A watershed is the area of land that drains toward a shared outlet, such as a stream, lake, or coastal water.
Definition
A watershed is the land area from which water drains to a common outlet, either over the ground or through connected drainage features. Its boundary is shaped by terrain and may also be affected by storm drains, ditches, culverts, diversions, or other infrastructure. A watershed may extend well beyond a project property and cross jurisdictional boundaries. Firms delineate and describe the area using available mapping, survey, drainage records, field observations, and project assumptions. The appropriate boundary depends on the outlet and the question being studied, so watershed and catchment labels may be used at different scales by different teams or agencies. A watershed is a geographic drainage area, not a single channel, a property boundary, or a prediction of how much runoff a specific storm will produce. EPA watershed resources provide broader context for watershed-based planning.
How owners use it
- Define the drainage area contributing water to a project outlet or receiving water.
- A site drains 4 acres and an upstream area contributes 11 acres, so the combined watershed area is 4 + 11 = 15 acres.
- Identify upstream land uses and drainage connections that may affect a site analysis.
- Set boundaries for hydrologic modeling and coordinate drainage assumptions across project teams.
