View source for Outwash plain

{{Short description|Plain formed from glacier sediment transported by meltwater}}
{{About|the glacial geographical feature|the non-glacier related feature|Alluvial plain}}
{{more footnotes|date=March 2018}}
[[File:Skeiðarársandur, Iceland.jpg|thumb|right|260px|{{lang|is|Skeiðarársandur|italic=no}} in Iceland, viewed from its eastern margin at the terminus of Svínafellsjökull glacier]]

An '''outwash plain''', also called a '''sandur''' (plural: ''sandurs''<ref>Ritter, Dale F., R. Craig Kochel, & Jerry Russell Miller. 1995. ''Process geomorphology''. Dubuque, IA: Wm C. Brown, p. 349.</ref>), '''sandr'''<ref>Whittow, John (1984). ''[[iarchive:penguindictionar0000whit_b9d8|Dictionary of Physical Geography]]''. London: Penguin, 1984, p. 467. {{ISBN|0-14-051094-X}}.</ref> or '''sandar''',<ref>Gornitz, Vivien (ed.). 2009. ''Encyclopedia of Paleoclimatology And Ancient Environments''. Springer: Dordrecht, p. 665.</ref> is a [[plain]] formed of [[glaciofluvial deposits]] due to [[meltwater]] outwash at the [[glacier terminus|terminus]] of a [[glacier]]. As it flows, the glacier grinds the underlying rock surface and carries the debris along. The meltwater at the snout of the glacier deposits its load of [[sediment]] over the outwash plain, with larger boulders being deposited near the terminal [[moraine]], and smaller particles travelling further before being deposited. Sandurs are common in Iceland where geothermal activity accelerates the melting of ice flows and the deposition of sediment by meltwater.

== Formation ==
Sandurs are found in glaciated areas, such as [[Svalbard]], [[Kerguelen Islands]], and [[Iceland]].  Glaciers and [[icecaps]] contain large amounts of [[silt]] and sediment, picked up as they [[erosion|erode]] the underlying rocks when they move slowly downhill, and at the snout of the glacier, meltwater can carry this sediment away from the glacier and deposit it on a broad plain. The material in the outwash plain is often size-sorted by the water runoff of the melting glacier with the finest materials, like silt, being the most distantly re-deposited, whereas larger boulders are the closest to the original terminus of the glacier.

An outwash plain might contain surficial [[braided stream]] complexes that rework the original deposits.  They may also contain [[kettle lake]]s, locations where blocks of ice have melted, leaving a depression that fills with water. The flow pattern of glacial rivers across sandar is typically diffuse and unchannelized, but in situations where the glacial snout has retreated from the terminal [[moraine]], the flow is more channelized.

Sandurs are most common in Iceland, where [[Geothermal activity|geothermal]] activity beneath ice caps speeds up the deposition of sediment by meltwater.  As well as regular geothermal activity, [[volcano|volcanic]] activity gives rise to large [[jökulhlaup|glacial burst]]s several times a century, which carry down large volumes of sediment.

The [[Gaspé Peninsula]] that makes up the essential part of southern [[Quebec]] (Lower St-Lawrence and Gaspé areas) also contains several examples of paleo-sandar, dating from the [[Pleistocene]] ice melt.
[[Image:Lómagnú and Skeiðarárjökull.jpg|thumb|600px|center|The western edge of {{lang|is|Skeiðarársandur|italic=no}} in Iceland shows the diffuse drainage channels typical of sandur.]]

== The prototype sandur ==
[[Image:Lómagnú with sand blowing.jpg|thumb|right|260px|Unimpeded by topographic obstructions, sand-bearing [[katabatic wind]]s can be fierce enough on {{lang|is|Skeiðarársandur|italic=no}} to strip paint from cars.<ref>{{cite book
|url=https://books.google.com/books?id=YHJWc7jcJaIC&pg=PA28
|title=The Rough Guide to Iceland
|last1=Leffman
|first1=David
|last2=Proctor
|first2=James
|publisher=Rough Guides
|year=2004
|page=28|isbn=9781843532897
}}</ref> Here, sand is blown in the air in front of the peak {{lang|is|[[Lómagnúpur]]|italic=no}}.]]

One of the sandurs from which the general name is derived is {{lang|is|[[Skeiðarársandur]]|italic=no}}, a broad sandy wasteland along Iceland's south-eastern coast, between the {{lang|is|[[Vatnajökull]]|italic=no}} icecap and the sea. Volcanic eruptions under the icecap have given rise to many large glacial bursts (''{{lang|is|jökulhlaups}}'' in [[Icelandic language|Icelandic]]), most recently in 1996, when the [[Hringvegur|Ring Road]] was washed away (minor floods have also occurred since then). This road, which encircles Iceland and was completed in 1974, has since been repaired. The 1996 [[jökulhlaup]] was caused by the eruption of the {{lang|is|[[Grímsvötn]]|italic=no}} volcano, with peak flow estimated to be {{convert|50,000|m3/s|cuft/s|abbr=on}} compared to the normal summer peak flow of {{convert|200|to(-)|400|m3/s|cuft/s|abbr=on}}. Net deposition of sediment was estimated to be {{convert|12800000|m3|cuft|abbr=on}}.

