flood plain
From Longman Dictionary of Contemporary EnglishRelated topics: Geography, Geologyflood 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 fullExamples from the Corpusflood 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.