
A drainage basin is any area of land where
precipitation
In meteorology
Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...

collects and drains off into a common outlet, such as into a
river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

,
bay
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean
The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface ...

, or other
body of water
A body of water or waterbody (often spelled water body) is any significant accumulation of water, generally on a planet's surface. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetla ...

. The drainage basin includes all the
surface water
Surface water is water
Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of ...
from
rain runoff,
snowmelt
In hydrology
Hydrology (from Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece
Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
, hail, sleet and nearby streams that run downslope towards the shared outlet, as well as the
groundwater
Groundwater is the water
Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

underneath the earth's surface. 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.
Other terms for ''drainage basin'' are
catchment
A drainage basin is any area of land where precipitation
In meteorology
Meteorology is a branch of the atmospheric sciences
Atmospheric science is the study of the Earth's atmosphere
File:Atmosphere gas proportions.svg, Composi ...
area, catchment basin, drainage area, river basin, water basin, and impluvium.
In
North America
North America is a continent
A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

, 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 topography, a drainage divide, water divide, divide, ridgeline, watershed, water parting or ''height of land'' is elevated terrain that separates neighboring drainage basins. On rugged land, the divide lies along topographical ridges, and may ...
.
In a closed drainage basin, or
endorheic basin
An endorheic basin (; also spelled endoreic basin or endorreic basin) is a drainage basin
A drainage basin is any area of land where precipitation
In meteorology
Meteorology is a branch of the atmospheric sciences
Atmospheri ...
, the water converges to a single point inside the basin, known as a
sink
A sink – also known by other names including sinker, washbowl, hand basin, wash basin, and simply basin – is a bowl-shaped plumbing fixture
A plumbing fixture is an exchangeable device which can be connected to a plumbing system to deli ...
, which may be a permanent lake, a
dry lake
A dry lake bed, also known as a playa, is a basin or depression that formerly contained a standing surface water body, which disappeared when evaporation processes exceeded recharge. If the floor of a dry lake is covered by deposits of alkaline ...

, or a point where surface water is
lost underground.
The drainage basin acts as a
funnel
A ceramic Roman kitchen funnel (1st–3rd century AD)
A funnel is a tube or pipe that is wide at the top and narrow at the bottom, used for guiding liquid or powder into a small opening.
Funnels are usually made of stainless steel
Stainles ...

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
In topography, a drainage divide, water divide, divide, ridgeline, watershed, water parting or ''height of land'' is elevated terrain that separates neighboring drainage basins. On rugged land, the divide lies along topographical ridges, and may ...
, making up a succession of higher geographical features (such as a
ridge
A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. The sides of the ridge slope away from narrow top on either side. The lines along th ...

,
hill
A hill is a landform
A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body
A planet is an astronomical body
Astronomy (from el, ἀστρονομία, literally meaning the scie ...

or
mountain
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

s) 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.
[
]
Major drainage basins of the world
Ocean basins
The following is a list of the major ocean basins:
* About 48.71% of the world's land drains to the . In North America
North America is a continent
A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

, surface water drains to the Atlantic via the Saint Lawrence River
The St. Lawrence River is a large river
A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its c ...
and Great Lakes
The Great Lakes also called the Great Lakes of North America or the Laurentian Great Lakes, is a series of large interconnected freshwater lake
A lake is an area filled with water, localized in a basin, surrounded by land
Land ...

basins, the Eastern Seaboard of the United States, the Canadian Maritimes
The Maritimes, also called the Maritime provinces, is a region
In geography, regions are areas that are broadly divided by physical characteristics ( physical geography), human impact characteristics ( human geography), and the interaction of h ...

, and most of Newfoundland and Labrador
Newfoundland and Labrador (; sometimes abbreviated as NL) is the easternmost province of Canada
The Province of Canada (or the United Province of Canada or the United Canadas) (french: link=no, Province du Canada) was a British North Am ...
. Nearly all of South America
South America is a continent
A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continent ...

east of the Andes
The Andes, Andes Mountains or Andean Mountains ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sou ...

also drains to the Atlantic, as does most of Western
Western may refer to:
Places
*Western, Nebraska, a village in the US
*Western, New York, a town in the US
*Western Creek, Tasmania, a locality in Australia
*Western Junction, Tasmania, a locality in Australia
*Western world, countries that ide ...

and Central Europe
Central Europe is an area of Europe
Europe is a which is also recognised as part of , located entirely in the and mostly in the . It comprises the westernmost peninsulas of the of Eurasia, it shares the continental landmass of with both ...

and the greatest portion of western Sub-Saharan Africa
Sub-Saharan Africa (commonly called Black Africa) is, geographically, the area of the continent of Africa that lies south of the Sahara. According to the United Nations, it consists of all list of sovereign states and dependent territories i ...

