Drainage basin

A drainage basin is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. The drainage basin includes all the surface water from rain runoff, snowmelt, and nearby streams that run downslope towards the shared outlet, as well as the groundwater underneath the earth's surface.[1] Drainage basins connect into other drainage basins at lower elevations in a hierarchical pattern, with smaller sub-drainage basins, which in turn drain into another common outlet.[2]

Other terms used interchangeably with drainage basin are catchment area, catchment basin, drainage area, river basin, and water basin.[3] In North America, the term watershed is commonly used to mean a drainage basin, though in other English-speaking countries, it is used only in its original sense, that of a drainage divide.

In a closed drainage basin, or endorheic basin, the water converges to a single point inside the basin, known as a sink, which may be a permanent lake, a dry lake, or a point where surface water is lost underground.[4]

The drainage basin acts as a funnel by collecting all the water within the area covered by the basin and channelling it to a single point. Each drainage basin is separated topographically from adjacent basins by a perimeter, the drainage divide, making up a succession of higher geographical features (such as a ridge, hill or mountains) forming a barrier.

Drainage basins are similar but not identical to hydrologic units, which are drainage areas delineated so as to nest into a multi-level hierarchical drainage system. Hydrologic units are defined to allow multiple inlets, outlets, or sinks. In a strict sense, all drainage basins are hydrologic units but not all hydrologic units are drainage basins.[4]

Hydrographic basin
Illustration of a drainage basin. The dashed line is the main water divide of the hydrographic basin.
EN Bazinul hidrografic al Raului Latorita, Romania
Digital terrain map of the Latorița River's drainage basin in Romania
Modelare 3D pentru Bazinul Hidrografic al Paraului Latorita
Digital terrain model of the Latorița River's drainage basin in Romania

Major drainage basins of the world


Major continental divides, showing drainage into the major oceans and seas of the world.

Ocean basins

The following is a list of the major ocean basins:

Largest river basins

The five largest river basins (by area), from largest to smallest, are the basins of the Amazon (7M km2), the Congo (4M km2), the Nile (3.4M km2), the Río de la Plata (3.2M km2), and the Mississippi (3M km2). The three rivers that drain the most water, from most to least, are the Amazon, Ganga, and Congo rivers.[5]

Endorheic drainage basins

Endorheic drainage basins are inland basins that do not drain to an ocean. Around 18% of all land drains to endorheic lakes or seas or sinks. The largest of these consists of much of the interior of Asia, which drains into the Caspian Sea, the Aral Sea, and numerous smaller lakes. Other endorheic regions include the Great Basin in the United States, much of the Sahara Desert, the drainage basin of the Okavango River (Kalahari Basin), highlands near the African Great Lakes, the interiors of Australia and the Arabian Peninsula, and parts in Mexico and the Andes. Some of these, such as the Great Basin, are not single drainage basins but collections of separate, adjacent closed basins.

In endorheic bodies of standing water where evaporation is the primary means of water loss, the water is typically more saline than the oceans. An extreme example of this is the Dead Sea.

Importance of drainage basins

Geopolitical boundaries

Drainage basins have been historically important for determining territorial 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 fur trade in the entire Hudson Bay basin, an area called Rupert's Land. Bioregional political organization today includes agreements of states (e.g., international treaties and, within the U.S.A., interstate compacts) or other political entities in a particular drainage basin to manage the body or bodies of water into which it drains. Examples of such interstate compacts are the Great Lakes Commission and the Tahoe Regional Planning Agency.


Drainage basin of the Ohio River, part of the Mississippi River drainage basin

In hydrology, the drainage basin is a logical unit of focus for studying the movement of water within the hydrological 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 drainage basin 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.

Isochrone maps can be used to show the time taken for runoff water within a drainage basin to reach a lake, reservoir or outlet, assuming constant and uniform effective rainfall.[6][7][8][9]


Drainage basins are the principal hydrologic unit considered in fluvial geomorphology. A drainage basin is the source for water and sediment that moves from higher elevation through the river system to lower elevations as they reshape the channel forms.


