Overbank

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. == Depositional processes and facies ==

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. Crevasse splays Crevasse-splay deposits form during flooding events when a river cuts a levee to form a smaller channel away from the main channel. These crevasse channels are essentially miniature distributary systems and can have many of the features that larger fluvial bodies possess, like levees. A crevasse-splay sequence typically begins with an erosive base, followed by the deposition of coarse bed load sediment and transitioning to finer suspended sediment as energy decreases, resulting a graded bedding pattern when viewed in cross-section. Crevasse channels are ephemeral, and their deposits commonly show terrestrial or desiccation features near the top such as mudcracks or roots. == Relation to paleosols ==

Because overbank deposits often overlie areas that are normally exposed to weathering, they can bury soils, allowing those soils to be preserved as paleosols. 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. The degree of soil horizon development can be used as a proxy for this process. == Controls on depositional system evolution ==

When a river changes course (avulsion), former floodplains can be stranded far from their former channel. They can be covered by new overbank deposits, cut by a channel, eroded completely, or converted into non-fluvial terrestrial deposits like soils. Overbank deposits are climate-dependent. Of course, the frequency of floods has a major impact on overbank deposits. 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. ==See also==