, as well as Western Sahara
Western Sahara ( '; ; ) is a disputed territory
Controversy is a state of prolonged public dispute or debate, usually concerning a matter of conflicting opinion or point of view. The word was coined from the Latin ''controversia'', as a com ...

and part of Morocco
)
, image_map = Morocco (orthographic projection, WS claimed).svg
, map_caption = Location of Morocco in northwest Africa.Dark green: Undisputed territory of Morocco.Lighter green: Western Sahara, a United Nations lis ...

. The two major mediterranean seas of the world also flow to the Atlantic:
** The Caribbean Sea
The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lamè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is an Americas, American Mediterranean sea (oceanography), mediterranean sea of the Atlantic Ocean ...
and Gulf of Mexico
The Gulf of Mexico ( es, Golfo de México) is an ocean basin
400px, Diagrammatic cross-section of an ocean basin, showing the various geographic features
In hydrology
Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning ...

basin includes most of the U.S. interior between the AppalachianAppalachian may refer to:
* The Appalachian Mountains, a major mountain range in eastern United States and Canada
* The Appalachian Trail, a hiking trail in the eastern United States
* The people of Appalachia and their culture
** Appalachian Americ ...

and Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range
A mountain range is a series of mountains
ranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with simila ...

, a small part of the Canadian provinces of Alberta
("Strong and free")
, image_map = Alberta in Canada 2.svg
, Label_map = yes
, coordinates =
, official_lang = English
, capital = Edmonton
Edmonton ( ) is the capital ...

and Saskatchewan
("From Many Peoples Strength")
, image_map = Saskatchewan in Canada 2.svg
, Label_map = yes
, coordinates =
, official_lang = English language, English
, capital = Regina, S ...
, eastern Central America
Central America ( es, América Central, , ''Centroamérica'' ) is a region of the Americas
The Americas (also collectively called America) is a landmass comprising the totality of North
North is one of the four compass points or ...

, the islands of the Caribbean and the Gulf, and a small part of northern South America.
** The Mediterranean Sea
The Mediterranean Sea is a connected to the , surrounded by the and almost completely enclosed by land: on the north by and and , on the south by , and on the east by the . The Sea has played a central role in the . Although the Mediterrane ...
basin includes much of North Africa
North Africa or Northern Africa is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region, and it is sometimes defined as stretching from the Atlantic shores of Mauritania in th ...

, east-central Africa
Africa is the world's second-largest and second-most populous continent
A continent is any of several large landmass
A landmass, or land mass, is a large region
In geography
Geography (from Greek: , ''geographia'', ...

(through the Nile River
The Nile, , Bohairic , lg, Kiira , Nobiin
Nobiin, or Mahas, is a Northern Nubian languages, Nubian language of the Nilo-Saharan languages, Nilo-Saharan language family. "Nobiin" is the genitive case, genitive form of ''Nòòbíí'' ("Nub ...
), Southern
The name Southern may refer to:
* South
South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east and west.
Etymology
The word ''south'' comes from Old English ''sūþ'', from earl ...

, Central, and Eastern Europe
Eastern Europe is the eastern region of Europe
Europe is a continent
A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical reg ...

, Turkey
Turkey ( tr, Türkiye ), officially the Republic of Turkey, is a country located mainly on Anatolia
Anatolia,, tr, Anadolu Yarımadası), and the Anatolian plateau. also known as Asia Minor, is a large peninsula in Western Asia an ...

, and the coastal areas of Israel
Israel (; he, יִשְׂרָאֵל, translit=Yīsrāʾēl; ar, إِسْرَائِيل, translit=ʾIsrāʾīl), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a ...

, Lebanon
Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon or the Lebanese Republic, is a country in Western Asia
Western Asia, West Asia, or Southwest Asia, is the westernmost subregion
A subregion is a part ...