Mississippi River basin
The Mississippi River drains the largest area of any U.S. river, much of it agricultural regions. Agricultural runoff and other water pollution that flows to the outlet is the cause of the hypoxic, or dead zone in the Gulf of Mexico.

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

Modern use of artificial fertilizers, containing nitrogen, phosphorus, and potassium, has affected the mouths of drainage basins. The minerals are carried by the drainage basin to the mouth, and may accumulate there, disturbing the natural mineral balance. This can cause eutrophication where plant growth is accelerated by the additional material.

Resource management

Because drainage basins are coherent entities in a hydro-logical sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state of Minnesota, 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". In Brazil, the National Policy of Water Resources, regulated by Act n° 9.433 of 1997, establishes the drainage basin as the territorial division of Brazilian water management.

When a river basin crosses at least one political border, either a border within a nation or an international boundary, it is identified as a transboundary river. Management of such basins becomes the responsibility of the countries sharing it. Nile Basin Initiative, OMVS for Senegal River, Mekong River Commission are a few examples of arrangements involving management of shared river basins.

Management of shared drainage basins is also seen as a way to build lasting peaceful relationships among countries.[10]

Catchment factors

The catchment is the most significant factor determining the amount or likelihood of flooding.

Catchment factors are: topography, shape, size, soil type, and land use (paved or roofed areas). Catchment topography and shape determine the time taken for rain to reach the river, while catchment size, soil type, and development determine the amount of water to reach the river.


Generally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep mountainous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient).


Shape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment.


Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It is also determined on the basis of length and width of the drainage basin.

Soil type

Soil type will help determine how much water reaches the river. Certain soil types such as sandy soils are very free-draining, and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free-draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground. If the surface is impermeable the precipitation will create surface run-off which will lead to higher risk of flooding; if the ground is permeable, the precipitation will infiltrate the soil.

Land use

Land use can contribute to the volume of water reaching the river, in a similar way to clay soils. For example, rainfall on roofs, pavements, and roads will be collected by rivers with almost no absorption into the groundwater.

See also



  1. ^ "drainage basin". The Physical Environment. University of Wisconsin–Stevens Point. Archived from the original on March 21, 2004.
  2. ^ "What is a watershed and why should I care?". university of delaware. Retrieved 2008-02-11.
  3. ^ Lambert, David (1998). The Field Guide to Geology. Checkmark Books. pp. 130–13. ISBN 0-8160-3823-6.
  4. ^ a b "Hydrologic Unit Geography". Virginia Department of Conservation & Recreation. Archived from the original on 14 December 2012. Retrieved 21 November 2010.
  5. ^ Encarta Encyclopedia articles on Amazon River, Congo River, and Ganges Published by Microsoft in computers.
  6. ^ Bell, V. A.; Moore, R. J. (1998). "A grid-based distributed flood forecasting model for use with weather radar data: Part 1. Formulation" (PDF). Hydrology and Earth System Sciences. Copernicus Publications. 2 (2/3): 265–281. doi:10.5194/hess-2-265-1998.
  7. ^ Subramanya, K (2008). Engineering Hydrology. Tata McGraw-Hill. p. 298. ISBN 0-07-064855-7.
  8. ^ "EN 0705 isochrone map". UNESCO. Archived from the original on November 22, 2012. Retrieved March 21, 2012.
  9. ^ "Isochrone map". Webster's Online Dictionary. Retrieved March 21, 2012.
  10. ^ "Articles". www.strategicforesight.com.


  • DeBarry, Paul A. (2004). Watersheds: Processes, Assessment and Management. John Wiley & Sons.

External links

Columbia River drainage basin

The Columbia River drainage basin is the drainage basin of the Columbia River in the Pacific Northwest region of North America. It covers 668,000 km2 or 258,000 sq mi. In common usage, the term often refers to a smaller area, generally the portion of the drainage basin that lies within eastern Washington.Usage of the term "Columbia Basin" in British Columbia generally refers only to the immediate basins of the Columbia and Kootenay Rivers and excludes that of the Okanagan, Kettle and Similkameen Rivers.

