River bifurcation

River bifurcation (from Latin: furca, fork) occurs when a river flowing in a single stream separates into two or more separate streams (called distributaries) which then continue downstream. Some rivers form complex networks of distributaries, typically in their deltas. If the streams eventually merge again or empty into the same body of water, then the bifurcation forms a river island.

Bifurcation in Hövelhof, Germany
River deltas such as the pictured delta of the Salween River in Myanmar often show bifurcations. The water flows in from the lower section of the image and passes on both sides of the large island in the center.

River bifurcation may be temporary or semi-permanent, depending on the strength of the material that is dividing the two distributaries. For example, a mid-stream island of soil or silt in a delta is most likely temporary, due to low material strength. A location where a river divides around a rock fin, e.g. a volcanically formed dike, or a mountain, may be more lasting as a result of higher material strength and resistance to weathering and erosion. A bifurcation may also be man-made, for example when two streams are separated by a long bridge pier.

Scientific study of bifurcationEdit

River bifurcation commonly occurs in meandering and braided rivers, but is not uncommon in other types of rivers. In meandering rivers, bifurcations are often unstable in their configuration, and usually result in channel avulsion.[1] The stability of bifurcation is dependent on the rate of flow of the river upstream as well as the sediment transport of the upper reaches of the branches just after bifurcation occurs.[2] The evolution of bifurcation is highly dependent on the discharge of the river upstream of the bifurcation.[3] Unstable bifurcations are bifurcations in which only one channel receives water. Within deltas, these typically create channels with relatively large widths, and are also known as channel avulsions. Stable bifurcations are bifurcations in which both channels receive water.[4]

In deltas, the directions of distributaries resulting from bifurcation are easily changeable by processes like aggradation, or differential subsidence and compaction.[5] The number of distributaries that are present is in part determined by the rate of sediment discharge,[6] and increased sediment discharge leads to more river bifurcation. This then leads to increased numbers of distributaries in deltas.

Delta bifurcation has a typical angle at which it is observed, with a critical angle of approximately 72º.[7] However, observations and experiments show that many distributary channel bifurcations do not actually exhibit a bifurcation angle of 72º, but rather grow towards this angle over time after initiation of bifurcation.[8] This implies that bifurcations that occur in deltas are semi-permanent, as many observed channels do not exhibit this angle due to their relatively recent initiation, or because some of the channels that do reach this bifurcation angle did not last to be observed.

ImportanceEdit

As is the case with river confluence, bifurcation is important in dividing land and morphological areas. Rivers are abundantly used as political boundaries, marking borders between regions of opposing countries, states and peoples, among other things. Sudden river bifurcation, even temporary, can disturb terranes that would otherwise be considered the same region. Bifurcations are different from confluences in that many confluences are considered important sites for cities and trade. But due to the semi-permanence of most bifurcated rivers, and their uncommon occurrences, the concept of construction is not largely exhibited at sites of river bifurcation.

Distributaries are common components of deltas, and are the opposite of tributaries. These distributaries, that are a result of river bifurcation, are important for the deposition and movement of water, sediment and nutrients from farther inland to the larger body of water that it empties into.[9] Deltas are very important to humans, as the delta distributary regions provide homes to roughly half a billion people, and are exceptionally biologically rich.[10]

EvolutionEdit

 
Progression of bifurcated river systems can be modeled in stages. The figure above gives a rough picture of what it would actually look like. It depicts that, gradually, a stable bifurcation will deteriorate until one of the channels no longer receives flow from upstream, thus becoming an unstable bifurcation.

Bifurcated rivers are largely semi-permanent, and are subject to constant change in their configuration from evolving terranes and flow rates. As a result of this, observation of the process by which rivers bifurcate and then gradually deteriorate has been poorly documented. The evolution of river bifurcations from single channel to multi-channeled and back again is largely dependent on discharge rate from the backwater regions of the channel.[11] The bifurcation of channel systems begins when a single channel is forced to split when a bar of sediment causes initiation of the two channel system, however, this does not always result in a system in which both channels receive flow. In braided systems, evolution of bifurcate systems is largely determined by the water level of adjacent branches of the system.[12] The water level differences in braided systems are themselves caused by closure of branch entrances as a result of bar growth.[13] In addition to bar growth, differences in direction of bifurcated river flows from compound bar shapes and backwater effects also influence the evolution of the braided system.

Bifurcations move largely as a result of migration of the upstream channel.[14] The configuration of the bifurcated system is also modified by the migration of bars within the system.[15] This can cause sudden variations in channel widths, as well as width asymmetry in the system.[16] Over time, the stable channel system will eventually deteriorate until only one channel receives flow from upstream, this then creates an unstable channel, one in which no flow passes through.

ImpactsEdit

River bifurcations impact the surrounding area in a plethora of ways, namely, redistributing flow of water, sediment and nutrients throughout a watershed and delta. In addition to this, migrating bifurcations and landforms can alter the terranes in a given region affected by this process. Sudden bifurcation initiation can cause small scale flooding of the surrounding area. The opposite, deterioration of a stable bifurcation to an unstable one, can have similar effects, as flow that was split through two channels now being directed through one can cause the stable channel to surpass bank-full stage, or the point at which the water level is above the river bank. This can also cause flooding, and is a prominent issue in regions where levees are in use. Bifurcations are a major distributor of nutrients and mineral particulates to biologically rich areas in deltas. Sudden deterioration or initiation of bifurcated systems can disrupt the deposition of material required for various organisms to live, and thus has an indirect impact on surrounding ecosystems via flow patterns.

