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{{short description|Depositional landform in a river which splits a channel}}
{{short description|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<ref name=":0">{{Cite journal|last=Bridge|first=John S.|date=1993|title=The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers|url=https://doi.org/10.1144/GSL.SP.1993.075.01.02|journal=Geological Society, London, Special Publications|volume=75|issue=1|pages=13–71|doi=10.1144/gsl.sp.1993.075.01.02|issn=0305-8719}}</ref>.  Braid bars are distinguished from [[Point bar|point bars]] due to their presence in the middle of a flow channel, rather than along a bank of the river channel.
'''Braid bars''', or '''mid-channel bars''', are landforms in a [[river]] that begin to form when the [[discharge (hydrology)|discharge]] is low and the river is forced to take the route of less resistance by means of flowing in locations of lowest elevation. Over time, the river begins to erode the outer edges of the bar, causing it to become a higher elevation than the surrounding areas. The water level decreases even more as the river laterally erodes the less cohesive bank material resulting in a widening of the river and a further exposure of the braid bar. As the discharge increases, material may deposit about the braid bar since it is an area in the river of low velocity due to its increased elevation in relation to surrounding areas. During times of extremely high flow, the bars may become covered; only to resurface when the flow decreases. Most braid bars do not remain stable or in one location. However, vegetation succession on braid bars can increase the stability of the landform.<ref>{{cite journal | title=Evolution of mid-channel bars in a braided river and complex response to reservoir construction: an example from the middle Hanjiang River, China|author=Xu Jiongxin|journal=[[Earth Surface Processes and Landforms]] |volume= 22 |issue=10 |pages=953–965 |date=October 1997 |doi=10.1002/(SICI)1096-9837(199710)22:10<953::AID-ESP789>3.0.CO;2-S |issn=1096-9837}}</ref> They are commonly composed of [[sand]] or [[gravel]] and typically occur in [[braided river]]s.
[[File:Braided river, Denali NP.jpg|thumb|303x303px|A braided river in Denali National Park, AK, showing a number of braid bars.]]
Braid bars often originate from remnants of point bars or the growth of mid-channel unit bars in braided rivers<ref name=":0" />. These features typically form in rivers with a high sediment load, within channels characterized by a large bed load and easily-eroded bank material<ref name=":1">{{Cite web|title=Fluvial Features—Braided Stream (U.S. National Park Service)|url=https://www.nps.gov/articles/braided-stream.htm|access-date=2020-11-26|website=www.nps.gov|language=en}}</ref>.  There are several mechanisms of formation.  One explanation is that flow in the river may be redirected over a point bar, wherein the bar may be separated from the bank and thus become isolated within the channel.  This forms a small ‘island’ of sediment, which can evolve into a braid bar.  Alternatively, an obstacle such as a wedged log can result in the formation of a mid-channel bar if sediment is deposited along the feature.

As sediment is transported downriver, material can alternatively accrete onto the existing bar or erode away. Therefore, such features tend to be ephemeral and fluid, and can change shape quite frequently<ref name=":0" /> . Braided rivers can have variable, erratic flows, which can lead to successive periods of greater deposition or erosion<ref name=":1" />.  Entire bars can be created, destroyed, or migrate across a channel over the span of several years<ref name=":2">{{Cite journal|last=Ashworth|first=Philip J.|last2=Best|first2=James L.|last3=Roden|first3=Julie E.|last4=Bristow|first4=Charles S.|last5=Klaassen|first5=Gerrit J.|date=2000|title=Morphological evolution and dynamics of a large, sand braid-bar, Jamuna River, Bangladesh|url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-3091.2000.00305.x|journal=Sedimentology|language=en|volume=47|issue=3|pages=533–555|doi=10.1046/j.1365-3091.2000.00305.x|issn=1365-3091}}</ref>.

These formations are often composed of poorly-sorted sand and/or gravel<ref>{{Cite journal|last=Lunt|first=I. A.|last2=Bridge|first2=J. S.|date=2004|title=Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-3091.2004.00628.x|journal=Sedimentology|language=en|volume=51|issue=3|pages=415–432|doi=10.1111/j.1365-3091.2004.00628.x|issn=1365-3091}}</ref>.  The generally coarse material is the result of the processes common to braid bar formation, and more broadly, braided rivers.  Braided rivers are defined by relatively high flow power, as well as significant rates of erosion<ref name=":2" />.  These high rates of flow will transport fine-grained materials further downstream, while coarser material remains near the eroded bank and is eventually deposited onto mid-channel bars.

