From Longman Dictionary of Contemporary Englishflood plainˈflood plain noun [countable] SGthe large area of flat land on either side of a river that is sometimes covered with water when the river becomes too full
Examples from the Corpus
flood plain• Her dammed-up intelligence overflowed like a flood plain.• The little town crowns a low plateau just out of reach of the flood plain of the nearby Deerfield River. 