A tributary is a freshwater stream that feeds into a larger stream or river. The larger, or parent, river is called the mainstem. The point where a tributary meets the mainstem is called the confluence. Tributaries, also called affluents, do not flow directly into the ocean.
 
Most large rivers are formed from many tributaries. Each tributary drains a different watershed, carrying runoff and snowmelt from that area. Each tributary's watershed makes up the larger watershed of the mainstem. 
 
The Missouri River's massive watershed, for example, is created by the watersheds of dozens of tributaries extending from the provinces of Alberta and Saskatchewan, Canada, through seven states in the Upper Midwest of the U.S. The Missouri, in turn, is the largest tributary of the Mississippi River, which it meets at a confluence in St. Louis, Missouri. The Mississippi River watershed is the fourth-largest in the world.
 
A "left-bank tributary" or "right-bank tributary" indicates the side of the river a tributary enters. When identifying a left-bank or right-bank tributary, a geographer looks downstream (the direction the river is flowing). 
 
The Euphrates River, the longest river in southwestern Asia, stretches 2,700 kilometers (1,678 miles). The tiny streams that feed the Euphrates originate in the mountains of eastern Turkey. These streams become the Balikh and the Sajur Rivers, which join the Euphrates at different confluences in Syria. The Balikh is a left-bank tributary of the Euphrates, while the Sajur is a right-bank tributary.
 
Sometimes, tributaries have the same name as the river into which they drain. These tributaries are called forks. Different forks are usually identified by the direction in which they flow into the mainstem. 
 
The Shenandoah River, for example, flows through the U.S. states of West Virginia and Virginia. It has two long tributaries, the North Fork and South Fork, which meet at a confluence in Harpers Ferry, West Virginia.
 
The opposite of a tributary is a distributary. A distributary is a stream that branches off and flows apart from the mainstem of a stream or river. The process is called river bifurcation. 
 
At the Continental Divide in the U.S. state of Wyoming, the small North Two Ocean Creek bifurcates into Pacific Creek and Atlantic Creek. The water from each of these distributaries flows into the ocean for which it is named.
 
Classifying Tributaries
 
There are two leading methods geographers and potamologists (people who study rivers) use to classify tributaries. 
 
The first method lists a river's tributaries starting with those closest to the source, or headwaters, of the river. The earliest tributaries of the Rhine River, for example, include the Thur River of Switzerland and the Ill River of Austria. The Rhine, one of the longest rivers in Europe, has its source in the Alps and its mouth in the North Sea.
 
The second method lists a river's tributaries by their flow. Small streams are identified with low numbers, while larger tributaries have higher numbers. The Tshuapa and Kasai Rivers are both left-bank tributaries of the Congo River, the deepest river in the world. The Tshuapa is a smaller river, and has a lower tributary ranking, than the Kasai. 
 
People and Tributaries
 
Human activity in tributaries is often responsible for polluting the mainstem. The river carries all the runoff and pollution from all its tributaries. 
 
Rivers with tributaries that drain land that is not used for agriculture or development are usually less polluted than rivers with tributaries in agricultural or urban areas. 
 
Development, not size, determines the pollution of rivers. The Amazon River, with the largest drainage basin in the world, is much cleaner than the Hudson River, for instance. Tributaries to the Amazon flow through undeveloped regions of the Andes Mountains and rain forests in Brazil, Venezuela, Colombia, Peru, and Bolivia. The Hudson River flows through one of the largest urban areas on Earth, New York City.
 
tributary
The Ganges has more than a dozen tributaries, including the Alakananda and Bhagirathi, above.

Three Rivers
The Allegheny and Monongahela rivers are tributaries of the Ohio River. The confluence of these rivers is in Pittsburgh, Pennsylvania. Three Rivers is the name of a neighborhood in Pittsburgh, as well as the sports stadium used by the Pittsburgh Steelers and Pittsburgh Pirates until 2000.

Spiritual Tributary
People have built many great cities at points where tributaries join major rivers. In India, Allahabad is a city of more than 1 million people built where the Yamuna and the Saraswati rivers join the Ganges River. Allahabad is considered one of the holiest places in the Hindu religion, and some believe it is where Brahma, the god of creation, offered his first sacrifice after creating the world.

affluent
Noun

stream that feeds, or flows, into a larger stream. Also called a tributary.
agriculture
Noun

the art and science of cultivating land for growing crops (farming) or raising livestock (ranching).
bifurcate
Verb

divide or split into two branches.
confluence
Noun

place where two rivers join and flow together.
continental divide
Noun

point or area that separates which directions a continent's river systems flow.
development
Noun

construction or preparation of land for housing, industry, or agriculture.
distributary
Noun

stream that branches off from the main stem of a river or other flowing fluid.
downstream
Noun

in the direction of a flow, toward its end.
drainage basin
Noun

an entire river system or an area drained by a river and its tributaries. Also called a watershed.
fork
Noun

branch or tributary of a river, usually having the same name as the river itself.
freshwater
Noun

water that is not salty.
geographer
Noun

person who studies places and the relationships between people and their environments.
headwater
Noun

source of a river.
mainstem
Noun

largest river or channel in a watershed or drainage basin.
Midwest
Noun

area of the United States consisting of the following states: Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin.
mouth
Noun

place where a river empties its water. Usually rivers enter another body of water at their mouths.
originate
Verb

to begin or start.
pollution
Noun

introduction of harmful materials into the environment.
potamologist
Noun

person who studies rivers.
province
Noun

division of a country larger than a town or county.
rainforest
Noun

area of tall, mostly evergreen trees and a high amount of rainfall.
religion
Noun

a system of spiritual or supernatural belief.
responsible
Adjective

primary cause of something.
river
Noun

large stream of flowing fresh water.
runoff
Noun

overflow of fluid from a farm or industrial factory.
snowmelt
Noun

water supplied by snow.
source
Noun

beginning of a stream, river, or other flow of water.
stream
Noun

body of flowing water.
tributary
Noun

stream that feeds, or flows, into a larger stream.
urban area
Noun

developed, densely populated area where most inhabitants have nonagricultural jobs.
watershed
Noun

entire river system or an area drained by a river and its tributaries.

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    Editor

    Jeannie Evers, Emdash Editing

    Producer

    Caryl-Sue, National Geographic Society

    Last Updated

    April 18, 2013

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