Braided river

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The Rakaia River in the South Island of New Zealand is braided over most of its course Rakaia River NZ aerial braided.jpg
The Rakaia River in the South Island of New Zealand is braided over most of its course

A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.

Contents

Braided streams tend to occur in rivers with high sediment loads or coarse grain sizes, and in rivers with steeper slopes than typical rivers with straight or meandering channel patterns. They are also associated with rivers with rapid and frequent variation in the amount of water they carry, i.e., with "flashy" rivers, and with rivers with weak banks.

Braided channels are found in a variety of environments all over the world, including gravelly mountain streams, sand bed rivers, on alluvial fans, on river deltas, and across depositional plains. [1]

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 interweaved strands of a braid. [2] [3] The braid bars, also known as channel bars, [4] branch islands, [5] or accreting islands, are usually unstable and may be completely covered at times of high water. [3] The channels and braid bars are usually highly mobile, with the river layout often changing significantly during flood events. [6] When the islets separating channels are stabilized by vegetation, so that they are more permanent features, they are sometimes called aits or eyots. [7]

A braided river differs from a meandering river, which has a single sinuous channel. It is also distinct from an anastomosing river. Anastomosing rivers are similar to braided rivers in that they consist of multiple interweaving channels. However, anastomosing rivers consist of semi-permanent channels which are separated by floodplain rather than channel bars. These channels may themselves be braided. [3]

Formation

The White River in the U.S. state of Washington transports a large sediment load from the Emmons Glacier of Mount Rainier, a young, rapidly eroding volcano. White River 5965.JPG
The White River in the U.S. state of 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. [8] [9] However, there is wide agreement that a river becomes braided when it carries an abundant supply of sediments. [2] [8] [10]

Experiments with flumes 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 (ft/ft) for a 0.15 cu ft/s (0.0042 m3/s) stream with poorly sorted coarse sand. Any slope over this threshold created a braided stream, while any slope under the threshold created a meandering 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 meandering profile. [11]

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, [8] and numerical simulations have become increasingly important for understanding braided rivers. [12] [13]

Aggradation (net deposition of sediments) favors braided rivers, but is not essential. For example, the Rakaia and Waitaki Rivers 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, [14] 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. [15]

Numerical models suggest that bedload transport (movement of sediment particles by rolling or 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. [13] 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 meandering profile. [11] 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. [16]

Occurrences

Brahmaputra River seen from the Space Shuttle STS065-96-048.jpg
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 piedmonts [15] or in areas of coarse-grained sediments and limited growth of vegetation near the river banks. [17] They are also found on fluvial (stream-dominated) alluvial fans. [18] Extensive braided river systems are found in Alaska, Canada, New Zealand's South Island, and the Himalayas, which all contain young, rapidly eroding mountains.

Tagliamento River seen from the Pinzano bridge Le Tagliamento, pris du pont di Pisano.jpg
Tagliamento River seen from the Pinzano bridge

See also

Related Research Articles

<span class="mw-page-title-main">Sediment</span> Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.

<span class="mw-page-title-main">River delta</span> Silt deposition landform at the mouth of a river

A river delta is a landform shaped like a triangle, created by deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the Greek letter Delta. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution.

<span class="mw-page-title-main">Alluvial fan</span> Fan-shaped deposit of sediment

An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than 1 square kilometer (0.4 sq mi) to almost 20,000 square kilometers (7,700 sq mi).

<span class="mw-page-title-main">Fluvial processes</span> Processes associated with rivers and streams

In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

<span class="mw-page-title-main">Oxbow lake</span> U-shaped lake or pool

An oxbow lake is a U-shaped lake or pool that forms when a wide meander of a river is cut off, creating a free-standing body of water. In South Texas, oxbows left by the Rio Grande are called resacas. In Australia, oxbow lakes are called billabongs. The word "oxbow" can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream.

