watershed

The entire area of land that drains water to a single point — a river mouth, a lake, a confluence. Every drop of rain that falls within a watershed eventually reaches the same destination (or soaks into the ground trying). Watersheds are the natural units of hydrology: they are defined not by political boundaries but by topography — the ridgelines that divide one drainage from the next. The word also means a turning point, which is fitting: a watershed ridge is the point where water decides which way to go.