The relatively flat land stretching from either side of a river to the bottom of the valley walls. Flood plains are periodically inundated by the river water; hence the name. Flood plains are often ill drained and marshy, and characteristic fluvial features include meanders, levées, and ox-bow lakes.

The flood-plain hydrology system is dependent on interactions among dynamic, non-linear physical and biological processes linking water, heat, and materials (biota, sediment, plant-growth nutrients), flux, and retention to fluvial landscape change. The key processes driving biogeochemical patterns and cycles include flood-caused scour and sedimentation (cut and fill alluviation), routing of river water and nutrients above and below ground, channel movement (avulsion). Groundwater routing through the flood plain and upwelling back to the surface involves penetration of river water into zones of high hydraulic conductivity (subsurface palaeochannels) within the bed sediments that are created by channel scour and subsequent filling with sorted gravel and cobbles. Strong interactions between short-duration, high stream-power floods, channel and sediment movement, increased roughness due to presence of vegetation and dead wood, and upwelling of groundwater creates a complex, dynamic distribution of resource patches, referred to as the shifting habitat mosaic (University of Montana Floodplain biocomplexity site).

See Poole et al. (2002) Freshwater Biol. 48, 10 on geomorphic controls on flood-plain hydrology and connectivity. For flood-plain management, see Evers and Urban (2003) COFIS abstracts.