Atributary,[1] or anaffluent,[2] is astream orriver that flows into a larger stream (main stem or"parent"), river, or alake.[3] A tributary does not flow directly into asea orocean.[4] Tributaries, and the main stem river into which they flow, drain the surroundingdrainage basin of its surface water andgroundwater, leading the water out into an ocean, another river, or into anendorheic basin.
TheIrtysh, a tributary of theOb river, is the longest tributary river in the world with a length of 4,248 km (2,640 mi).TheMadeira River is the largest tributary river by volume in the world with an average discharge of 31,200 m3/s (1.1 million cu ft/s).
The opposite to a tributary is adistributary, a river or stream that branches off from and flows away from the main stream.[5] Distributaries are most often found inriver deltas.
Right tributary, orright-bank tributary, andleft tributary, orleft-bank tributary, describe the orientation of the tributary relative to the flow of the main stem river. These terms are defined from the perspective of looking downstream, that is, facing the direction thewater current of the main stem is going. In anavigational context, if one were floating on araft or other vessel in the main stream, this would be the side the tributary enters from as one floats past; alternately, if one were floating down the tributary, the main stream meets it on theopposite bank of the tributary. This information may be used to avoidturbulent water by moving towards the opposite bank before approaching the confluence.[6]
Anearly tributary is a tributary that joins the main stem river closer to its source than its mouth, that is, before the river'smidpoint; alate tributary joins the main stem further downstream, closer to its mouth than to its source, that is, after the midpoint.
In the United States, where tributaries sometimes have the same name as the river into which they feed, they are calledforks. These are typically designated by compass direction. For example, theAmerican River in California receives flow from its North, Middle, and South forks. TheChicago River's North Branch has the East, West, and Middle Fork; the South Branch has its South Fork, and used to have a West Fork as well (now filled in). Forks are sometimes designated as right or left. Here, the handedness is from the point of view of an observer facing upstream. For instance,Steer Creek has a left tributary which is called Right Fork Steer Creek.
These naming conventions are reflective of the circumstances of a particular river's identification and charting: people living along the banks of a river, with a name known to them, may then float down the river in exploration, and each tributary joining it as they pass by appears as a new river, to be given its own name, perhaps one already known to the people who live upon its banks. Conversely, explorers approaching a new land from the sea encounter its rivers at their mouths, where they name them on their charts, then, following a river upstream, encounter each tributary as a forking of the stream to the right and to the left, which then appear on their charts as such; or the streams are seen to diverge by thecardinal direction (north, south, east, or west) in which they proceed upstream, sometimes a third stream entering between two others is designated themiddle fork; or the streams are distinguished by the relative height of one to the other, as one stream descending over acataract into another becomes theupper fork, and the one it descends into, thelower; or by relative volume: the smaller stream designated thelittle fork, the larger either retaining its name unmodified, or receives the designationbig.[7]
Tributaries are sometimes listed starting with those nearest to thesource of the river and ending with those nearest to themouth of the river. TheStrahler stream order examines the arrangement of tributaries in ahierarchy of first, second, third and higher orders, with the first-order tributary being typically the least in size. For example, a second-order tributary would be the result of two or more first-order tributaries combining to form the second-order tributary.[6]
The basin of theAmazon River is a system made up of many tributary streams. The streams shown on the map besides the Amazon are tributaries of the Amazon.
TheBenue River is fed by multiple tributaries originating in theAdamawa Plateau; many of the highest tributaries are seasonal streams. The Benue is itself a major tributary of theNiger.
TheMekong is a trans-boundary river, originating in theTibetan Plateau. Its upper tributary river systems (e.g.ngom chu [zh]) are restricted to narrowgorges, but the tributaries that feed its lower reaches (e.g. theMun River) cover larger areas.
The water basin of theWabash River; the other rivers (not including theOhio River) are tributaries of the Wabash River. The Vermillion River (and itsforks) is a highlighted example of a tributary of the Wabash River. The Wabash River is also a tributary of the Ohio River, which in turn is a tributary of theMississippi River.
TheYamuna is the second-largest tributary river of theGanges and the longest tributary inIndia. It flows almost parallel to the Ganges about its right bank for 1,376 kilometres (855 mi) before merging with it at theTriveni Sangam,Allahabad.
^Stewart, George R. (1939). "Nomenclature of Stream-Forks on the West Slope of the Sierra Nevada".American Speech.14 (3):191–197.doi:10.2307/451418.JSTOR451418.
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![The Mekong is a trans-boundary river, originating in the Tibetan Plateau. Its upper tributary river systems (e.g. ngom chu [zh]) are restricted to narrow gorges, but the tributaries that feed its lower reaches (e.g. the Mun River) cover larger areas.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMekongbasin.jpg)
