Avulsion (river)

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.[1]

Deltaic and net-depositional settings

Mississippi Delta 1976
Plumes of sediment enter the ocean from several mouths of the Mississippi River bird's-foot delta. This sediment is responsible for building the delta and allowing it to advance into the sea. As it extends further offshore, the channel slope will decrease and its bed will aggrade, promoting an avulsion.

Avulsions are common in river deltas, where sediment deposits as the river enters the ocean and channel gradients are typically very small.[2] This process is also known as delta switching.

Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta of the Mississippi River, pictured at right with its sediment plumes. As the deltaic lobe advances, the slope of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope or gradient). As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course downslope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[1] Second, as its slope is reduced, the amount of shear stress on the bed will decrease, resulting in deposition of more sediment within the channel and thus raising of the channel bed relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope.

When this avulsion occurs, the new channel carries sediment out to the ocean, building a new deltaic lobe.[3][4] The abandoned delta eventually subsides.[5]

This process is also related to the distributary network of river channels that can be observed within a river delta. When the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events happen repeatedly over time, a mature delta will gain a distributary network.[6]

Subsidence of the delta and/or sea-level rise can further cause backwater and deposition in the delta. This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basins. On average, an avulsion will occur every time the bed of a river channel aggrades enough that the river channel is superelevated above the floodplain by one channel-depth. In this situation, enough hydraulic head is available that any breach of the natural levees will result in an avulsion.[7][8]

Erosional avulsions

Rivers can also avulse due to the erosion of a new channel that creates a straighter path through the landscape. This can happen during large floods in situations in which the slope of the new channel is significantly greater than that of the old channel. Where the new channel's slope is about the same as the old channel's slope, a partial avulsion will occur in which both channels are occupied by flow.[9] An example of an erosional avulsion is the 2006 avulsion of the Suncook River in New Hampshire, in which heavy rains caused flow levels to rise. The river level backed up behind an old mill dam, which produced a shallowly-sloping pool that overtopped a sand and gravel quarry, connected with a downstream section of channel, and cut a new shorter channel at 25–50 meters per hour.[10] Sediment mobilised by this erosional avulsion produced a depositionally-forced meander cutoff further downstream by superelevating the bed around the meander bend to nearly the level of the floodplain.[11]

Meander cutoffs

An example of a minor avulsion is known as a meander cutoff, where the high-sinuosity meander bend is abandoned in favour of the high-slope (i.e Large bending meander has river cut through a straighter course, and the meander has water drain away) This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than about 1/5.[1]

Occurrence

Avulsion typically occurs during large floods which carry the power necessary to rapidly change the landscape. Dam removal could also lead to avulsion.

Avulsions usually occur as a downstream to upstream process via head cutting erosion. If a bank of a current stream is breached a new trench will be cut into the existing floodplain. It either cuts through floodplain deposits or reoccupies an old channel.[12]

Avulsions have been investigated in the deltas or coastal plain channels as a result of obstructions such as log-jams and possible tectonic influences.[13]

See also

  • Aggradation – The increase in land elevation due to the deposition of sediment
  • Alluvial fan – A fan- or cone-shaped deposit of sediment crossed and built up by streams
  • Lake Ragunda – A former whitewater rapid on the river Indalsälven, Sweden (the Ragunda event in Sweden)
  • Megafan
  • Meander – A sinuous bend in a series in the channel of a river
  • River delta – Silt deposition landform at the mouth of a river
  • plume
  • Stream capture

