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. The controls on flood frequency are complex, but rainfall frequency is a major contributing factor. 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. References See also Deposition (sediment) Sedimentary rock Sedimentology Deposition (geology)