Avulsion (river)
In
Deltaic and net-depositional settings
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
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]
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]
Another example is the Cheslatta River, once a small tributary of the Nechako River in British Columbia. In the 1950s the Cheslatta River was made to be the spillway of the then new Nechako Reservoir. The discharge of the spills far exceeds the original flow of the Cheslatta River, which has resulted in major erosion in the upper Cheslatta valley, with the scoured sediment being deposited in the lower valley. Large reservoir spills caused the lower Cheslatta River to avulse in 1961 and again in 1972, carving a new route to the Nechako River and depositing a fan of sediment called the Cheslatta Fan in the Nechako River. After 1972 a cofferdam was built to restore the river to its original course.[12]
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
Avulsions have been investigated in deltas or coastal plain channels as a result of obstructions such as log-jams and possible tectonic influences.[14]
See also
- Aggradation – Increase in land elevation due of the deposition of sediment
- Alluvial fan – Fan-shaped deposit of sediment
- Lake Ragunda – Former whitewater rapid on the river Indalsälven, Sweden
- Meander – One of a series of curves in a channel of a matured stream
- Megafan – Large fan-shaped deposit
- River delta – Silt deposition landform at the mouth of a river
- River plume – Mix of fresh river water and seawater
- Stream capture – Geomorphological phenomenon
References
- ^ .
- ISBN 0-393-97423-5pp. 528–9
- ISBN 0-7167-2882-6p. 136
- ^ Marshak, pp. 528–9
- ^ Stanley, p. 136
- ^ Easterbrook, Don J.Surface Processes and Landforms Second EditionPrentice Hall, New Jersey: 1999.
- .
- .
- .
- ^ 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.
- hdl:1721.1/45792.
- ^ "Nechako River Geomorphic Assessment, Phase I" (PDF). BC Ministry of Water, Land and Air Protection. pp. 3, 10. Archived from the original (PDF) on 3 March 2016. Retrieved 4 August 2013.
- .
- S2CID 128660882.