noun
flood plain
A flat area adjacent to a river that is subject to flooding. Used in geography and environmental contexts.
この川の下流には広大な氾濫原が広がっている。
A vast flood plain stretches along the lower reaches of this river.
氾濫 means 'flooding' or 'overflow', while 氾濫原 specifically refers to the geographical area that floods.
Compound of 氾濫 (はんらん, 'flooding') and 原 (げん, 'plain' or 'field').