<span class="mw-page-title-main">Meander</span> One of a series of curves in a channel of a matured stream

A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank and deposits sediments on an inner, convex bank which is typically a point bar. The result of this coupled erosion and sedimentation is the formation of a sinuous course as the channel migrates back and forth across the axis of a floodplain.

The terms river morphology and its synonym stream morphology are used to describe the shapes of river channels and how they change in shape and direction over time. The morphology of a river channel is a function of a number of processes and environmental conditions, including the composition and erodibility of the bed and banks ; erosion comes from the power and consistency of the current, and can effect the formation of the river's path. Also, vegetation and the rate of plant growth; the availability of sediment; the size and composition of the sediment moving through the channel; the rate of sediment transport through the channel and the rate of deposition on the floodplain, banks, bars, and bed; and regional aggradation or degradation due to subsidence or uplift. River morphology can also be affected by human interaction, which is a way the river responds to a new factor in how the river can change its course. An example of human induced change in river morphology is dam construction, which alters the ebb flow of fluvial water and sediment, therefore creating or shrinking estuarine channels. A river regime is a dynamic equilibrium system, which is a way of classifying rivers into different categories. The four categories of river regimes are Sinuous canali- form rivers, Sinuous point bar rivers, Sinuous braided rivers, and Non-sinuous braided rivers.

<span class="mw-page-title-main">Sediment transport</span> Movement of solid particles, typically by gravity and fluid entrainment

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

<span class="mw-page-title-main">Sedimentary budget</span>

Sedimentary budgets are a coastal management tool used to analyze and describe the different sediment inputs (sources) and outputs (sinks) on the coasts, which is used to predict morphological change in any particular coastline over time. Within a coastal environment the rate of change of sediment is dependent on the amount of sediment brought into the system versus the amount of sediment that leaves the system. These inputs and outputs of sediment then equate to the total balance of the system and more than often reflect the amounts of erosion or accretion affecting the morphology of the coast.

<span class="mw-page-title-main">Point bar</span> Landform related to streams and rivers

A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than, towheads, or river islands.

<span class="mw-page-title-main">River</span> Natural flowing watercourse

A river is a natural flowing watercourse, usually freshwater stream, flowing on the surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland or another river. In some cases, a river flows into the ground or becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as creek, brook, rivulet and rill. There are no official definitions for the generic term river as applied to geographic features, although in some countries or communities a stream is defined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and Northeast England, and "beck" in Northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.

<span class="mw-page-title-main">Avulsion (river)</span> Rapid abandonment of a river channel and formation of a new channel

In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.

<span class="mw-page-title-main">Bar (river morphology)</span> Elevated region of sediment in a river that has been deposited by the flow

A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.

River channel migration is the geomorphological process that involves the lateral migration of an alluvial river channel across its floodplain. This process is mainly driven by the combination of bank erosion of and point bar deposition over time. When referring to river channel migration, it is typically in reference to meandering streams. In braided streams, channel change is driven by sediment transport.

Three components that are included in the load of a river system are the following: dissolved load, wash load and bed material load. The bed material load is the portion of the sediment that is transported by a stream that contains material derived from the bed. Bed material load typically consists of all of the bed load, and the proportion of the suspended load that is represented in the bed sediments. It generally consists of grains coarser than 0.062 mm with the principal source being the channel bed. Its importance lies in that its composition is that of the bed, and the material in transport can therefore be actively interchanged with the bed. For this reason, bed material load exerts a control on river channel morphology. Bed load and wash load together constitute the total load of sediment in a stream. The order in which the three components of load have been considered – dissolved, wash, bed material – can be thought of as progression: of increasingly slower transport velocities, so that the load peak lags further and further behind the flow peak during any event.

<span class="mw-page-title-main">Alluvial river</span> Type of river

An alluvial river is one in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. For this reason, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology; and the amount, size, and type of sediment that they carry.

