FloodplainsFlooding & SedimentationLeveesPoint Bar DepositsPoint Bar DepositsFloodplain landformsOxbow lakeSplayMeander scrollNatural leveeBackswampAlluvial FansCoalescing Alluvial FansRiver TerracesRiver TerracesFloodplainsA floodplain is the flat land immediately surrounding a stream channel andinnundated at times of high flow.Aggradation occurs when deposition is greater than erosion.Aggradation of the Rivière des Ha! Ha! in QuebecIncision occurs when erosion is greater than deposition.Broadstreet HollowStream, NYFloodplains can form either by deposition of overbank suspended sediment or by deposition of bedload as the channel migrates across its valley.Flooding & SedimentationFiner sedimentFiner sedimentThe area adjacent to and outside of the channel serves as an overflow area for excess water and sedimentFlood stageCoarser sediment Levee DepositsLeveesThe boundary between channel and floodplain may be the site of a natural levee (a broad, low ridge of alluvium built along the side of a channel by debris-laden floodwater).Levees form when debris-laden floodwater overflows the channel and slows as it moves onto the floodplain.Formation of MeandersPoint bar depositsPoint Bar DepositsPoint Bar DepositsPoint bar deposit grows laterally through timeCut bank erosionPoint bar depositsPoint Bar DepositsOxbow FormationOxbow channelsMeandering streamflowing fromtop of screento bottomMaximum erosionMaximum depositionOxbow LakeOxbow cuttoffMeander scarsThe river cuts downward to form a ‘V’-shaped valley.The river startsto meanderRiver cuts from side to side eating intothe valley wallsFloodplain starts to form meandersFloodplain starts to form, enhances meanderingFloodplainAlluvialdepositsFloodplain landformsOxbow lakeA crescent-shaped lake formed in an abandoned river bend which has become separated from the main stream by a change in the course of the river.Oxbow lake near the Chippewa River, Eau Claire, WisconsinSplayA deposit of coarse material resulting from a levee breach during a flood.Meander scrollA meander scroll consists of long, curving, parallel ridges (scrolls) that during stages of high water have been aggraded against the inner bank of the meandering channel, while the opposite bank experienced erosion.Side looking radar (SLAR) image of floodplain of an Amazon River tributary in 1971/2; flow is toward lower right.Natural leveeA bank confining a stream channel or limiting areas subject to flooding.BackswampA low area of swampy ground beyond a river’s natural levees.Alluvial FansWhen streams encounter a sudden change in gradient, e.g. leaving mountains, they deposit alluvial fans.Alluvial fans tend to be coarse-grained at their head. At their edges, however, they can be relatively fine-grained.Coalescing Alluvial FansMany stream valleys contain one or more relatively flat alluvial terraces that lie above the floodplain.A terrace is a remnant of an abandoned floodplain.• Streams may create depositional landforms (especially floodplains) and then start to down-cut.•Terracesare abandoned floodplains.River TerracesRiver Terraces• Changes in whether a stream is eroding or depositing in a given location can be caused by several different phenomena:– Tectonic uplift– Change in base level downstream– Change in climate modifying system’s dischargeRiver
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