Editing Braided river

{{short description|Network of river channels}}

[[File:Rakaia River NZ aerial braided.jpg|thumb|The [[Rakaia River]] in the [[South Island]] of New Zealand is braided over most of its course]]

A '''braided river''' (also called '''braided channel''' or '''braided stream''') consists of a network of river [[channel (geography)|channel]]s separated by small, often temporary, [[island]]s called ''[[braid bar]]s'' or, in British English usage, ''[[ait]]s'' or ''eyots''.

Braided streams tend to occur in rivers with high [[sediment]] loads or coarse grain sizes, and in rivers with steeper [[Stream slope|slopes]] than typical rivers with straight or [[meander]]ing channel patterns. They are also associated with rivers with rapid and frequent variation in the amount of water they carry, i.e., with "[[Flash flood|flashy]]" rivers, and with rivers with weak [[River bank|banks]].

Braided channels are found in a variety of environments all over the world, including gravelly mountain streams, sand bed rivers, on [[alluvial fan]]s, on [[river delta]]s, and across depositional plains.<ref name=BristowBest>{{cite journal |title=Braided rivers: perspectives and problems |first1=C. S. |last1=Bristow |first2=J. L. |last2=Best |journal=Geological Society, London, Special Publications |volume=75 |pages=1–11 |date=1 January 1993  |issue=1 |doi=10.1144/GSL.SP.1993.075.01.01|bibcode=1993GSLSP..75....1B |s2cid=129232374 }}</ref>

==Description==
A braided river consists of a network of multiple shallow channels that diverge and rejoin around [[ephemeral]] ''braid bars''. This gives the river a fancied resemblance to the interwoven strands of a [[braid]].<ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=braided stream}}</ref><ref name=Leeder2011>{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins : from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=9781405177832 |edition=2nd |pages=247–252}}</ref> The braid bars, also known as channel bars,{{sfn|Jackson|1997|loc="channel bar"}} branch islands,{{sfn|Jackson|1997|loc="branch island"}} or accreting islands, are usually unstable and may be completely covered at times of high water.<ref name=Leeder2011/> The channels and braid bars are usually highly mobile, with the river layout often changing significantly during [[flood]] events.<ref>{{cite journal |last1=Hickin |first1=E |last2=Sichingabula |first2=H |title=The geomorphic impact of the catastrophic October 1984 flood on the planform of the Squamish River, southwestern British Columbia |journal=Canadian Journal of Earth Sciences |year=1988 |volume=25 |issue=7 |pages=1078–1087 |doi=10.1139/e88-105|bibcode=1988CaJES..25.1078H }}</ref> When the islets separating channels are stabilized by vegetation, so that they are more permanent features, they are sometimes called [[ait]]s or eyots.<ref>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth}}</ref>

A braided river differs from a ''[[meandering river]]'', which has a single sinuous channel. It is also distinct from an ''[[anastomosing river]]'', which consist of multiple interweaving semi-permanent channels which are separated by floodplain rather than channel bars; these channels may themselves be braided.<ref name=Leeder2011/>

==Formation==
[[File:White River 5965.JPG|thumb|right|The [[White River (Washington)|White River]] in the [[U.S. state]] of [[Washington (state)|Washington]] transports a large sediment load from the [[Emmons Glacier]] of [[Mount Rainier]], a young, rapidly eroding volcano.]]
The physical processes that determine whether a river will be braided or meandering are not fully understood.{{sfn|Leeder|2011|p=248}}<ref name="MurrayPaola1994"/> However, there is wide agreement that a river becomes braided when it carries an abundant supply of sediments.<ref name=Jackson1997/>{{sfn|Leeder|2011|p=248}}<ref name=Gray>{{cite journal |last1=Gray |first1=D. |last2=Harding |first2=J.S. |title=Braided river ecology: A literature review of physical habitats and aquatic invertebrate communities |journal=Science for Conservation |year=2007 |issue=279 |url=https://dcon01mstr0c21wprod.azurewebsites.net/globalassets/documents/science-and-technical/sfc279.pdf |access-date=9 May 2022}}</ref>

