watershed

(noun) a ridge of land that separates two adjacent river systems. Synonyms: water parting, divide.

(noun) the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet. Synonyms: river basin, basin, drainage basin, catchment area, catchment basin, drainage area. Example: "Flood control in the missouri basin."

(noun) an event marking a unique or important historical change of course or one on which important developments depend. Synonyms: landmark, turning point. Example: "The agreement was a watershed in the history of both nations."