There is some fluidity in the distinction between braid bar and island<ref name=":0" />.  Compared to a braid bar, an island is regarded as having a more permanent and potentially larger structure that can influence, to some degree, the path of the river.  Vegetation can be present on a braid bar, but is suggested that, on islands, vegetation has a significant stabilizing impact<ref>{{Cite journal|last=Brice|first=James Coble|date=1964|title=Channel patterns and terraces of the Loup Rivers in Nebraska|url=http://dx.doi.org/10.3133/pp422d|journal=Professional Paper|doi=10.3133/pp422d|issn=2330-7102}}</ref>. This is in part due to the nature of many braided rivers, where variable flow discharge can create a hostile environment for any significant vegetation<ref name=":1" />. In addition, the elevation of the island in regards to bankfull discharge should also be considered; an island will likely not be entirely submerged when bankfull discharge occurs, whereas a braid bar may be.


==See also==
==See also==

Revision as of 04:14, 26 November 2020

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[1].  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.

A braided river in Denali National Park, AK, showing a number of braid bars.

Braid bars often originate from remnants of point bars or the growth of mid-channel unit bars in braided rivers[1]. These features typically form in rivers with a high sediment load, within channels characterized by a large bed load and easily-eroded bank material[2].  There are several mechanisms of formation.  One explanation is that flow in the river may be redirected over a point bar, wherein the bar may be separated from the bank and thus become isolated within the channel.  This forms a small ‘island’ of sediment, which can evolve into a braid bar.  Alternatively, an obstacle such as a wedged log can result in the formation of a mid-channel bar if sediment is deposited along the feature.

As sediment is transported downriver, material can alternatively accrete onto the existing bar or erode away. Therefore, such features tend to be ephemeral and fluid, and can change shape quite frequently[1] . Braided rivers can have variable, erratic flows, which can lead to successive periods of greater deposition or erosion[2].  Entire bars can be created, destroyed, or migrate across a channel over the span of several years[3].

These formations are often composed of poorly-sorted sand and/or gravel[4].  The generally coarse material is the result of the processes common to braid bar formation, and more broadly, braided rivers.  Braided rivers are defined by relatively high flow power, as well as significant rates of erosion[3].  These high rates of flow will transport fine-grained materials further downstream, while coarser material remains near the eroded bank and is eventually deposited onto mid-channel bars.

There is some fluidity in the distinction between braid bar and island[1].  Compared to a braid bar, an island is regarded as having a more permanent and potentially larger structure that can influence, to some degree, the path of the river.  Vegetation can be present on a braid bar, but is suggested that, on islands, vegetation has a significant stabilizing impact[5]. This is in part due to the nature of many braided rivers, where variable flow discharge can create a hostile environment for any significant vegetation[2]. In addition, the elevation of the island in regards to bankfull discharge should also be considered; an island will likely not be entirely submerged when bankfull discharge occurs, whereas a braid bar may be.

See also

  • Point bar – Landform related to streams and rivers
  • Mouth bar – A river bar that forms in the middle of a channel in a river delta

References

  1. ^ a b c d Bridge, John S. (1993). "The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers". Geological Society, London, Special Publications. 75 (1): 13–71. doi:10.1144/gsl.sp.1993.075.01.02. ISSN 0305-8719.
  2. ^ a b c "Fluvial Features—Braided Stream (U.S. National Park Service)". www.nps.gov. Retrieved 2020-11-26.
  3. ^ a b Ashworth, Philip J.; Best, James L.; Roden, Julie E.; Bristow, Charles S.; Klaassen, Gerrit J. (2000). "Morphological evolution and dynamics of a large, sand braid-bar, Jamuna River, Bangladesh". Sedimentology. 47 (3): 533–555. doi:10.1046/j.1365-3091.2000.00305.x. ISSN 1365-3091.
  4. ^ Lunt, I. A.; Bridge, J. S. (2004). "Evolution and deposits of a gravelly braid bar, Sagavanirktok River, Alaska". Sedimentology. 51 (3): 415–432. doi:10.1111/j.1365-3091.2004.00628.x. ISSN 1365-3091.
  5. ^ Brice, James Coble (1964). "Channel patterns and terraces of the Loup Rivers in Nebraska". Professional Paper. doi:10.3133/pp422d. ISSN 2330-7102.
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