floodplain, level land along the course of a river formed by the deposition of sediment during periodic floods. Floodplains contain such features as levees, backswamps, delta plains, and oxbow lakes. Floodplains may be extensive, such as below the conflux of the Ohio and the Mississippi, where they have a width up to 80 mi (130 km). Rivers with extensive floodplains are the Nile, Ganges, Danube, and Po. Floodplains are generally very fertile, thus making them rich agricultural lands. The disadvantage of farming on a floodplain is the natural hazard of floods. In the United States there has been extensive house construction on floodplains in recent years, necessitating the construction of new dams to control small, annual floods.