Experiments with [[flume]]s suggest that a river becomes braided when a threshold level of [[sediment]] load or slope is reached. On timescales long enough for the river to evolve, a sustained increase in sediment load will increase the bed slope of the river, so that a variation of slope is equivalent to a variation in sediment load, provided the amount of water carried by the river is unchanged. A threshold slope was experimentally determined to be 0.016&nbsp;(ft/ft) for a {{convert|0.15|cuft/s|m3/s|abbr=on}} stream with poorly sorted coarse sand. Any slope over this threshold created a braided stream, while any slope under the threshold created a [[meander]]ing stream or – for very low slopes – a straight channel. Also important to channel development is the proportion of [[suspended load]] sediment to [[bed load]]. An increase in suspended sediment allowed for the deposition of fine [[erosion]]-resistant material on the inside of a curve, which accentuated the curve and in some instances, caused a river to shift from a braided to a [[meander]]ing profile.<ref name=Schumm>{{cite journal |last1=Schumm |first1=S |last2=Kahn |first2=H |title=Experimental Study of Channel Patterns |journal=Bulletin of the Geological Society of America |year=1972 |volume=83 |issue=6 |pages=1755–1770 |doi=10.1130/0016-7606(1972)83[1755:esocp]2.0.co;2}}</ref>

These experimental results were expressed in formulas relating the critical slope for braiding to the discharge and grain size. The higher the discharge, the lower the critical slope, while larger grain size yields a higher critical slope. However, these give only an incomplete picture,{{sfn|Leeder|2011|p=248}} and numerical simulations have become increasingly important for understanding braided rivers.<ref>{{cite journal |last1=Williams |first1=Richard D. |last2=Brasington |first2=James |last3=Hicks |first3=D. Murray |title=Numerical Modelling of Braided River Morphodynamics: Review and Future Challenges: Modelling Braided River Morphodynamics |journal=Geography Compass |date=March 2016 |volume=10 |issue=3 |pages=102–127 |doi=10.1111/gec3.12260|url=http://eprints.gla.ac.uk/114536/7/114536.pdf }}</ref><ref name=MurrayPaola1994>{{cite journal |last1=Murray |first1=A. Brad |last2=Paola |first2=Chris |title=A cellular model of braided rivers |journal=Nature |date=September 1994 |volume=371 |issue=6492 |pages=54–57 |doi=10.1038/371054a0|bibcode=1994Natur.371...54M |s2cid=4276051 }}</ref>

[[Aggradation]] (net deposition of sediments) favors braided rivers, but is not essential. For example, the [[Rakaia River|Rakaia]] and [[Waitaki River]]s of New Zealand are not aggrading, due to retreating shorelines, but are nonetheless braided rivers. Variable discharge has also been identified as important in braided rivers,<ref name=Leopold>{{cite journal |last1=Leopold |first1=L.B. |last2=Wolman |first2=M.G. |title=River channel patterns: Braiding, meandering, and straight |journal=U.S. Geological Survey Professional Papers |series=Professional Paper |year=1957 |volume=282-B |pages=39–85 |doi=10.3133/pp282B|doi-access=free |bibcode=1957usgs.rept....7L }}</ref> but this may be primarily due to the tendency for frequent floods to reduce bank vegetation and destabilize the banks, rather than because variable discharge is an essential part of braided river formation.{{sfn|Williams|Brasington|Hicks|2016|p=104}}

Numerical models suggest that bedload transport (movement of sediment particles by rolling or [[Saltation (geology)|bouncing]] along the river bottom) is essential to formation of braided rivers, with net erosion of sediments at channel divergences and net deposition at convergences. Braiding is reliably reproduced in simulations whenever there is little lateral constraint on flow and there is significant bedload transport. Braiding is not observed in simulations of the extreme cases of pure scour (no deposition taking place), which produces a dendritic system, or of cohesive sediments with no bedload transport. Meanders fully develop only when the river banks are sufficiently stabilized to limit lateral flow.<ref name=MurrayPaola1994/> An increase in suspended sediment relative to bedload allows the deposition of fine [[erosion]]-resistant material on the inside of a curve, which accentuated the curve and in some instances, causes a river to shift from a braided to a [[meander]]ing profile.<ref name=Schumm/> A stream with cohesive banks that are resistant to erosion will form narrow, deep, meandering channels, whereas a stream with highly erodible banks will form wide, shallow channels, preventing the helical flow of the water necessary for meandering and resulting in the formation of braided channels.<ref name="Easterbrook, Don J. 1999">{{cite book |last1=Easterbrook |first1=Don J. |title=Surface processes and landforms |date=1999 |publisher=Prentice Hall |location=Upper Saddle River, New Jersey |isbn=978-0138609580 |edition=2nd}}</ref>

