Editing Floodplain (section)

==Formation==
Most floodplains are formed by deposition on the inside of river [[meanders]] and by overbank flow.<ref name="wolman-leopold-1957">{{cite journal |last1=Wolman |first1=M. Gordon |last2=Leopold |first2=Luna B. |title=River Flood Plains: Some Observations On Their Formation |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1957 |volume=282-C |doi=10.3133/pp282C |page=87|doi-access=free }}</ref>

Wherever the river meanders, the flowing water erodes the river bank on the outside of the meander. At the same time, sediments are simultaneously deposited in a  [[point bar|bar]] on the inside of the meander. This is described as ''lateral accretion'' since the deposition builds the point bar laterally into the river channel. Erosion on the outside of the meander usually closely balances deposition on the inside so that the channel shifts in the direction of the meander without changing significantly in width. The point bar is built up to a level very close to that of the river banks.  Significant net erosion of sediments occurs only when the meander cuts into higher ground. The overall effect is that, as the river meanders, it creates a level flood plain composed mostly of point bar deposits. The rate at which the channel shifts varies greatly, with reported rates ranging from too slow to measure to as much as {{convert|2400|feet|m|sp=us}} per year for the [[Kosi River]] of India.{{sfn|Wolman|Leopold|1957|pp=91-97}}

Overbank flow takes place when the river is flooded with more water than can be accommodated by the river channel. Flow over the banks of the river deposits a thin veneer of sediments that is coarsest and thickest close to the channel. This is described as ''vertical accretion'', since the deposits build upwards. In undisturbed river systems, overbank flow is frequent, typically occurring every one to two years, regardless of climate or topography.{{sfn|Wolman|Leopold|1957|pp=88-91}} Sedimentation rates for a three-day flood of the [[Meuse]] and [[Rhine River]]s in 1993 found average sedimentation rates in the floodplain of between 0.57 and 1.0&nbsp;kg/m<sup>2</sup>. Higher rates were found on the levees (4&nbsp;kg/m<sup>2</sup> or more) and on low-lying areas (1.6&nbsp;kg/m<sup>2</sup>).<ref>{{cite journal |last1=Asselman |first1=Nathalie E. M. |last2=Middelkoop |first2=Hans |title=Floodplain sedimentation: Quantities, patterns and processes |journal=Earth Surface Processes and Landforms |date=September 1995 |volume=20 |issue=6 |pages=481–499 |doi=10.1002/esp.3290200602|bibcode=1995ESPL...20..481A }}</ref>

Sedimentation from the overbank flow is concentrated on natural levees, [[crevasse splay]]s, and in wetlands and shallow lakes of flood basins. Natural levees are ridges along river banks that form from rapid deposition from the overbank flow. Most of the suspended sand is deposited on the levees, leaving the silt and clay sediments to be deposited as floodplain mud further from the river. Levees are typically built up enough to be relatively well-drained compared with nearby wetlands, and levees in non-arid climates are often heavily vegetated.<ref>{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins : from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=9781405177832 |edition=2nd |pages=265–266}}</ref>

Crevasses are formed by breakout events from the main river channel. The river bank fails, and floodwaters scour a channel. Sediments from the crevasse spread out as [[River delta|delta]]-shaped deposits with numerous distributary channels. Crevasse formation is most common in sections of rivers where the river bed is accumulating sediments ([[Aggradation|aggrading]]).{{sfn|Leeder|2011|pp=266-267}}

Repeated flooding eventually builds up an alluvial ridge, whose natural levees and abandoned [[Oxbow lake|meander loops]] may stand well above most of the floodplain.{{sfn|Leeder|2011|pp=267}} The alluvial ridge is topped by a channel belt formed by successive generations of channel migration and meander cutoff. At much longer intervals, the river may abandon the channel belt and build a new one at another position on the floodplain. This process is called avulsion and occurs at intervals of 10–1000 years. Historical avulsions leading to catastrophic flooding include the [[1851–1855 Yellow River floods|1855 Yellow River flood]] and the [[2008 Bihar flood|2008 Kosi River flood]].{{sfn|Leeder|2011|pp=269-271}}

Floodplains can form around rivers of any kind or size. Even relatively straight stretches of river are capable of producing floodplains. [[Braid bar|Mid-channel bars]] in braided rivers migrate downstream through processes resembling those in point bars of meandering rivers and can build up a floodplain.{{sfn|Wolman|Leopold|1957|pp=105-106}}

The quantity of sediments in a floodplain greatly exceeds the river load of sediments. Thus, floodplains are an important storage site for sediments during their transport from where they are generated to their ultimate depositional environment.<ref>{{cite journal |last1=Lewin |first1=John |title=Floodplain geomorphology |journal=Progress in Physical Geography: Earth and Environment |date=October 1978 |volume=2 |issue=3 |pages=408–437 |doi=10.1177/030913337800200302|bibcode=1978PrPG....2..408L |s2cid=220950870 }}</ref>

When the rate at which the river is [[river incision|cutting downwards]] becomes great enough that overbank flows become infrequent, the river is said to have abandoned its floodplain. Portions of the abandoned floodplain may be preserved as [[fluvial terrace]]s.{{sfn|Wolman|Leopold|1957|p=105}}