ExamplesEdit

See alsoEdit

Notes and referencesEdit

Notes:

  1. ^ Kosovo is the subject of a territorial dispute between the Republic of Kosovo and the Republic of Serbia. The Republic of Kosovo unilaterally declared independence on 17 February 2008. Serbia continues to claim it as part of its own sovereign territory. The two governments began to normalise relations in 2013, as part of the 2013 Brussels Agreement. Kosovo is currently recognized as an independent state by 97 out of the 193 United Nations member states. In total, 112 UN member states have recognized Kosovo at some point, of which 15 later withdrew their recognition.

References:

  1. ^ Kleinhans, Maarten. "River bifurcations in meandering rivers on lowland deltaic plains", 2005-2008.
  2. ^ Le, T.B; Crosato, A; Mosselman, E.; Uijttewaal, W.S.J. "On the stability of river bifurcations created by longitudinal training walls. Numerical investigation", Advances in Water Resources, Volume 113, p.112-125, March 2018.
  3. ^ Edmonds, D.A. “Stability of backwater‐influenced river bifurcations: A study of the Mississippi‐Atchafalaya system”, April, 2012.
  4. ^ Edmonds, D.A. “Stability of backwater‐influenced river bifurcations: A study of the Mississippi‐Atchafalaya system”, April, 2012.
  5. ^ Olariu, Cornel; Bhattacharya, Janok P. “Terminal Distributary Channels and Delta Front Architecture of River-dominated Delta Systems”, Journal of Sedimentary Research, v. 76, p. 212–233, 2006.
  6. ^ Olariu, Cornel; Bhattacharya, Janok P. “Terminal Distributary Channels and Delta Front Architecture of River-dominated Delta Systems”, Journal of Sedimentary Research, v. 76, p. 212–233, 2006.
  7. ^ Coffey, Thomas S.; Shaw, John B. “Congruent Bifurcation Angles in River Delta and Tributary Channel Networks”, November, 2017.
  8. ^ Coffey, Thomas S.; Shaw, John B. “Congruent Bifurcation Angles in River Delta and Tributary Channel Networks”, November, 2017.
  9. ^ Olariu, Cornel; Bhattacharya, Janok P. “TERMINAL DISTRIBUTARY CHANNELS AND DELTA FRONT ARCHITECTURE OF RIVER-DOMINATED DELTA SYSTEMS”, Journal of Sedimentary Research, v. 76, p. 212–233, 2006.
  10. ^ Edmonds, D.A. “Stability of backwater‐influenced river bifurcations: A study of the Mississippi‐Atchafalaya system”, April, 2012.
  11. ^ Le, T.B; Crosato, A; Mosselman, E.; Uijttewaal, W.S.J. "On the stability of river bifurcations created by longitudinal training walls. Numerical investigation", Advances in Water Resources, Volume 113, p.112-125, March 2018.
  12. ^ Schuurman, F.; Kleinhans, M.G. “3D modelling of bar and bifurcation evolution”, Utrecht University, Faculty of Geosciences, Utrecht, The Netherlands. Royal HaskoningDHV, Dep. Rivers, Deltas and Coasts, Amersfoort, The Netherlands. 2013.
  13. ^ Schuurman, F.; Kleinhans, M.G. “3D modelling of bar and bifurcation evolution”, Utrecht University, Faculty of Geosciences, Utrecht, The Netherlands. Royal HaskoningDHV, Dep. Rivers, Deltas and Coasts, Amersfoort, The Netherlands. 2013.
  14. ^ Bertoldi, Walter. “Life of a bifurcation in a gravel‐bed braided river”, May, 2012.
  15. ^ Bertoldi, Walter. “Life of a bifurcation in a gravel‐bed braided river”, May, 2012.
  16. ^ Bertoldi, Walter. “Life of a bifurcation in a gravel‐bed braided river”, May, 2012.
  17. ^ Tomović, Gordana (2006). "Kosovo on old maps from the XV to the XVIII century". Belgrade: Serbian Academy of Sciences and Arts.
  18. ^ Alexander Anatolievich Bazelyuk (Базелюк Александр Анатольевич), "АНТРОПОГЕННОЕ ИЗМЕНЕНИЕ ГИДРОГРАФИЧЕСКОЙ СЕТИ КУМО-МАНЫЧСКОЙ ВПАДИНЫ Archived 2009-03-05 at the Wayback Machine" (Anthropogenic changes in the Hydrographic Network of the Kuma-Manych Depression), summary of the Cand. Sci. dissertation. Rostov-on-Don, 2007. (in Russian) Includes maps.
  19. ^ http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=dairut&aq=&sll=27.556982,30.836563&sspn=0.14549,0.264187&ie=UTF8&hq=&hnear=Dairut,+Assiut,+Egypt&ll=27.561966,30.809011&spn=0.036219,0.066047&t=h&z=15
  20. ^ http://www.pajala.se/mun/pajala/www.nsf/English/764B6DC8BFD894E2C1256FB30024F22D?
  21. ^ "River and Drainage System". Banglapedia. 5 May 2014.
  22. ^ Kester Freriks, Langs de IJssel, natuur en cultuur in de IJsselvallei Zutphen: Walburg pers, 2017 (Dutch book)