Meander cutoff

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Animation of the formation of an oxbow lake OxbowAnimation.gif
Animation of the formation of an oxbow lake

A meander cutoff is a natural form of a cutting or cut in a river occurs when a pronounced meander (hook) in a river is breached by a flow that connects the two closest parts of the hook to form a new channel, a full loop. The steeper drop in gradient (slope) causes the river flow gradually to abandon the meander which will silt up with sediment from deposition. Cutoffs are a natural part of the evolution of a meandering river. Rivers form meanders as they flow laterally downstream, see sinuosity. [1]

Contents

Meandering rivers flow higher and hence with more total flow, pressure and erosion on the outside of their bends due to forming a vortex as in a stirred coffee cup and consequently the river erodes more the outer bank. On the inside bend of a river, the level is lower, secondary flow moves sand and gravel across the river bed creating shallows and point bars, and friction of air and perturbances of the bed act against a higher proportion of the column of water, being shorter, slowing the water to varying degrees. [2] Rivers are commonly described and interpreted by their sinuosity. The term is equally used to describe the actual incidence of and potential tendency of a river to curve or meander over its length. [3] It is expressed as the ratio of the distance between two distant points in a river following the middle-of-the-river course of the river as compared with the straight distance between those points. [4] Three conventional categorizations of rivers or their reaches exist. Meandering rivers have a sinuosity value/ratio of greater than 1.5. A sinuosity value of less than 1.1 is a “straight” river. Between these values, a river is described as sinuous which describes those in a transitory state between the two states. Braided rivers do not follow this same convention. [3] Meandering rivers trend in the direction of increasing sinuosity. [5]

Cutoff channel

A river constantly evolves and as it does, meanders that were once a part of the river are abandoned in favor of a route that is more efficient for a river to take. As these old meanders are cutoff from the rest of the river, a new channel, or cutoff channel, is formed.

Formation

A chute cutoff channel can form during a flood resulting in an overbank flow where water goes over the banks of the river, creating erosion of the surrounding landscape. More studies need to be done on how the magnitude of these floods and their recurrence interval are related to how often these chute cutoff channels form. [6] Neck cutoff channels are commonly formed the same way when an overbank flow occurs during a flood and the narrow piece of land between a bend in a meander is eroded away; this is known as rush-cutting. [5] A meander can also be cutoff by a channel due to excess sediment upstream as a result of high erosion rates. This leads to a cutoff channel forming since a river might no longer be able to carry that sediment through the bend efficiently, so the river forms a new path for it to flow. [7] Meander cutoffs can also be formed by humans; by removing a beaver dam, the likelihood that meander cutoff channels will be formed downstream increases. [7] A cutoff channel can be engineered for the purpose of navigation, traditionally for water mill leats and for controlling the possibility of any future flood were done on the lower reaches away from the tide. These meander cutoffs straighten a river. [8] Many rivers are transformed by humans, becoming less sinuous. [6]

Oxbow lake

When either of these meander cutoff processes takes place a bend of the river is left behind forming, in many instances, an oxbow lake. An oxbow lake forms after there has been deposition of sediment, by the new cutoff channel flowing adjacent to it, at the entrances of the abandoned bend; this seals the bend off from the rest of the river. Oxbow lakes have been shown to be an important habitat for various species of wildlife. Recent efforts have been made to protect these important bodies of water from harmful practices such as agricultural use. [8] One proposed method to restore these oxbow lakes has been dredging. Dredging will remove sediment from the lake’s floor and will increase the lake’s depth. Collins Lake in Scotia, New York is an example of this method. Oxbow lakes can be valuable for recreational purposes and in Salix, Iowa, Browns Lake’s water level was increased for recreational use. [8]

Example

View along the former Mississippi River riverbed at the Tennessee/Arkansas state line near Reverie, Tennessee (2007) Reverie TN 08 former MS river S.jpg
View along the former Mississippi River riverbed at the Tennessee/Arkansas state line near Reverie, Tennessee (2007)

