watershed

nouns

a ridge of land that separates two adjacent river systems
divide, water parting, watershed
the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet
"flood control in the Missouri basin"
basin, catchment area, catchment basin, drainage area, drainage basin, river basin, watershed
an event marking a unique or important historical change of course or one on which important developments depend
"the agreement was a watershed in the history of both nations"
landmark, turning point, watershed