A watershed is the area of land where all of the water that falls
in it and drains off of it goes to the same place.
Watersheds can be as small as a footprint or large enough to
encompass all the land that drains water into rivers.
Larger watersheds contain many smaller watersheds. It all
depends on the outflow point; all of the land that drains water
to the outflow point is the watershed for that outflow location.
Watersheds are important because the streamflow and the
water quality of a river are affected by things, human-induced
or not, happening in the land area "above" the river-outflow