Overbank

Alluvial geological deposit

An overbank is an alluvial geological deposit consisting of sediment that has been deposited on the floodplain of a river or stream by flood waters that have broken through or overtopped the banks. The sediment is carried in suspension, and because it is carried outside of the main channel, away from faster flow, the sediment is typically fine grained. An overbank deposit usually consists primarily of fine sand, silt and clay. Overbank deposits can be beneficial because they refresh valley soils. Overbank deposits can also be referred to as floodplain deposits. Examples include natural levees and crevasse splays.

Floodplains are far wider than the channel they border, reaching widths of up to 100 kilometers, and their length is 10 times that. They are thin and roughly planar in shape.

Overbank deposits are fine grained and accumulate vertically. The disturbance of adjacent environments during flooding events leads to deposits containing terrestrial organic debris such as plant matter, and the intervening dry periods allow subaerial bioturbation by roots and burrowing animals. In the stratigraphic record, natural levee deposits typically consist of thinly layered sandstones overlying mud to clay sized beds. Paleosols can serve as bounds for overbank depositional sequences or alternate with overbank deposits where flooding is episodic. Paleosols tend to show more maturity at a greater distance from the channel, where there is less sediment flux. In humid environments, crevasse channels may empty into long standing lakes or marshes, whereas in arid environments any drainage areas can dry up between flooding events. Tectonism can also affect the fluvial system by altering relative sea level, exposing floodplains or covering new areas with overbank deposits.