a region of land where water from rain or snowmelt drains downhill into a body of water, such as a river, lake, estuary, wetland, sea or ocean. The drainage basin includes both the streams and rivers that convey the water as well as the land surfaces from which water drains into those channels. The drainage basin acts like a funnel - collecting all the water within the area covered by the basin and channeling it into a waterway. Each drainage basin is separated topographically from adjacent basins by a ridge, hill or mountain, which is known as a water divide or sometimes a watershed (in those parts of the world where the drainage basin itself is not called a watershed).

In addition to drainage basin and watershed, the following terms can be used to describe the same concept:

catchment, catchment area, or catchment basin
drainage area
river basin
water basin

The largest drainage basin in the world is the Amazon, covering 6,144,727 km² (a third of the area of South America).

1 Physical description and formation
2 Ocean drainage basins
3 Endorheic drainage basins
4 Geopolitical boundaries
5 Hydrology
6 Ecology
7 Resource management
8 References
9 External links

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Physical description and formation

The two key defining characteristics of a drainage basin is the drainage divide that traces the perimeter of the basin, and the river or drainage network that conveys the surface water out of the basin. The two are closely related. Over geological time, outward movement of the divide would generally force the river network to expand as well, and growth of the river network generally forces the drainage divide to shift outward.

The shape of a drainage basin is determined by tectonics and geomorphic processes. Mountain ranges are natural drainage divides — the Amazon is bounded on the west by the Andes; the Mississippi is bounded on the west by the Rocky Mountains and on the east by the Appalachian Mountains. The shape of smaller basins is determined by erosion, and by those environmental factors that control erosion, such as the dip of any underlying geological feature.

Sediment eroded from a drainage basin is conveyed to the outlet, along with the water that transports it, along the glaciers, streams and rivers that comprise the drainage network. Over geologic time, this network changes. For example, gullies eroding into a hillside may erode so far as to lower the ridge or peak at the top of the hill. As this happens the position of the drainage divide shifts in the direction of the gully growth, and the drainage basin expands. If a river network erodes so much, it may start to drain pre-existing rivers by a process called stream capture. Because erosion tends to lead to the expansion of the river network, and hence of the drainage basin as well, adjacent drainage basins are in a way competing for land area. The drainage divide between the two becomes essentially fixed once the rate of erosion on one side of the divide equals the rate of erosion on the other. Therefore, as the intervening ridge erodes away, it simply lowers its elevation (unlike when a gully erodes it from just one side).

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Ocean drainage basins

Most of the Earth's surface drains into one of the five oceans. Basins that do not drain into an ocean are called endorheic (see below).

The Arctic Ocean drains Rupert's Land in northern Canada, and most of Russia.
The Atlantic Ocean drains the Saint Lawrence River and Great Lakes drainage basins, 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; the greatest portion of western Sub-Saharan Africa; and the drainage basins of the Caribbean and Mediterranean seas:

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

The Indian Ocean drains 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 basin includes 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 Southern Ocean drains Antarctica.

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Endorheic drainage basins

Endorheic drainage basins are inland basins that do not drain into an 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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Geopolitical boundaries

Drainage basins 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 vast majority of Canada's land area.

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

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Hydrology

In hydrology, the drainage basin is a logical unit of focus for studying the movement of water within the hydrologic cycle, because the majority of water that discharges from the basin outlet originated as precipitation falling on the basin. A portion of the water that enters the groundwater system beneath the drainage basin may flow towards the outlet of another drainage basin because groundwater flow directions do not always match those of their overlying drainage network. Measurement of the discharge of water from a basin may be made by a stream gauge located at the basin's outlet.

Rain gauge data is used to measure total precipitation over a drainage basin, 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.

Measurement of the discharge of water from a basin may be made by a stream gauge located at the basin's outlet.

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Ecology

Image:Mississippi Drainage Basin.jpg

Drainage basins are important elements to condsider also in ecology. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants. Like the water, they get transported towards the outlet of the basin, and can affect the ecological processes along the way as well as in the receiving water body.

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Resource management

Because drainage basins are coherent entites in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In Minnesota, U.S.A., governmental entities that perform this function are called watershed districts. In New Zealand, they are called catchment boards. Comparable community groups based in Ontario, Canada, are called conservation authorities. In North America this function is referred to as watershed management.

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