noun (n)

• a long depression in the surface of the land that usually contains a river(noun.object)

  Synonym:
  vale
  

  source: wordnet30
• The space inclosed between ranges of hills or mountains; the strip of land at the bottom of the depressions intersecting a country, including usually the bed of a stream, with frequently broad alluvial plains on one or both sides of the stream. Also used figuratively.(noun)

  source: webster1913