==Occurrences==
<!-- This section is not meant to be a comprehensive list of all braided rivers. Include only particularly notable examples. -->
[[File:STS065-96-048.jpg|thumb|[[Brahmaputra River]] seen from the Space Shuttle]]
Braided rivers occur in many environments, but are most common in wide valleys associated with mountainous regions or their [[Piedmont (geography)|piedmonts]]{{sfn|Williams|Brasington|Hicks|2016|p=104}} or in areas of coarse-grained sediments and limited growth of vegetation near the river banks.<ref name="Ashmore2013">{{cite journal |last1=Ashmore |first1=P. |title=9.17 Morphology and Dynamics of Braided Rivers |journal=Treatise on Geomorphology |date=2013 |pages=289–312 |doi=10.1016/B978-0-12-374739-6.00242-6|isbn=9780080885223 }}</ref> They are also found on fluvial (stream-dominated) [[alluvial fan]]s.<ref>{{cite book |last1=Boggs |first1=Sam Jr. |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, New Jersey |isbn=0131547283 |edition=4th |page=248}}</ref> Extensive braided river systems are found in [[Alaska]], [[Canada]], [[New Zealand]]'s [[South Island]], and the [[Himalayas]], which all contain young, rapidly eroding mountains.

* The enormous [[Brahmaputra River|Brahmaputra]] river in Northeastern India is a classic example of a braided river.<ref name="Catling1992">{{cite book |last=Catling |first=David |title=Rice in deep water |url=https://books.google.com/books?id=N5JxwKx1RAgC&pg=PA177 |access-date=23 April 2011 |year=1992 |publisher=[[International Rice Research Institute]] |isbn=978-971-22-0005-2 |page=177}}</ref>
* A notable example of a large braided stream in the contiguous United States is the [[Platte River]] in central and western [[Nebraska]].<ref>{{cite journal |last1=Blodgett |first1=R.H. |last2=Stanley |first2=K.O. |title=Stratification, Bedforms, and Discharge Relations of the Platte Braided River System, Nebraska |journal=SEPM Journal of Sedimentary Research |date=1980 |volume=50 |issue=1 |doi=10.1306/212F7987-2B24-11D7-8648000102C1865D}}</ref> Platte-type braided rivers are characterized by abundant linguoid (tonguelike) bar and dune deposits.<ref name=Miall1977>{{cite journal |last1=Miall |first1=Andrew D. |title=A review of the braided-river depositional environment |journal=Earth-Science Reviews |date=May 1977 |volume=13 |issue=1 |pages=1–62 |doi=10.1016/0012-8252(77)90055-1|bibcode=1977ESRv...13....1M }}</ref>
* The Scott River of southern [[Alaska]] is the type for braided glacial outwash rivers characterized by longitudinal gravel bars and by sand lenses deposited in scours from times of high water.<ref name=Miall1977/>
* The Donjek River of the [[Yukon River|Yukon Basin]] is the type for braided rivers showing repeated cycles of deposition, with finer sediments towards the top of each cycle.<ref name=Miall1977/>
* The [[Bijou Creek]] of Colorado is the type for braided rivers characterized by laminated sand deposits emplaced during floods.<ref name=Miall1977/>
* A portion of the lower [[Yellow River]] takes a braided form.<ref>{{cite journal |last=Chien |first=N. |year=1961 |title=The braided stream of the lower Yellow River |journal=Scientia Sinica |volume=10 |pages=734–754}}</ref>
* The Sewanee Conglomerate, a Pennsylvanian coarse sandstone and conglomerate unit<ref>{{cite conference |last=Fraley |first=Thear Kirk |title=Depositional Environment of Lower Pennsylvanian Sewanee Conglomerate, Lookout Mountain, Georgia |conference=Geological Society of America Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting |date=13–16 March 2010}}</ref> present on the [[Cumberland Plateau]] near the [[University of the South]], may have been deposited by an ancient braided and meandering river that once existed in the eastern United States.<ref>{{cite book |last1=Churnet |first1=Habte Giorgis |last2=Bergenback |first2=Richard E. |title=Depositional Systems of Pennsylvanian Rocks in the Cumberland Plateau of Southern Tennessee |year=1986 |publisher=Georgia Geological Society}}</ref> Others have interpreted the depositional environment for this unit as a tidal delta.<ref>{{cite journal |last1=Ferm |first1=J. C. |last2=Miliei |first2=R. C. |last3=Eason |first3=J.E. |year=1972 |title=Carboniferous depositional environments in the Cumberland Plateau of Southern Tennessee and Northern Alabama |journal=Tennessee Div. Geology Rept. Inv. |issue=33}}</ref>
[[File:Le Tagliamento, pris du pont di Pisano.jpg|thumb|Tagliamento River seen from the Pinzano bridge<ref>{{cite web | url=https://www.google.com/maps/place/Ponte+di+Pinzano+al+Tagliamento/@46.1846956,12.9556653,17.17z/data=!4m13!1m7!3m6!1s0x477a296db4e7c121:0xdcf471d684c8b8b1!2s33094+Pinzano+Al+Tagliamento+PN!3b1!8m2!3d46.1830944!4d12.9455901!3m4!1s0x477a293ff75c5077:0x14600045fa75c25a!8m2!3d46.1847356!4d12.9569417 | title=Google Maps }}</ref>]]
* The [[Tagliamento]] of Italy is an example of a gravel bed braided river.<ref>{{cite journal |last1=Bertoldi |first1=W. |last2=Zanoni |first2=L. |last3=Tubino |first3=M. |title=Assessment of morphological changes induced by flow and flood pulses in a gravel bed braided river: The Tagliamento River (Italy) |journal=Geomorphology |date=January 2010 |volume=114 |issue=3 |pages=348–360 |doi=10.1016/j.geomorph.2009.07.017|bibcode=2010Geomo.114..348B }}</ref>
* The [[Piave (river)|Piave]], also in Italy, is an example of a river that is transitioning from braided to meandering due to human interventions.<ref>{{cite journal |last1=Surian |first1=Nicola |title=Channel changes due to river regulation: the case of the Piave River, Italy |journal=Earth Surface Processes and Landforms |date=8 November 1999 |volume=24 |issue=12 |pages=1135–1151 |doi=10.1002/(SICI)1096-9837(199911)24:12<1135::AID-ESP40>3.0.CO;2-F|bibcode=1999ESPL...24.1135S }}</ref>
* The [[Waimakariri River]] of New Zealand is an example of a braided river with an extensive floodplain.<ref>{{cite journal |last1=Reinfelds |first1=Ivars |last2=Nanson |first2=Gerald |title=Formation of braided river floodplains, Waimakariri River, New Zealand |journal=Sedimentology |date=December 1993 |volume=40 |issue=6 |pages=1113–1127 |doi=10.1111/j.1365-3091.1993.tb01382.x|bibcode=1993Sedim..40.1113R }}</ref>

