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. 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 used interchangeably with drainage basin are catchment
area, catchment basin, drainage area, river basin, and water basin.
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
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
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
1 Major drainage basins of the world
1.2 Ocean basins
1.3 Largest river basins
1.4 Endorheic drainage basins
2 Importance of drainage basins
2.1 Geopolitical boundaries
2.5 Resource management
3 Catchment factors
3.5 Land use
4 See also
6 External links
Major drainage basins of the world
See also: List of drainage basins by area
Drainage basins of the principal oceans and seas of the world. Grey
areas are endorheic basins that do not drain to the oceans.
The following is a list of the major ocean basins:
About 48.7% of the world's land drains to the Atlantic Ocean.[citation
needed] In North America, surface water drains to the Atlantic via the
Great Lakes basins, the Eastern Seaboard of
the United States, the Canadian Maritimes, and most of Newfoundland
and Labrador. Nearly all of
South America east of the
drains to the Atlantic, as does most of Western and
Central Europe and
the greatest portion of western Sub-Saharan Africa, as well as Western
Sahara and part of Morocco. The two major mediterranean seas of the
world also flow to the Atlantic:
Caribbean Sea and
Gulf of Mexico
Gulf of Mexico basin includes most of the U.S.
interior between the Appalachian and Rocky Mountains, a small part of
the Canadian provinces of
Alberta and Saskatchewan, eastern Central
America, the islands of the Caribbean and the Gulf, and a small part
of northern South America.
Mediterranean Sea basin includes much of North Africa,
Africa (through the Nile River), Southern, Central, and
Eastern Europe, Turkey, and the coastal areas of Israel, Lebanon, and
Arctic Ocean drains most of Western and
Northern Canada east of
the Continental Divide, northern
Alaska and parts of North Dakota,
South Dakota, Minnesota, and
Montana in the United States, the north
shore of the
Scandinavian peninsula in Europe, central and northern
Russia, and parts of
Mongolia in Asia, which totals to
about 17% of the world's land.
Just over 13% of the land in the world drains to the Pacific Ocean.
Its basin includes much of China, eastern and southeastern Russia,
Japan, the Korean Peninsula, most of Indochina, Indonesia and
Malaysia, the Philippines, all of the Pacific Islands, the northeast
coast of Australia, 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's drainage basin also comprises about 13% of Earth's
land. It drains the eastern coast of Africa, the coasts of the Red Sea
and the Persian Gulf, the Indian subcontinent, Burma, and most of
Southern Ocean drains Antarctica.
approximately eight percent of the Earth's land.
Largest river basins
See also: List of drainage basins by area
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
Río de la Plata
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.
Endorheic drainage basins
Endorheic basin in Central Asia
Main article: Endorheic basin
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
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
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
Drainage basin of the Ohio River, part of the Mississippi River
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
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.
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.
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
Further information: Watershed 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.
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
Generally, topography plays a big part in how fast runoff will reach a
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
Size will help determine the amount of water reaching the river, as
the larger the catchment the greater the potential for flooding. It
also determined on the basis of length and width of the drainage
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 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.
Integrated catchment management
International Journal of
River Basin Management (JRBM)
International Network of Basin Organizations
River Basin Management Plans
Time of concentration
^ "drainage basin". The Physical Environment. University of
Wisconsin–Stevens Point. Archived from the original on March 21,
^ "What is a watershed and why should I care?". university of
delaware. Retrieved 2008-02-11.
^ Lambert, David (1998). The Field Guide to Geology. Checkmark Books.
pp. 130–13. ISBN 0-8160-3823-6.
^ a b "Hydrologic Unit Geography". Virginia Department of Conservation
& Recreation. Archived from the original on 14 December 2012.
Retrieved 21 November 2010.
^ Encarta Encyclopedia articles on Amazon River, Congo River, and
Ganges Published by Microsoft in computers.
^ Bell, V. A.; Moore, R. J. (1998). "A grid-based distributed flood
forecasting model for use with weather radar data: Part 1.
Hydrology and Earth System Sciences. Copernicus
Publications. 2 (2/3): 265–281. doi:10.5194/hess-2-265-1998.
^ Subramanya, K (2008). Engineering Hydrology. Tata McGraw-Hill.
p. 298. ISBN 0-07-064855-7.
^ "EN 0705 isochrone map". UNESCO. Archived from the original on
November 22, 2012. Retrieved March 21, 2012.
^ "Isochrone map". Webster's Online Dictionary. Retrieved March 21,
2012. [permanent dead link]
^ "Articles". www.strategicforesight.com.
DeBarry, Paul A. (2004). Watersheds: Processes, Assessment and
Management. John Wiley & Sons.
Science week catchment factsheet
Catchment Modelling Toolkit
Water Evaluation And Planning System (WEAP) - modeling hydrologic
processes in a drainage basin
Mexico State University - Water Task Force
Recommended Watershed Terminology
Watershed Condition Classification Technical Guide United States
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, WRI
What is a watershed and why should I care?
Cycleau - A project looking at approaches to managing catchments in
North West Europe
flash animation of how rain falling onto the landscape will drain into
a river depending on the terrain
StarHydro – software tool that covers concepts of fluvial
geomorphology and watershed hydrology
EPA Surf your watershed
Florida Watersheds and
River Basins - Florida DEP
Drainage system (geomorphology)
Strahler number (stream order)
Bar (river morphology)
River channel migration
Erosion and tectonics
A Directory of Important Wetlands in Australia
Clean Water Act
Delta Waterfowl Foundation
Flooded grasslands and savannas
Freshwater swamp forest
Peat swamp forest
Salt pannes and pools
Wildfowl & Wetlands Trust