Watersheds are like upside down umbrellas
Everyone lives in a watershed. This is because when it rains, it drains into a catchment basin.
Think of it like an upside-down umbrella. The canopy catches water to eventually end up at the lowest depression.
These depressions are often well-connected networks of tributaries.
They zig-zag all the way to a main body of water like a river or lake. That means everything upstream ends up downstream.