<gallery widths="200px" heights="160px">
File:Waimakariri03 gobeirne.jpg|[[Waimakariri River]] with the [[:en:Southern Alps|Southern Alps]] in the background, [[Canterbury Region|Canterbury]], New Zealand
File:Rakaia_River_NZ_aerial_closer.jpg|[[Rakaia River]], South Island, New Zealand
File:Braided_Stream_FairbanksTananaRiver.jpg|Tanana River, Fairbanks, Alaska, United States
File:Toklat_River_-_East_Fork_01.jpg|Toklat River, Denali National Park and Preserve, Alaska, United States
File:Intermittent_Medano_Creek_Seeps_through_Desert_Sands.jpg|Medano Creek, Great Sand Dunes National Park and Preserve, Colorado, United States
</gallery>

==See also==
* {{annotated link|Anabranch}}
* {{annotated link|Lagoon}}
* {{annotated link|Shoal}}

==References==
{{Reflist}}

==Further reading==
* {{Cite book |editor-last=Hibbert |editor-first=Barry |editor2-last=Brown |editor2-first=Kerry |date=2001 |title=Braided River Field Guide |location=Christchurch, N.Z. |publisher=Department of Conservation |isbn=9780478221213}}

==External links==
* {{Commons category-inline|Braided rivers}}

{{river morphology}}
{{Rivers, streams and springs}}
{{sediment transport}}
{{Authority control}}

[[Category:Rivers]]
[[Category:Geomorphology]]
[[Category:Fluvial landforms]]
[[Category:Sedimentology]]
[[Category:Water streams]]
[[Category:Braided rivers| ]]