Aggradation (or alluviation) is the term used in
geology
Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
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
Transport (in British English) or transportation (in American English) is the intentional Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land tr ...
. The mass balance between sediment being transported and sediment in the bed is described by the
Exner equation.
Typical aggradational environments include lowland
alluvial river
A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...
s,
river delta
A river delta is a landform, archetypically triangular, created by the deposition of the sediments that are carried by the waters of a river, where the river merges with a body of slow-moving water or with a body of stagnant water. The creat ...
s, and
alluvial fan
An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...
s. 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
Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...
,
land use, and geologic activity, such as
volcanic eruption,
earthquake
An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
s, 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. However, both degradation and aggradation are event driven.
[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]
JSTOR website
Retrieved 5 May 2025.
See also
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References
External links
Schlumberger Oilfield Glossary*
David MohrigMIT OpenCourseWare - 12.110: Sedimentary Geology - Fall 2004*John B. Southard
Geomorphology
Sedimentology
Deposition (geology)
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