Avulsion (river)

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 lower than the slope that the river could travel if it took a new course.[1]


Deltaic and net-depositional settings

Plumes of sediment enter the ocean from 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 deltaic settings, where sediment deposits as the river enters the ocean and channel gradients are typically very small.[2] This process of avulsion in deltaic settings 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 sediment within the channel and 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] Because this process results in the formation of a series of lobes, it is also known as delta switching.

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 flow level rise was pronounced 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 (and shorter) channel at a rate of 25–50 meters per hour.[10] Sediment mobilized 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 favor of the high-slope. This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than 1/6.[1]


Avulsion typically occurs during large floods which carry the power necessary to rapidly change the landscape.

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]

See also


  1. ^ a b c Slingerland, Rudy; Smith, Norman D. (1998). "Necessary conditions for a meandering-river avulsion". Geology 26 (5): 435–438. doi:10.1130/0091-7613(1998)026<0435:NCFAMR>2.3.CO;2. http://geology.gsapubs.org/content/26/5/435.abstract. 
  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., P. Falk, and C. Paola (1995), Experimental study of avulsion frequency and rate of deposition, Geology (Boulder), 23, 365–368.
  8. ^ Mohrig, D., P. L. Heller, C. Paola, and W. J. Lyons (2000), Interpreting avulsion process from ancient alluvial sequences; Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado), Geological Society of America Bulletin, 112, 1787–1803.
  9. ^ Slingerland, Rudy; Smith, Norman D. (2004). "RIVER AVULSIONS AND THEIR DEPOSITS". Annual Review of Earth and Planetary Sciences 32: 257. doi:10.1146/annurev.earth.32.101802.120201. 
  10. ^ Perignon, M. C. (2007) (S.B. Thesis). Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River Avulsion in Epsom, New Hampshire. Massachusetts Institute of Technology. 
  11. ^ Perignon, M. C. (2008) (S.B. Thesis). Sediment wave-induced channel evolution following the 2006 avulsion of the Suncook River in Epsom, New Hampshire. Massachusetts Institute of Technology. http://dspace.mit.edu/handle/1721.1/45792. 
  12. ^ Nanson, G.C.; Knighton, A.D. (1996). "Anabranching rivers: Their cause, character, and classification". Earth Surface Processes and Landforms 21 (3): 217–39. doi:10.1002/(SICI)1096-9837(199603)21:3<217::AID-ESP611>3.0.CO;2-U. 

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • River delta — Nile River delta, as seen from Earth orbit. The Nile is an example of a wave dominated delta that has the classic Greek delta (Δ) shape after which river deltas were named. A delta is a landform that is formed at the mouth of a river where that… …   Wikipedia

  • avulsion — avul·sion /ə vəl shən/ n [Latin avulsio act of tearing away, from avellere to tear away, from a off, away + vellere to pull, pluck]: a sudden cutting off of land by flood or change in the course of a body of water; esp: one that separates a… …   Law dictionary

  • Avulsion — in general refers to a tearing away. Specifically, it can refer to: * A form of amputation where the extremity is pulled off rather than cut off. * Avulsion fracture * Avulsion injury, the removal of all the layers of skin from abrasion * In the… …   Wikipedia

  • Avulsion — A*vul sion, n. [L. avulsio.] 1. A tearing asunder; a forcible separation. [1913 Webster] The avulsion of two polished superficies. Locke. [1913 Webster] 2. A fragment torn off. J. Barlow. [1913 Webster] 3. (Law) The sudden removal of lands or… …   The Collaborative International Dictionary of English

  • Avulsion (legal term) — In Property law, avulsion refers to a sudden loss or addition to land, which results from the action of water. It differs from accretion, which describes a gradual loss or addition to land resulting from the action of water. This distinction… …   Wikipedia

  • avulsion — /euh vul sheuhn/, n. 1. a tearing away. 2. Law. the sudden removal of soil by change in a river s course or by a flood, from the land of one owner to that of another. 3. a part torn off. [1615 25; < L avulsion (s. of avulsio), equiv. to avuls(us) …   Universalium

  • avulsion — noun a) The loss or separation of a body part, either by surgery or due to trauma b) An abrupt change in the course of a river, typically from one channel to another …   Wiktionary

  • avulsion — a•vul•sion [[t]əˈvʌl ʃən[/t]] n. 1) a tearing away 2) law the sudden removal of soil by change in a river s course or by a flood, from the land of one owner to that of another 3) a part torn off • Etymology: 1615–25; < L …   From formal English to slang

  • avulsion — /əˈvʌlʃən/ (say uh vulshuhn) noun 1. a tearing away. 2. Law the sudden removal of soil by change in a river s course or by a flood, from the land of one owner to that of another. 3. a part torn off. {Latin āvulsio} …   Australian English dictionary

  • Mojave River — Coordinates: 36°06′20″N 116°03′53″W / 36.10556°N 116.06472°W / 36.10556; 116.06472 …   Wikipedia

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.