, and Syria
Syria ( ar, سُورِيَا or ar, سُورِيَة, ''Sūriyā''), officially the Syrian Arab Republic ( ar, ٱلْجُمْهُورِيَّةُ ٱلْعَرَبِيَّةُ ٱلسُّورِيَّةُ, al-Jumhūrīyah al-ʻArabīyah as-S ...

.
* The Arctic Ocean
The Arctic Ocean is the smallest and shallowest of the world's five major s. It spans an area of approximately and is also known as the coldest of all the oceans. The (IHO) recognizes it as an ocean, although some call it the Arctic Medit ...

drains most of Western
Western may refer to:
Places
*Western, Nebraska, a village in the US
*Western, New York, a town in the US
*Western Creek, Tasmania, a locality in Australia
*Western Junction, Tasmania, a locality in Australia
*Western world, countries that ide ...
and Northern Canada
Northern Canada, colloquially the North or the Territories, is the vast northernmost region of Canada variously defined by geography and politics. Politically, the term refers to the three territories of Canada
The provinces and territories of ...

east of the Continental Divide
A continental divide is a drainage divide on a continent
A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regarded as co ...
, northern Alaska
Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state in the Western United States, on the northwest extremity of the country's West Coast of the United State ...

and parts of North Dakota
North Dakota () is a U.S. state
In the , a state is a , of which there are currently 50. Bound together in a , each state holds al jurisdiction over a separate and defined geographic territory where it shares its with the . Due to th ...
, South Dakota
South Dakota () (Sioux
The Sioux or Oceti Sakowin (; Dakota
Dakota may refer to:
* Dakota people, a sub-tribe of the Sioux
** Dakota language, their language
From this origin, Dakota may also refer to:
Places United States
* Dakot ...

, Minnesota
Minnesota () is a state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* The State (newspaper), ''The State'' (newspaper), a daily newspaper ...

, and Montana
Montana () is a state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* The State (newspaper), ''The State'' (newspaper), a daily newspaper ...

in the United States, the north shore of the Scandinavian peninsula
The Scandinavian Peninsula ( sv, Skandinaviska halvön; no, Den skandinaviske halvøy (Bokmål
Bokmål (, ; literally "book tongue") is an official written standard for the Norwegian language
Norwegian (Norwegian: ''norsk'') is a North Ge ...
in Europe, central and northern Russia, and parts of Kazakhstan
Kazakhstan ( kk, Қазақстан, Qazaqstan; russian: Казахстан, Kazakhstan), officially the Republic of Kazakhstan,; russian: Республика Казахстан, Respublika Kazakhstan, link=no) is a country located mainly in ...

and Mongolia
Mongolia (, mn, Монгол Улс, Mongol Uls, Mongolian script, Traditional Mongolian: '; literal translation, lit. "Mongol Nation" or "State of Mongolia") is a landlocked country in East Asia. It is bordered by Russia Mongolia–Russia ...

in Asia
Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hemispheres. It shares the continental landmass of Eurasia with the cont ...

, which totals to about 17% of the world's land.
* Just over 13% of the land in the world drains to the Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth
Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. ...

. Its basin includes much of China, eastern and southeastern Russia, Japan, the Korean Peninsula
Korea is a region
In geography, regions are areas that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and the interaction of humanity and the environment (environmental ...

, most of Indochina, Indonesia and Malaysia, the Philippines, all of the Pacific Islands
This is a list of islands in the Pacific Ocean, collectively called the Pacific Islands. Three major groups of island
An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as ...
, the northeast coast of Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

, and Canada and the United States west of the Continental Divide (including most of Alaska), as well as western Central America and South America west of the Andes.
* The Indian Ocean
The Indian Ocean is the third-largest of the world's five ocean
The ocean (also the or the world ocean) is the body of that covers approximately 70.8% of the surface of and contains 97% of . Another definition is "any of the large ...