Congo Basin

The Congo Basin is the sedimentary basin of the Congo River. The Congo Basin is located in Central Africa, in a region known as west equatorial Africa. The Congo Basin region is sometimes known simply as the Congo.

The basin begins in the highlands of the East African Rift system with input from the Chambeshi River, the Uele and Ubangi Rivers in the upper reaches and the Lualaba River draining wetlands in the middle reaches. Due to the young age and active uplift of the East African Rift at the headlands, the river's yearly sediment load is very large but the drainage basin occupies large areas of low relief throughout much of its area. The basin is a total of 3.7 million square kilometers and is home to some of the largest undisturbed stands of tropical rainforest on the planet, in addition to large wetlands. The basin ends where the river empties its load in the Gulf of Guinea on the Atlantic Ocean. The climate is equatorial tropical, with two rainy seasons including very high rainfalls, and high temperature year round. The basin is home to the endangered western lowland gorilla.

The basin was the watershed of the Congo River populated by pygmy peoples, and eventually Bantu peoples migrated there and founded the Kingdom of Kongo. Belgium, France, and Portugal later established colonial control over the entire region by the late 19th century. The General Act of the Berlin Conference of 1885 gave a precise definition to the "conventional basin" of the Congo, which included the entire actual basin plus some other areas. The General Act bound its signatories to neutrality within the conventional basin, but this was not respected during the First World War.

Drainage system (geomorphology)

In geomorphology, drainage systems, also known as river systems, are the patterns formed by the streams, rivers, and lakes in a particular drainage basin. They are governed by the topography of the land, whether a particular region is dominated by hard or soft rocks, and the gradient of the land. Geomorphologists and hydrologists often view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. The number, size, and shape of the drainage basins found in an area vary and the larger the topographic map, the more information on the drainage basin is available.

Endorheic basin

An endorheic basin (also endoreic basin or endorreic basin) (from the Ancient Greek: ἔνδον, éndon, "within" and ῥεῖν, rheîn, "to flow") is a limited drainage basin that normally retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but converges instead into lakes or swamps, permanent or seasonal, that equilibrate through evaporation. Such a basin may also be referred to as a closed or terminal basin or as an internal drainage system or interior drainage basin.

Endorheic regions, in contrast to exorheic regions which flow to the ocean in geologically defined patterns, are closed hydrologic systems. Their surface waters drain to inland terminal locations where the water evaporates or seeps into the ground, having no access to discharge into the sea. Endorheic water bodies include some of the largest lakes in the world, such as the Caspian Sea, the world's largest saline inland sea.Endorheic basins constitute local base levels, defining a limit of erosion and deposition processes of nearby areas.

List of rivers of Arkansas

List of rivers in Arkansas (U.S. state).

For a list of dams and reservoirs in Arkansas, see List of Arkansas dams and reservoirs Rivers are listed by drainage basin, by size, and alphabetically.

List of rivers of Georgia (U.S. state)

List of rivers in Georgia (U.S. state).

BY MAHAMAD SHEIK This list is arranged by drainage basin, with respective tributaries indented under each larger stream's name.

List of rivers of Iowa

The following is a list of rivers and creeks in Iowa. The rivers are listed by multiple arrangements:

those that form part of the boundaries of the U.S. state of Iowa;

ordered by drainage basin, with tributaries indented under each larger river's name;

ordered alphabetically.

List of rivers of Kentucky

List of rivers in Kentucky (U.S. state).

List of rivers of Minnesota

List of rivers in Minnesota (U.S. state).

List of rivers of Oklahoma

This is a list of rivers in the state of Oklahoma, listed by drainage basin, alphabetically, and by size. In mean flow of water per second, the Arkansas is Oklahoma's largest river, followed by the Red River of the South and the Neosho River.

