watershed
wa.ter.shed \'wo.t-*r-.shed, 'wa:t-\ n 1: WATER PARTING 2: a region or area bounded peripherally by a water parting and draining ul timately to a particular watercourse or body of water 3: a crucial dividing point or line
Webster's English Dictionarywa.ter.shed \'wo.t-*r-.shed, 'wa:t-\ n 1: WATER PARTING 2: a region or area bounded peripherally by a water parting and draining ul timately to a particular watercourse or body of water 3: a crucial dividing point or line