's drainage basin also comprises about 13% of Earth's land. It drains the eastern coast of Africa, the coasts of the Red Sea
The Red Sea ( ar, البحر الأحمر, translit=al-Baḥr al-ʾAḥmar; or ; Coptic
Coptic may refer to:
Afro-Asia
* Copts, an ethnoreligious group mainly in the area of modern Egypt but also in Sudan and Libya
* Coptic language, a North ...

and the Persian Gulf
The Persian Gulf ( fa, خلیج فارس, translit=xalij-e fârs, lit=Gulf of , ) is a in . The body of water is an extension of the () through the and lies between to the northeast and the to the southwest.United Nations Group of Exper ...
, the Indian subcontinent, Burma, and most of Australia.
* The Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. As such, it is regarded as the second-smallest of t ...

drains Antarctica
Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

. Antarctica comprises approximately eight percent of the Earth's land.
Largest river basins
The five largest river basins (by area), from largest to smallest, are the basins of the Amazon
Amazon usually refers to:
* Amazons
In Greek mythology, the Amazons (Ancient Greek: Ἀμαζόνες ''Amazónes'', singular Ἀμαζών ''Amazōn'') are portrayed in a number of ancient Greek, ancient epic poems and legends, such as the ...

(7M km), the Congo (4M km), the Nile
The Nile, , Bohairic , lg, Kiira , Nobiin
Nobiin, or Mahas, is a Northern Nubian languages, Nubian language of the Nilo-Saharan languages, Nilo-Saharan language family. "Nobiin" is the genitive case, genitive form of ''Nòòbíí'' ("Nub ...
(3.4M km), the Mississippi
Mississippi () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States, bordered to the north by Tennessee; to the east by Alabama; to the south by the Gulf of Mexico; to the southwest by Louisiana; a ...

(3.22M km), and the Río de la Plata
The Río de la Plata (, "river of silver"), called River Plate in British English
British English (BrE) is the standard dialect of the English language
English is a West Germanic languages, West Germanic language first spoken in H ...

(3.17M km). The three rivers that drain the most water, from most to least, are the Amazon, Ganga
The Ganges ( ) (in India: Ganga ( ); in Bangladesh
Bangladesh (, bn, বাংলাদেশ, ), officially the People's Republic of Bangladesh, is a country in South Asia
South Asia is the southern region of Asia, which is de ...

, and Congo rivers.
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
Asia () is Earth's largest and most populous continent, located primarily in the Eastern Hemisphere, Eastern and Northern Hemisphere, Northern Hemisphere of the Earth, Hemispheres. It shares the continental landmass of Eurasia with the cont ...

, which drains into the Caspian Sea
The Caspian Sea (also known as Mazandaran Sea, Hyrcanian Ocean, or Khazar Sea), tk, Hazar deňzi, az, Xəzər Dənizi, russian: Каспийское море, script=Latn, fa, دریای مازندران، دریای خزر, script=Latn, tly, ...

, the Aral Sea
The Aral Sea (Aral ; kk, Aral teńizi, Арал теңізі, uz, Orol dengizi, Орол денгизи, kaa, Aral ten'izi, Арал теңизи, russian: Аральское море) was an endorheic lake lying between Kazakhstan
Kaza ...

, and numerous smaller lakes. Other endorheic regions include the Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America
North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It can also be described as the ...
in the United States, much of the Sahara Desert
The Sahara (, ; ar, الصحراء الكبرى, ', 'the Greatest Desert') is a desert on the . With an area of , it is the largest hot in the world and the third largest desert overall, smaller only than the deserts of and the northern . ...

, the drainage basin of the Okavango River
The Okavango River (formerly spelled Okovango or Okovanggo) is a river in southwest Africa. It is the fourth-longest river system in southern Africa, running southeastward for . It begins at 1,300 m altitude in the sandy highlands of Angola, wher ...

(Kalahari Basin
The Kalahari Basin, also known as the Kalahari Depression or the Okavango Basin, is an endorheic basin and large lowland area covering over 2.5 million km2 covering most of Botswana and Namibia, as well as parts of Angola, South Africa, Zambia, ...
), highlands near the African Great Lakes
The African Great Lakes ( sw, Maziwa Makuu) are a series of lake
A lake is an area filled with water, localized in a Depression (geology), basin, surrounded by land, and set apart from any river or other outlet that serves to feed or drai ...

, the interiors of Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

and the Arabian Peninsula
The Arabian Peninsula (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plate. At , the ...
, and parts in Mexico
Mexico, officially the United Mexican States, is a country
A country is a distinct territorial body or political entity
A polity is an identifiable political entity—any group of people who have a collective identity, who are organi ...

and the Andes
The Andes, Andes Mountains or Andean Mountains ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sou ...

