noun (n)

• a ridge of land that separates two adjacent river systems(noun.location)

  Synonym:
  divide, water parting
  

  source: wordnet30
• the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet(noun.location)

  Synonym:
  basin, catchment area, catchment basin, drainage area, drainage basin, river basin
  

  source: wordnet30
• an event marking a unique or important historical change of course or one on which important developments depend(noun.event)

  Example:
  The agreement was a watershed in the history of both nations.
  

  Synonym:
  landmark, turning point
  

  source: wordnet30
• The whole region or extent of country which contributes to the supply of a river or lake.(noun)

  source: webster1913