FLOOD PLAIN

flat low-lying ground alongside a river that is flooded when the river overflows its banks

A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. It includes the floodway, which consists of the stream channel and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current.
See more at Wikipedia.org...

Noun
1. a low plain adjacent to a river that is formed chiefly of river sediment and is subject to flooding
(synonym) floodplain
(hypernym) plain, field, champaign

An area which is subject to periodic flooding. (DOI4)

The flat land that surrounds a stream and becomes sub merged when the stream overflows its banks.