Aggradation (oralluviation) is the term used ingeology for the increase in land elevation, typically in a river system, due to thedeposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able totransport. The mass balance between sediment being transported and sediment in the bed is described by theExner equation.
Typical aggradational environments include lowlandalluvialrivers,river deltas, andalluvial fans. Aggradational environments are often undergoing slowsubsidence which balances the increase in land surface elevation due to aggradation. After millions of years, an aggradational environment will become asedimentary basin, which contains the deposited sediment, includingpaleochannels and ancientfloodplains.
Aggradation can be caused by changes inclimate,land use, and geologic activity, such asvolcanic eruption,earthquakes, andfaulting. 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 itsfloodplain. 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 theUniversity of Colorado at Boulder in the journalNature Geoscience said that reduced aggradation was contributing to an increased risk of flooding in many river deltas.[1]
| Large-scale features | |
|---|---|
| Alluvial rivers | |
| Bedrock river | |
| Bedforms | |
| Regional processes | |
| Mechanics | |
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