Overbank sediments (also called alluvial soils, floodplain sediment or levee sediment) occur along rivers and streams with variable rates of water discharge The sediment is deposited on flood plains and levees from water suspensions during flooding, when the discharge exceeds the amounts that can be contained within the normal channel.
Even in moderate-sized streams, the water level can rise by several meters and thus cover large areas. Also, new sediment sources open up during flooding. Thereby the origin of the suspended materials in the streams is diverse.
During the flooding and especially its last phases, some of the suspended material will deposit on the flood plain, out of reach of the ordinary stream channel. Thereby, nearly horizontal strata of overbank sediment are built up over time. The total thickness of overbank sediment can be up to several meters. Overbank sediments thus represent an important medium in which natural and anthropogenic toxic substances are transported and stored on floodplains.