References

  1. ^ a b c Slingerland, Rudy; Smith, Norman D. (1998). "Necessary conditions for a meandering-river avulsion". Geology. 26 (5): 435–438. Bibcode:1998Geo....26..435S. doi:10.1130/0091-7613(1998)026<0435:NCFAMR>2.3.CO;2.
  2. ^ Marshak, Stephen (2001), Earth: Portrait of a Planet, New York: W.W. Norton & Company, ISBN 0-393-97423-5 pp. 528–9
  3. ^ Stanley, Steven M. (1999) Earth System History. New York: W.H. Freeman and Company, ISBN 0-7167-2882-6 p. 136
  4. ^ Marshak, pp. 528–9
  5. ^ Stanley, p. 136
  6. ^ Easterbrook, Don J.Surface Processes and Landforms Second EditionPrentice Hall, New Jersey: 1999.
  7. ^ Bryant, M.; Falk, P.; Paola, C. (1995). "Experimental study of avulsion frequency and rate of deposition". Geology. 23 (4): 365–368. Bibcode:1995Geo....23..365B. doi:10.1130/0091-7613(1995)023<0365:ESOAFA>2.3.CO;2.
  8. ^ Mohrig, D.; Heller, P. L.; Paola, C.; Lyons, W. J. (2000). "Interpreting avulsion process from ancient alluvial sequences: Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado)". Geological Society of America Bulletin. 112 (12): 1787–1803. Bibcode:2000GSAB..112.1787M. doi:10.1130/0016-7606(2000)112<1787:IAPFAA>2.0.CO;2.
  9. ^ Slingerland, Rudy; Smith, Norman D. (2004). "RIVER AVULSIONS AND THEIR DEPOSITS". Annual Review of Earth and Planetary Sciences. 32: 257. Bibcode:2004AREPS..32..257S. doi:10.1146/annurev.earth.32.101802.120201.
  10. ^ Perignon, M. C. (2007). Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River Avulsion in Epsom, New Hampshire (S.B. Thesis). Massachusetts Institute of Technology.
  11. ^ Perignon, M. C. (2008). Sediment wave-induced channel evolution following the 2006 avulsion of the Suncook River in Epsom, New Hampshire (S.B. Thesis). Massachusetts Institute of Technology.
  12. ^ Nanson, G.C.; Knighton, A.D. (1996). "Anabranching rivers: Their cause, character, and classification". Earth Surface Processes and Landforms. 21 (3): 217–39. Bibcode:1996ESPL...21..217N. doi:10.1002/(SICI)1096-9837(199603)21:3<217::AID-ESP611>3.0.CO;2-U.
  13. ^ Phillips, J. D. (2012). "Log-jams and avulsions in the San Antonio River Delta, Texas". Earth Surface Processes and Landforms. 37 (9): 936–950. Bibcode:2012ESPL...37..936P. doi:10.1002/esp.3209.
Aggradation

Aggradation (or alluviation) is the term used in geology for the increase in land elevation, typically in a river system, due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport. The mass balance between sediment being transported and sediment in the bed is described by the Exner equation.

Typical aggradational environments include lowland alluvial rivers, river deltas, and alluvial fans. Aggradational environments are often undergoing slow subsidence which balances the increase in land surface elevation due to aggradation. After millions of years, an aggradational environment will become a sedimentary basin, which contains the deposited sediment, including paleochannels and ancient floodplains.

Aggradation can be caused by changes in climate, land use, and geologic activity, such as volcanic eruption, earthquakes, and faulting. For example, volcanic eruptions may lead to rivers carrying more sediment than the flow can transport: this leads to the burial of the old channel and its floodplain. In another example, the quantity of sediment entering a river channel may increase when climate becomes drier. The increase in sediment is caused by a decrease in soil binding that results from plant growth being suppressed. The drier conditions cause river flow to decrease at the same time as sediment is being supplied in greater quantities, resulting in the river becoming choked with sediment.

In 2009, a report by researchers from the University of Colorado at Boulder in the journal Nature Geoscience said that reduced aggradation was contributing to an increased risk of flooding in many river deltas.

Avulsion

Avulsion in general refers to a tearing away. Specifically, it can refer to:

Avulsion fracture, when a fragment of bone tears away from the main mass of bone as a result of physical trauma

Avulsion injury, in which a body structure is detached from its normal point of insertion, either torn away by trauma or cut by surgery

Avulsion (legal term), the sudden loss of land by the action of water

Avulsion (river), abandonment of an old river channel and the creation of a new one

Floodplain

A floodplain or flood plain is an area of land adjacent to a stream or 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 levees, silts, and sands deposited during floods. Levees are the heaviest materials (usually pebble-size) and they are deposited first; silts and sands are finer materials.

Lilly Arbor Project

The Lilly Arbor Project is a part of an experimental riparian floodplain reforestation and ecological restoration program, located along the White River in Indiana, in the eastern United States.

Meander

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 (cut 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 degree of meandering of the channel of a river, stream, or other watercourse is measured by its sinuosity. The sinuosity of a watercourse is the ratio of the length of the channel to the straight line down-valley distance. Streams or rivers with a single channel and sinuosities of 1.5 or more are defined as meandering streams or rivers.

Mouth bar

A mouth bar is a bar in a river that is typically created in the middle of a channel in a river delta. It is created by a positive feedback between mid-channel deposition and flow divergence. As the flow diverges near the ocean, sediment settles out in the channel and creates an incipient mouth bar. As flow is routed around the incipient bar, additional sediment is deposited on the incipient bar. This continued process results in the formation of a full-fledged mouth bar, which causes the channel to bifurcate. This continued process leads to the characteristic fractal tree pattern found in some prograding river-dominated deltas.

Oxbow lake

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. This landform is so named for its distinctive curved shape, which resembles the bow pin of an oxbow. In Australia, an oxbow lake is called a billabong, from the indigenous Wiradjuri language. In south Texas, oxbows left by the Rio Grande are called resacas.

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.

River delta

A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment. 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. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position.

Stream pool

A stream pool, in hydrology, is a stretch of a river or stream in which the water depth is above average and the water velocity is below average.

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