. 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
The Dead Sea ( he, יָם הַמֶּלַח lit. Sea of Salt; ar, البحر الميت , lit. ''the Dead Sea'',The first article ''al-'' is unnecessary and usually not used. or Buhayrat, Bahret or Birket Lut, ''lit.'' "Lake/Sea of Lot") is a s ...

.
Importance
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
English usually refers to:
* English language
English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, which has eventually become the World language, leading lan ...

crown gave the Hudson's Bay Company
The Hudson's Bay Company (HBC; french: Compagnie de la Baie d'Hudson) is a Canadian, now American-owned, retail
Retail is the sale of goods
In economics
Economics () is the social science that studies how people interact with va ...
a monopoly on the fur trade
The fur trade is a worldwide industry dealing in the acquisition and sale of animal fur
Fur is a thick growth of hair
Hair is a protein filament
In biology
Biology is the natural science that studies life and living organi ...
in the entire Hudson Bay
Hudson Bay ( iu, text=ᑲᖏᖅᓱᐊᓗᒃ ᐃᓗᐊ, translit=Kangiqsualuk ilua or iu, text=ᑕᓯᐅᔭᕐᔪᐊᖅ, translit=Tasiujarjuaq; french: baie d'Hudson), sometimes called Hudson's Bay (usually historically), is a large body of sal ...
basin, an area called Rupert's Land
Rupert's Land (french: Terre de Rupert), or Prince Rupert's Land, was a territory in British North America comprising the Hudson Bay drainage basin, a territory in which a commercial monopoly was operated by the Hudson's Bay Company for 200 ...
. Bioregional political organization today includes agreements of states (e.g., international treaties
A treaty is a formal, legally binding written agreement between actors in international law
International law, also known as public international law and law of nations, is the set of rules, norms, and standards generally accepted in relati ...

and, within the US, interstate compact
In the United States
The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country Contiguous United States, primarily located in North America. It consists of 50 U.S. state, states, a Washin ...
s) 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.
Hydrology
In hydrology
Hydrology (from Ancient Greek, Greek wikt:ὕδωρ, ὕδωρ, ''hýdōr'' meaning "water" and wikt:λόγος, λόγος, ''lógos'' meaning "study") is the scientific study of the movement, distribution, and management of water on Earth and ...
, the drainage basin is a logical unit of focus for studying the movement of water within the hydrological cycle
The water cycle, also known as the hydrologic cycle or the hydrological cycle, describes the continuous movement of water
Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, ...
, because the majority of water that discharges from the basin outlet originated as precipitation
In meteorology
Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
falling on the basin. A portion of the water that enters the groundwater
Groundwater is the water
Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

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
A stream gauge, streamgage or stream gauging station is a location used by hydrologists
Hydrology (from Greek: ὕδωρ, "hýdōr" meaning "water" and wikt:λόγος, λόγος, "lógos" meaning "study") is the scientific study of the move ...
located at the basin's outlet. Depending on the conditions of the drainage basin, as rainfall occurs some of it seeps directly into the ground. This water will either remain underground, slowly making its way downhill and eventually reaching the basin, or it will permeate deeper into the soil and consolidate into groundwater aquifers.
As water flows through the basin, it can form tributaries that change the structure of the land. There are three different main types, which are affected by the rocks and ground underneath. Rock that is quick to erode forms dendritic patterns, and these are seen most often. The two other types of patterns that form are trellis patterns and rectangular patterns.
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
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
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 isohyet
A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve
In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to b ...

al 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 map
An isochrone map in geography
Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first pers ...
s 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.
Geomorphology
Drainage basins are the principal hydrologic unit considered in fluvial
In geography
Geography (from Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece
Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
geomorphology
Geomorphology (from Ancient Greek
Ancient Greek includes the forms of the Greek language
Greek ( el, label=Modern Greek
Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek ...

. A drainage basin is the source for water and sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

that moves from higher elevation through the river system to lower elevations as they reshape the channel forms.
Ecology
Drainage basins are important in ecology
Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms
In biol ...
. As water flows over the ground and along rivers it can pick up nutrients, sediment, and pollutants
A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. A pollutant may cause long- or short-term damage by changing the growth rate of plant or animal spec ...

