Watershed
A watershed is an area of land from which rainfall, snowmelt, surface runoff, and connected flows drain toward a common outlet, such as a stream, river, lake, estuary, or ocean. Watersheds are defined by topography and hydrologic connections, can be nested at different scales, and may cross political or administrative boundaries.
In simple terms
A small tributary watershed can sit inside a larger river-basin watershed, so the relevant boundary depends on the outlet and decision. Water moving across and beneath the land connects upstream activities with downstream quantity and quality. Depending on the method, closely related terms such as drainage basin or catchment may be used. Maps should state the outlet, scale, and treatment of groundwater because subsurface boundaries do not always match surface divides.
Why it matters
Water, sediment, nutrients, pollutants, and flood flows move through hydrologic systems rather than stopping at property or political borders. A watershed boundary helps organizations and communities identify shared dependencies, upstream causes, downstream effects, and the stakeholders needed for effective water stewardship. Effective action also needs water stress data to show when demand is pressing hardest on the shared system.
Example
For example, a reservoir's watershed includes the streams and land areas that drain to it. Farms, forests, roads, towns, and wetlands upstream can all influence inflow, sediment, nutrients, and water quality, even when they lie in different municipalities.
How it differs
Water stewardship
A watershed is the hydrologically connected area draining to a common outlet; water stewardship is a management approach that acts within shared site and catchment contexts.