Geography

So, what exactly is a watershed?
A watershed is a geographic area of land in which all surface and ground water flows downhill to a common point, such as a river, stream, pond, lake, wetland, or estuary.

Water flows over and through land before reaching streams, rivers and wetlands. And the land that yields water, that "sheds" water, is called a watershed. Based on the hills and valleys, you can trace the probable direction that water will flow across the landscape to the nearest stream. When you map the area for that includes all the water that will reach a stream, you've delineated the boundaries of that stream's watershed.