area of land adjacent to a stream or river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge

A floodplain is the flat area around a river that is covered in times of flood. A floodplain is a very fertile area due to the rich alluvium (fine sediment) deposited by floodwaters. This makes floodplains a good place for agriculture. A build-up of alluvium on the banks of a river can create levees, which raise the riverbank.

When a flood plain is not flooded it may be good farm land