The water that flows down from the top of a ridgeline will inevitably end up in a sea or an ocean. This is what we learn from the water cycle. The force of gravity causes liquid water to descend, whether by surface runoff or infiltration. On the surface, it travels by a stream, then one or more rivers, before joining a waterway that flows into a sea or an ocean.

A watershed is a geographic area bounded by water divide lines. All precipitation that falls within this area flows from upstream to downstream. The lowest point of the watershed is called the outlet or the mouth.