On 7 March 1876 a cutoff formed suddenly across the neck of a meander, known as the "Devil's Elbow", in the Mississippi River near Reverie, Tennessee, shortening the river's course and leaving the town connected to Arkansas, but across the new river channel from the rest of Tennessee. [9]

Importance

Some research has been done to show importance. Cutoffs have been shown to limit the age of a river meander and thus how large that meander can get, without which in areas of extreme sinuosity and low gradients means very long, slightly slowed, sections of rivers intensifying local flood risk. [10] Meander cutoffs influence the formation of a river’s floodplain and continue to do so as the river evolves. [11] Cutoffs can affect the way that other river bends adjacent evolve over time, increasing their height of flooding and the direct momentum of the water which can then create further cutoffs and affect other bends further downstream throughout the river. [1] Meander cutoffs directly reduce and tend to indirectly reduce a river’s sinuosity, thus straightening out a river’s channel. [5] Understanding the processes that form meander cutoffs can allow one to predict how a river will evolve in the future which is important for agricultural businesses and controlling future floods. [5]

Related Research Articles

Sediment 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.

Floodplain Land adjacent to a river which is flooded during periods of high discharge

A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge. The soils usually consist of clays, silts, sands, and gravels deposited during floods.

Braided river Network of river channels separated by small, and often temporary, islands

A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, aits or eyots. Braided streams tend to occur in rivers with high sediment loads and/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.

Fluvial processes 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.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

Oxbow lake part of meanders

An oxbow lake is a U-shaped lake 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.

Kobuk River

The Kobuk River is a river located in the Arctic region of northwestern Alaska in the United States. It is approximately 280 miles (451 km) long. Draining a basin with an area of 12,300 square miles (32,000 km2), the Kobuk River is among the largest rivers in northwest Alaska with widths of up to 1500 feet and flow at a speed of 3–5 miles per hour in its lower and middle reaches. The average elevation for the Kobuk River Basin is 1,300 feet (400 m) above sea level, ranging from near sea level to 11,400 feet. Topography includes low, rolling mountains, plains and lowlands, moderately high rugged mountainous land, and some gently sloped plateaus and highlands. The river contains an exceptional population of sheefish, a large predatory whitefish within the salmon family, found throughout the Arctic that spawns in the river's upper reaches during the autumn. A portion of the vast Western Arctic Caribou Herd utilize the Kobuk river valley as winter range.

Meander Sinuous bend in a series in the channel of a river

A meander is one of a series of regular sinuous curves, bends, loops, turns, or windings in the channel of a river, stream, or other watercourse. It is produced by a stream or river swinging from side to side as it flows across its floodplain or shifts its channel within a valley. A meander is produced by a stream or river as it erodes the sediments comprising an outer, concave bank and deposits this and other sediment downstream on an inner, convex bank which is typically a point bar. The result of sediments being eroded from the outside concave bank and their deposition on an inside convex bank is the formation of a sinuous course as a channel migrates back and forth across the down-valley axis of a floodplain. The zone within which a meandering stream shifts its channel across either its floodplain or valley floor from time to time is known as a meander belt. It typically ranges from 15 to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

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.

An overbank is an alluvial geological deposit consisting of sediment that has been deposited on the floodplain of a river or stream by flood waters that have broken through or overtopped the banks. The sediment is carried in suspension, and because it is carried outside of the main channel, away from faster flow, the sediment is typically fine-grained. An overbank deposit usually consists primarily of fine sand, silt and clay. Overbank deposits can be beneficial because they refresh valley soils.

Point bar Depositional feature of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope

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.

Drop structure

A drop structure, also known as a grade control, sill, or weir, is a manmade structure, typically small and built on minor streams, or as part of a dam's spillway, to pass water to a lower elevation while controlling the energy and velocity of the water as it passes over. Unlike most dams, drop structures are usually not built for water impoundment, diversion or raising the water level. Mostly built on watercourses with steep gradients, they serve other purposes such as water oxygenation and erosion prevention.