. 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
Eutrophication (from Greek ''eutrophos'', "well-nourished") is the process by which an entire body of water
(Lysefjord) in Norway
Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål
Bokm ...

where plant growth is accelerated by the additional material.
Resource management
Because drainage basins are coherent entities in a hydrological sense, it has become common to manage water resources on the basis of individual basins. In the U.S. state
In the United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state ...
of Minnesota
Minnesota () is a state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* The State (newspaper), ''The State'' (newspaper), a daily newspaper ...

, 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 managementWatershed management is the study of the relevant characteristics of a watershed aimed at the sustainable distribution of its resources and the process of creating and implementing plans, programs and projects to sustain and enhance watershed funct ...
".
In Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At 8.5 million square kilometers (3.2 million square miles) and with over 211 mill ...

, 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
A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the highest number of these rivers, including two of the world's largest rivers, the Ganges and ...
. Management of such basins becomes the responsibility of the countries sharing it. Nile Basin Initiative, for Senegal River
The Senegal River ( ar, نهر السنغال ''Nahr As-sinigāl'', french: Fleuve Sénégal) is a long river in West Africa that forms the Mauritania–Senegal border, border between Senegal and Mauritania. It has a drainage basin of , a mean ...
, Mekong River Commission
The Mekong River Commission (MRC) is an "...inter-governmental organisation that works directly with the governments of Cambodia, Laos, Thailand, and Vietnam
, image_map =
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, capital ...

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.
Catchment factors
The catchment is the most significant factor determining the amount or likelihood of flooding
A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide
Tides are the rise and fall of s caused by the combined effects of the fo ...

.
Catchment factors are: topography
Topography is the study of the forms and features of land surface
Relief map of Sierra Nevada, Spain
Terrain or relief (also topographical
Topography is the study of the forms and features of land surfaces. The topography of an ...
, shape, size, soil
Soil is a mixture
In chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

type, and land use
Land use involves the management and modification of natural environment
The natural environment or natural world encompasses all life, living and non-living things occurring nature, naturally, meaning in this case not Artificiality, artifi ...
(paved or roof
A roof is the top covering of a building
A building, or edifice, is a structure with a roof and walls standing more or less permanently in one place, such as a house
A house is a single-unit residential building, which may range in co ...

ed areas). Catchment topography and shape determine the time taken for rain
Rain is liquid water in the form of droplet
Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, Van der Waals force">Cohesion_(chemistry).html" ;"title="surface tension, Cohesion (chemistry)">cohesion, ...

to reach the river, while catchment size, soil type, and development determine the amount of water to reach the river.
Topography
Generally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep mountain
A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

ous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient).
Shape
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
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. The runoff from the drainage area is dependent on the soil type. Certain soil types such as sand
Sand is a granular material composed of finely divided rock (geology), rock and mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer ...

y soils are very free-draining, and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay
Clay is a type of fine-grained natural soil
Surface-water- gley developed in glacial till, Northern Ireland.">Northern_Ireland.html" ;"title="glacial till, Northern Ireland">glacial till, Northern Ireland.
Soil is a mixture of organic m ...

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, sidewalk, pavements, and roads will be collected by rivers with almost no absorption into the groundwater
Groundwater is the water
Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

.
See also
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*Catchment hydrology
References
Citations
Sources
* DeBarry, Paul A. (2004). ''Watersheds: Processes, Assessment and Management.'' John Wiley & Sons.
External links
Instructional video: Manual watershed delineation is a five-step process
Instructional video: To delineate a watershed you must identify land surface features from topographic contours
Science week catchment factsheet
Catchment Modelling Toolkit
Water Evaluation And Planning System (WEAP) - modeling hydrologic processes in a drainage basin
New Mexico State University - Water Task Force
Watershed Condition Classification Technical Guide
United States Forest Service
''Science in Your Watershed''
USGS
''Studying Watersheds: A Confluence of Important Ideas''
''Water Sustainability Project'' Sustainable water management through demand management and ecological governance, with the POLIS Project at the University of Victoria
Map of the Earth's primary watersheds
World Resources Institute, WRI
What is a watershed and why should I care?
Cycleau - A project looking at approaches to managing catchments in North West Europe
* [http://web.mit.edu/star/hydro StarHydro – software tool that covers concepts of fluvial geomorphology and watershed hydrology]
EPA Surf your watershed
Florida Watersheds and River Basins - Florida DEP
{{Authority control
Drainage basins,
Fluvial landforms
Freshwater ecology
Geomorphology
Hydrology
Rivers
Water and the environment
Water streams