# Aggradation

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{{Short description|Increase in land elevation due to the deposition of sediment}}
[[Image:Aggradation.svg|thumb|Schematic of sediment accumulation (aggradation) in a [river](/source/river) channel. The sediment is brown. The river is flowing on bedrock in the upper image, but because sediment was deposited over time the riverbed has risen. This has caused the house to be buried in the lower image.]]

'''Aggradation''' (or '''alluviation''') in [geology](/source/geology) is the increase in land elevation, typically in a river system, due to the [deposition](/source/deposition_(geology)) 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](/source/sediment_transport). The mass balance between sediment being transported and sediment in the bed is described by the [Exner equation](/source/Exner_equation).

Typical aggradational environments include lowland [alluvial](/source/alluvial) [river](/source/river)s, [river delta](/source/river_delta)s, and [alluvial fan](/source/alluvial_fan)s. Aggradational environments are often undergoing slow [subsidence](/source/subsidence) which balances the increase in land surface elevation due to aggradation. After millions of years, an aggradational environment will become a [sedimentary basin](/source/sedimentary_basin), which contains the deposited sediment, including [paleochannel](/source/paleochannel)s and ancient [floodplain](/source/floodplain)s.

Aggradation can be caused by changes in [climate](/source/climate), [land use](/source/land_use), and geologic activity, such as [volcanic eruption](/source/volcanic_eruption), [earthquake](/source/earthquake)s, and [faulting](/source/fault_(geology)). 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](/source/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](/source/University_of_Colorado_at_Boulder) in the journal [Nature Geoscience](/source/Nature_Geoscience) said that reduced aggradation was contributing to an increased risk of flooding in many river deltas.<ref>{{cite news | last = Black | first = Richard | title = 'Millions at risk' as deltas sink | publisher = [BBC News Online](/source/BBC_News_Online) | date = 2009-09-21 | url = https://newsvote.bbc.co.uk/2/hi/science/nature/8266500.stm | access-date = 2009-09-23 }}</ref> However, both degradation and aggradation are event driven.<ref>Moore, James E., et al. “Short-Term Assessment of Morphological Change on Five Lower Mississippi River Islands.” ''Southeastern Naturalist'', vol. 10, no. 3, 2011, pp. 459–76. [http://www.jstor.org/stable/41262925 JSTOR website] Retrieved 5 May 2025.</ref>

== See also ==
* {{annotated link|Avulsion (river)}}
* {{annotated link|Progradation}}
* {{annotated link|Sedimentary basin}}

== References ==
{{reflist}}

== External links ==
*[http://www.glossary.oilfield.slb.com/Display.cfm?Term=aggradation Schlumberger Oilfield Glossary] {{Webarchive|url=https://web.archive.org/web/20120716211655/http://www.glossary.oilfield.slb.com/Display.cfm?Term=aggradation |date=2012-07-16 }}
*[https://web.archive.org/web/20051216143857/http://www.uwsp.edu/geo/faculty/ritter/glossary/a_d/aggradation.html "The Physical Environment" Glossary definition]
* [David Mohrig](/source/David_Mohrig), [http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-110Fall-2004/CourseHome/index.htm MIT OpenCourseWare - 12.110: Sedimentary Geology - Fall 2004]
*John B. Southard, [https://web.archive.org/web/20090106060837/http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-110Spring-2007/CourseHome/index.htm MIT OpenCourseWare - 12.110: Sedimentary Geology - Spring 2007]

{{River morphology}}
{{Rivers, streams and springs}}

Category:Geomorphology
Category:Sedimentology
Category:Deposition (geology)

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Adapted from the Wikipedia article [Aggradation](https://en.wikipedia.org/wiki/Aggradation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Aggradation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.