Editing Braided river (section)

==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>