Editing Alluvial plain (section)

{{Short description|Region on which rivers have deposited sediment}}
{{About||the glacial feature|Outwash plain}}{{Mcn|date=November 2024}}[[File:Waimakariri02 gobeirne.jpg|thumb|Floodplain (centre) within the alluvial plain of the [[Waimakariri River]], [[New Zealand]] (part of the [[Canterbury Plains]]).]]
[[Image:AlluvialPlain.JPG|thumb|A small, [[River incision|incised]] alluvial plain from [[Red Rock Canyon State Park (California)]].]]

An '''alluvial plain''' is a [[plain]] (an essentially flat [[landform]]) created by the deposition of [[sediment]] over a long period by one or more [[river]]s coming from highland regions, from which [[Alluvium|alluvial]] soil forms. A ''[[floodplain]]'' is part of the process, being the smaller area over which the rivers flood at a particular time. In contrast, the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.

As the highlands [[erosion|erode]] due to [[weathering]] and water flow, the sediment from the hills is [[sediment transport|transported]] to the lower plain. Various [[stream|creek]]s will carry the water further to a river, [[lake]], [[bay]], or [[ocean]]. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a [[meander]] (a curved path). The leftover higher locations, typically natural [[levee]]s at the margins of the flood channel, will be eroded by lateral stream erosion, local rainfall, and possibly wind transport if the climate is arid and does not support soil-holding grasses. These processes, over geologic time, will form the plain, a region with little [[relief]] (local changes in elevation) yet with a constant but slight slope.

The ''Glossary of Landform and Geologic Terms'', maintained by the United States [[National Cooperative Soil Survey]] (NCSS), defines an "alluvial plain" as "a large assemblage of fluvial landforms (braided streams, terraces, etc.) that form a low gradient, regional ramps along the flanks of mountains and extend great distances from their sources (e.g., High Plains of North America)".  Use of "alluvial plain" as a general, informal term for a broad flood plain or a low-gradient delta is explicitly discouraged.  The NCSS glossary instead suggests "flood plain".<ref name="LandformGloss">{{cite web | url=https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=44260.wba | title=Glossary of Landform and Geologic Terms | publisher=[[National Cooperative Soil Survey]] | work=National Soil Survey Handbook—Part 629 | date=April 2013 | format=PDF | access-date=1 March 2023 | archive-date=28 March 2023 | archive-url=https://web.archive.org/web/20230328113202/https://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=44260.wba | url-status=dead }}</ref>

Alluvial plains have similar traits to a [[river delta]]; however, the river delta will flow into a larger body of water. Alluvial plains generally don't have this.