Channel patterns are found in rivers, streams, and other bodies of water that transport water from one place to another. Systems of branching river channels dissect most of the sub-aerial landscape, each in a valley proportioned to its size. Whether formed by chance or necessity, by headward erosion or downslope convergence, whether inherited or newly formed. Depending on different geological factors such as weathering, erosion, depositional environment, and sediment type, different types of channel patterns can form.

<span class="mw-page-title-main">Braid bar</span> Depositional landform in a river which splits a channel

Braid bars, or mid-channel bars, are river landforms typically present in braided river channels. These formations have many names, including medial, longitudinal, crescentic, and transverse bars, as well as the more colloquial sandflat. Braid bars are distinguished from point bars due to their presence in the middle of a flow channel, rather than along a bank of the river channel.

<span class="mw-page-title-main">Slip-off slope</span> Depositional landform on the inside convex bank of a meandering river

A slip-off slope is a depositional landform that occurs on the inside convex bank of a meandering river. The term can refer to two different features: one in a freely meandering river with a floodplain and the other in an entrenched river.

<span class="mw-page-title-main">Glaciofluvial deposits</span> Sediments/deposits formed from ice sheets or glaciers

Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers. They are transported, sorted and deposited by streams of water. The deposits are formed beside, below or downstream from the ice. They include kames, kame terraces and eskers formed in ice contact and outwash fans and outwash plains below the ice margin. Typically the outwash sediment is carried by fast and turbulent fluvio-glacial meltwater streams, but occasionally it is carried by catastrophic outburst floods. Larger elements such as boulders and gravel are deposited nearer to the ice margin, while finer elements are carried farther, sometimes into lakes or the ocean. The sediments are sorted by fluvial processes. They differ from glacial till, which is moved and deposited by the ice of the glacier, and is unsorted.