List of rivers of Texas

The list of rivers of Texas is a list of all named waterways, including rivers and streams that partially pass through or are entirely located within the U.S. state of Texas. Across the state, there are 3,700 named streams and 15 major rivers accounting for over 80,000 mi (130,000 km) of waterways. All of the state's waterways drain towards the Mississippi River, the Texas Gulf Coast, or the Rio Grande, with mouths located in seven major estuaries.

List of rivers of Virginia

This is a list of rivers in the U.S. state of Virginia.

Main stem

In hydrology, a main stem (or trunk) is "the primary downstream segment of a river, as contrasted to its tributaries". Water enters the main stem from the river's drainage basin, the land area through which the main stem and its tributaries flow. A drainage basin may also be referred to as a watershed or catchment.

Hydrological classification systems assign numbers to tributaries and main stems within a drainage basin. In the Strahler number, a modification of a system devised by Robert E. Horton in 1945, channels with no tributaries are called "first-order" streams. When two first-order streams meet, they are said to form a second-order stream; when two second-order streams meet, they form a third-order stream, and so on. In the Horton system, the entire main stem of a drainage basin was assigned the highest number in that basin. However, in the Strahler system, adopted in 1957, only that part of the main stem below the tributary of the next highest rank gets the highest number.In the United States, the Mississippi River main stem achieves a Strahler number of 10, the highest in the nation. Eight rivers, including the Columbia River, reach 9. Streams with no tributaries, assigned the Strahler number 1, are most common. More than 1.5 million of these small streams, with average drainage basins of only 1 square mile (2.6 km2), have been identified in the United States alone. Outside of the United States, the Amazon River reaches a Strahler number of 12, making it the highest-order river in the world.

Möller Ice Stream

Möller Ice Stream (82°20′S 63°30′W) is an Antarctic ice stream that drains an area of 66,000 square kilometres (25,000 sq mi) of the West Antarctic Ice Sheet as it flows north-northeast into the Ronne Ice Shelf to the west of Foundation Ice Stream. The drainage basin of this ice stream is separated by the Rambo Nunataks from the drainage basin of Foundation Ice Stream.The feature was delineated from U.S. Landsat imagery commissioned by the Institut für Angewandte Geodäsie, Frankfurt am Main, Germany, recorded January–March, 1986. It was named after German engineer Professor Dietrich Möller, Director of the Institute for Land Survey at the Technical University of Braunschweig from 1972, and Deputy Leader and in charge of geodetic work at Filchner Station on the Ronne Ice Shelf in 1979–80.

River Camlad

The River Camlad (or just Camlad) is a minor river in Powys and Shropshire. It forms part of the border between Wales and England in places, before flowing into the River Severn. It is notable for being the only river to cross from England into Wales and does so twice.

The river originates in England, in the area between Snead and Lydham, flows west, forming part of the border between Wales and England, before flowing northwest into Wales. It passes through Church Stoke, where the River Caebitra flows into the Camlad. From Church Stoke it flows north, crossing the border back into England, and flows just east of Chirbury before turning west again, and forms the border (for the second time) between England and Wales. It turns northwest into Wales (for the second time) to join the River Severn to the west of Forden.

Senegal River

The Senegal River (Arabic: نهر السنغال‎, French: Fleuve Sénégal) is a 1,086 km (675 mi) long river in West Africa that forms the border between Senegal and Mauritania.

Tigris–Euphrates river system

The Tigris and Euphrates, with their tributaries, form a major river system in Western Asia. From sources originating in eastern Turkey, they flow by/through Syria through Iraq into the Persian Gulf. The system is part of the Palearctic Tigris–Euphrates ecoregion, which includes Iraq and parts of Turkey, Syria, Iran, Saudi Arabia, Kuwait, and Jordan.

From their sources and upper courses in the mountains of eastern Anatolia, the rivers descend through valleys and gorges to the uplands of Syria and northern Iraq and then to the alluvial plain of central Iraq. The rivers flow in a south-easterly direction through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf.The region has historical importance as part of the Fertile Crescent region, in which civilization is believed to have first emerged.

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