what is a watershed?
A watershed is an area of land that captures water from above and below ground and channels it into a main body of water, such as a river or lake. The term drainage basin can be used interchangeably with watershed.
If you think about a bowl, the rim of the bowl acts as the defining ridge of its watershed. If you stick that bowl outside in the rain, all the water that collects inside the bowl is part of the bowl's watershed. All the water that falls outside the bowl is part of a different watershed. Mountains, hills or other areas of high elevation act as the defining edges of a watershed.
Watersheds can be very small, such as the area of ground that collects water into a puddle. Or they can be a bit larger and drain water into a stream or river. Even larger watersheds include multiple rivers that all flow into one main river, lake or ocean.