References

  1. ↑ Bristow, C. S.; Best, J. L. (1 January 1993). "Braided rivers: perspectives and problems". Geological Society, London, Special Publications. 75 (1): 1–11. Bibcode:1993GSLSP..75....1B. doi:10.1144/GSL.SP.1993.075.01.01. S2CID   129232374.
  2. 1 2 Jackson, Julia A., ed. (1997). "braided stream". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN   0922152349.
  3. 1 2 3 Leeder, M. R. (2011). Sedimentology and sedimentary basins : from turbulence to tectonics (2nd ed.). Chichester, West Sussex, UK: Wiley-Blackwell. pp. 247–252. ISBN   9781405177832.
  4. ↑ Jackson 1997, "channel bar".
  5. ↑ Jackson 1997, "branch island".
  6. ↑ Hickin, E; Sichingabula, H (1988). "The geomorphic impact of the catastrophic October 1984 flood on the planform of the Squamish River, southwestern British Columbia". Canadian Journal of Earth Sciences. 25 (7): 1078–1087. Bibcode:1988CaJES..25.1078H. doi:10.1139/e88-105.
  7. ↑ Allaby, Michael (2013). A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN   9780199653065.
  8. 1 2 3 Leeder 2011, p. 248.
  9. ↑ Murray, A. Brad; Paola, Chris (September 1994). "A cellular model of braided rivers". Nature. 371 (6492): 54–57. Bibcode:1994Natur.371...54M. doi:10.1038/371054a0. S2CID   4276051.
  10. ↑ Gray, D.; Harding, J.S. (2007). "Braided river ecology: A literature review of physical habitats and aquatic invertebrate communities" (PDF). Science for Conservation (279). Retrieved 9 May 2022.
  11. 1 2 Schumm, S; Kahn, H (1972). "Experimental Study of Channel Patterns". Bulletin of the Geological Society of America. 83 (6): 1755–1770. doi:10.1130/0016-7606(1972)83[1755:esocp]2.0.co;2.
  12. ↑ Williams, Richard D.; Brasington, James; Hicks, D. Murray (March 2016). "Numerical Modelling of Braided River Morphodynamics: Review and Future Challenges: Modelling Braided River Morphodynamics" (PDF). Geography Compass. 10 (3): 102–127. doi:10.1111/gec3.12260.
  13. 1 2 Murray, A. Brad; Paola, Chris (September 1994). "A cellular model of braided rivers". Nature. 371 (6492): 54–57. Bibcode:1994Natur.371...54M. doi:10.1038/371054a0. S2CID   4276051.
  14. ↑ Leopold, L.B.; Wolman, M.G. (1957). "River channel patterns: Braiding, meandering, and straight". U.S. Geological Survey Professional Papers. Professional Paper. 282-B: 39–85. doi: 10.3133/pp282B .
  15. 1 2 Williams, Brasington & Hicks 2016, p. 104.
  16. ↑ Easterbrook, Don J. (1999). Surface processes and landforms (2nd ed.). Upper Saddle River, N.J.: Prentice Hall. ISBN   978-0138609580.
  17. ↑ Ashmore, P. (2013). "9.17 Morphology and Dynamics of Braided Rivers". Treatise on Geomorphology: 289–312. doi:10.1016/B978-0-12-374739-6.00242-6. ISBN   9780080885223.
  18. ↑ Boggs, Sam Jr. (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. p. 248. ISBN   0131547283.
  19. ↑ Catling, David (1992). Rice in deep water. International Rice Research Institute. p. 177. ISBN   978-971-22-0005-2 . Retrieved 23 April 2011.
  20. ↑ Blodgett, R.H.; Stanley, K.O. (1980). "Stratification, Bedforms, and Discharge Relations of the Platte Braided River System, Nebraska". SEPM Journal of Sedimentary Research. 50 (1). doi:10.1306/212F7987-2B24-11D7-8648000102C1865D.
  21. 1 2 3 4 Miall, Andrew D. (May 1977). "A review of the braided-river depositional environment". Earth-Science Reviews. 13 (1): 1–62. Bibcode:1977ESRv...13....1M. doi:10.1016/0012-8252(77)90055-1.
  22. ↑ Chien, N. (1961). "The braided stream of the lower Yellow River". Scientia Sinica. 10: 734–754.
  23. ↑ Fraley, Thear Kirk (13–16 March 2010). Depositional Environment of Lower Pennsylvanian Sewanee Conglomerate, Lookout Mountain, Georgia. Geological Society of America Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting.
  24. ↑ Churnet, Habte Giorgis; Bergenback, Richard E. (1986). Depositional Systems of Pennsylvanian Rocks in the Cumberland Plateau of Southern Tennessee. Georgia Geological Society.
  25. ↑ Ferm, J.C.; Miliei, R.C.; Eason, J.E. (1972). "Carboniferous depositional environments in the Cumberland Plateau of Southern Tennessee and Northern Alabama". Tennessee Div. Geology Rept. Inv. (33).
  26. ↑ "Google Maps".
  27. ↑ Bertoldi, W.; Zanoni, L.; Tubino, M. (January 2010). "Assessment of morphological changes induced by flow and flood pulses in a gravel bed braided river: The Tagliamento River (Italy)". Geomorphology. 114 (3): 348–360. Bibcode:2010Geomo.114..348B. doi:10.1016/j.geomorph.2009.07.017.
  28. ↑ Surian, Nicola (8 November 1999). "Channel changes due to river regulation: the case of the Piave River, Italy". Earth Surface Processes and Landforms. 24 (12): 1135–1151. Bibcode:1999ESPL...24.1135S. doi:10.1002/(SICI)1096-9837(199911)24:12<1135::AID-ESP40>3.0.CO;2-F.
  29. ↑ Reinfelds, Ivars; Nanson, Gerald (December 1993). "Formation of braided river floodplains, Waimakariri River, New Zealand". Sedimentology. 40 (6): 1113–1127. Bibcode:1993Sedim..40.1113R. doi:10.1111/j.1365-3091.1993.tb01382.x.

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