watershed

noun

1
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
2
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
3
a ridge of land that separates two adjacent river systems

Also useful for “watershed”:Unscrambler— Rearrange the letters in “watershed”