The main braided channels of {{lang|is|Skeiðarársandur|italic=no}} are the [[Gígjukvísl]] and {{lang|is|[[Skeiðará]]|italic=no}} rivers, which incurred net gains of {{convert|29|and|24|cm|in|abbr=on}} respectively during the 1996 jökulhlaup. In the Gígjukvísl there was massive sediment deposition of up to {{convert|12|m|ft|abbr=on}}, which occurred closest to the terminus of the glacier. The erosional patterns of {{lang|is|Skeiðarársandur|italic=no}} can be seen by looking at the centimetre-scale elevation differences measured with repeat-pass laser altimetry ([[LIDAR]]) flown in 1996 (pre-flood), 1997, and 2001. Of the overall deposition during the 1996 jökulhlaup, nearly half of the net gain had been eroded 4 years after the flood. These two rivers on the sandur display drastically different erosional patterns. The difference in sediment erosion can be attributed to the {{convert|2|km|mi|abbr=on}} wide trench near the terminus where the Gígjukvísl flows, in contrast with the {{lang|is|Skeiðará|italic=no}}, which has braided flows directly onto the outwash plain. The Gígjukvísl river is where some of the highest level of sediment deposit occurred and also where the largest erosion happened afterward. This indicates that these massive jökulhlaup deposits may have a large geomorphic impact in the short term, but the net change on the surface relief could be minimal after a couple of years to a decade.

The observed change of {{lang|is|Skeiðarársandur|italic=no}} from a diffuse to a channelized distributary system where it has the most observed sediment deposit has a significant impact on the development of the fluvial succession in the proximal zone. However, in order to have sustained active accretion across the entire sandur there needs to be a diffuse, multipoint distribution system. The system of accumulation on {{lang|is|Skeiðarársandur|italic=no}}, which is a product of glacier retreat, can be seen as multiple regions of differing [[channel patterns]] that distribute sediment across the plain in dynamic configurations.

== Fossil sandar ==
Fossil sandar (i.e. no longer active) are found in areas which were formerly glaciated. An example would be the [[River Usk|Usk Valley]] of South [[Wales]] where, towards the end of the [[Last Glacial Period|last ice age]], the receding Usk valley glacier left behind a series of [[recessional moraines]] and sandar deposits down-valley of them. Many of the sandar surfaces are still visible, albeit degraded over succeeding millennia.<ref>{{cite book |last1=Thomas |first1=G.S.P |title=Geomorphology of the Middle Usk Valley (chapter) in The Quaternary of the South Midlands and the Welsh Marches (Field Guide) |date=1997 |publisher=Quaternary Research Association |isbn=0907780520 |editor-last=Lewis |editor-first=S.G. |edition=1st |editor-last2=Maddy |editor-first2=D.}}</ref> Extensive sandar are also recorded in the eastern part of the [[Cheshire Plain]] and beneath [[Morecambe Bay]], both in northwest [[England]]. 'Valley sandur' deposits are recorded from various localities in that same region.<ref>{{cite book |last1=Aitkenhead |first1=N |display-authors=etal |title=British regional geology: the Pennines and adjacent areas |date=2002 |publisher=British Geological Survey |location=Nottingham |isbn=0852724241 |pages=109–110 |edition=4th}}</ref>

== See also ==
*{{annotated link|Kankakee Outwash Plain}}
*{{annotated link|Terminal moraine}}

== References ==

{{Reflist}}

== Further reading ==
* Church, Michael A. (1972), Baffin Island sandurs: a study of arctic fluvial processes.
* Garvin J.B. (2001), ''Topographic Dynamics of Kerguelen Island: A Preliminary SRTM Analysis'', American Geophysical Union, Fall Meeting 2001
* Gomez B., Russell A.J., Finnegan D.C., Smith L.C., Knudsen O. (2001), ''Sediment Distribution on Skeidararsandur, Southeast Iceland'', American Geophysical Union, Fall Meeting 2001
* Hardardottir J., Snorrason A., Zophoniasson S., Jonsson P., Sigurdsson O., Elefsen S.O. (2003), ''Glacial Outburst Floods (Jökulhlaups) in Iceland'', EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6–11 April 2003
* Magilligan F.J., Gomez B., Mertes L.A.K., Smith, L.C. Smith N.D., Finnegan D., Garvin J.B., ''Geomorphic effectiveness, sandur development, and the pattern of landscape response during jökulhlaups: {{lang|is|Skeiðarársandur}}, southeastern Iceland'', Geomorphology 44 (2002) 95–113
* Hétu, B., ''La déglaciation de la région de Rimouski, Bas-Saint-Laurent (Québec): Indices d'une récurrence glaciaire dans la mer de Goldthwait entre 12400 et 12000 BP'', Géographie physique et Quaternaire, 1998, vol. 52, n.3, p.&nbsp;325-347
* Smith L.C., Sheng Y., Magilligan F.J., Smith N.D., Gomez B., Mertes L., Krabill W.B., Garven J.B., ''Geomorphic impact and rapid subsequent recovery from the 1996 {{lang|is|Skeiðarársandur}} jökulhlaup, Iceland, measured with multi-year airborne lidar''. Geomorphology vol. 75 Is. 1-2 (2006) 65-75

== External links ==
* [https://web.archive.org/web/20100611222902/http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_9/GEO_PLATE_G-22.shtml NASA page about the Skeiðarársandur]
* [https://web.archive.org/web/20211020104711/http://instaar.colorado.edu/news-events/conferences/abstract_detail.php?abstract_id=44 A study of sandur formation in western Greenland]

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[[Category:Outwash plains| ]]
[[Category:Glacial landforms]]
[[Category:Glaciology]]