Floodplain (noun)

1. A flat area of land adjacent to a river, subject to flooding.
2. An area of low-lying land adjacent to a river that is formed mainly of river sediments and is subject to flooding.

Origin:

Late 19th century: from flood + plain.

Examples:

1. The floodplain was covered with water during the flood.
2. The houses were built on the floodplain and were often damaged by floods.
3. The farmers use the floodplain for irrigation.
4. The floodplain was used as a recreation area.
5. The floodplain was a rich source of fertile soil.