It is the geographical area of land that collects, stores, and transfers water or the land area from which surface water flows into a shared stream channel, lake, reservoir, or other body of water.
A watershed can be huge, like the Mississippi River watershed, or small, like the water that flows into a little pond.
Basins are large watersheds that incorporate several smaller watersheds.
Hundreds, if not thousands, of creeks and streams flow from higher terrain to rivers, which finally empty into a bigger body of water.
Pollutants are picked up by the water as it flows, which can have negative consequences for the ecology of the watershed and ultimately, the reservoir, bay, or ocean where it finishes up.
However, not all water flows directly to the sea. Rain can sink into or enter the earth when it falls on dry ground. This groundwater will eventually flow into the nearest stream after remaining in the soil.
A watershed's size is determined on numerous scales, which are referred to as Hydrologic Unit Codes (HUC).