Abyssal channels are channels in Earth's sea floor. They are formed by fast-flowing floods of turbid water caused by avalanches near the channel's head, with the sediment carried by the water causing a build-up of the surrounding abyssal plains. Submarine channels and the turbidite systems which form them are responsible for the accumulation of most sandstone deposits found on continental slopes and have proven to be one of the most common types of hydrocarbon reservoirs found in these regions.

River Natural flowing watercourse

A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as stream, 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.

Backswamp Environment on a floodplain where deposits settle after a flood

In geology, a backswamp is a type of depositional environment commonly found in a floodplain. It is where deposits of fine silts and clays settle after a flood. These deposits create a marsh-like landscape that is often poorly drained and usually lower than the rest of the floodplain.

Avulsion (river) 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.

Bar (river morphology) 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.

Inverted relief Landscape features that have reversed their elevation relative to other features

Inverted relief, inverted topography, or topographic inversion refers to landscape features that have reversed their elevation relative to other features. It most often occurs when low areas of a landscape become filled with lava or sediment that hardens into material that is more resistant to erosion than the material that surrounds it. Differential erosion then removes the less resistant surrounding material, leaving behind the younger resistant material, which may then appear as a ridge where previously there was a valley. Terms such as "inverted valley" or "inverted channel" are used to describe such features. Inverted relief has been observed on the surfaces of other planets as well as on Earth. For example, well-documented inverted topographies have been discovered on Mars.

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.

Slip-off slope 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.

References

  1. 1 2 "NSF Award Search: Award#0852865 - SGER: Fluvial Dynamics of a Large-River Meander Cutoff". www.nsf.gov. Retrieved 2017-05-12.
  2. ↑ "Meandering rivers | Rivers | Earth processes | OneGeology Kids | eXtra | OneGeology". www.onegeology.org. Retrieved 2017-05-12.
  3. 1 2 Subhasisch., Dey (2013-01-01). Fluvial hydrodynamics : sediment transport and scour phenomena. Springer. ISBN   9783642190612. OCLC   810950525.
  4. ↑ "Sinuosity". forest.mtu.edu. Retrieved 2017-05-12.
  5. 1 2 3 4 Wang, Zhaoyin; Li, Zhiwei; Xu, Mengzhen; Yu, Guoan (2016-03-30). River Morphodynamics and Stream Ecology of the Qinghai-Tibet Plateau. CRC Press. ISBN   9781315682983.
  6. 1 2 3 4 Eekhout, J. P. C.; Hoitink, A. J. F. (2015). "Chute cutoff as a morphological response to stream reconstruction: The possible role of backwater" (PDF). Water Resources Research. 51 (5): 3339–3352. Bibcode:2015WRR....51.3339E. doi: 10.1002/2014WR016539 .
  7. 1 2 "Meander Cutting" (PDF). The Montana Department of Natural Resources and Conservation.
  8. 1 2 3 Julien, Pierre (2008). Restoration of Abandoned Channels (PDF). Fort Collins, CO: Colorado State. pp. 1–42. S2CID   126584638. Archived from the original (PDF) on 2017-12-11.
  9. ↑ Christine A. Klein, Sandra B. Zellmer (2014). Mississippi River Tragedies: A Century of Unnatural Disaster. NYU Press. p. 19. ISBN   9781479856169.
  10. ↑ Camporeale, C.; Perucca, E.; Ridolfi, L. (2008). "Significance of cutoff in meandering river dynamics" (PDF). Journal of Geophysical Research. 113 (F1): F01001. Bibcode:2008JGRF..113.1001C. doi:10.1029/2006JF000694.
  11. ↑ Constantine, J. A.; Dunne, T. (2008). "Meander cutoff and the controls on the production of oxbow lakes". Geology. 36 (1): 23–26. Bibcode:2008Geo....36...23C. doi:10.1130/G24130A.1.
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