A watershed (also called a catchment or drainage basin) is the area of land where all surface runoff drains to a single common outlet — a river mouth, lake, or specified point on a stream. Watersheds are bounded by drainage divides, the ridgelines separating one basin from its neighbours.
What it is and why it matters
Watersheds are a fundamental unit for hydrology, flood and erosion risk, water resources, and environmental baseline work, because water, sediment, and contaminants move within basin boundaries rather than political ones. In GIS, watersheds are derived from a DEM through a standard chain of terrain analysis:
- Fill sinks — remove spurious pits that would trap flow.
- Flow direction — compute the downslope direction for each cell (commonly the D8 method, where flow goes to the steepest of eight neighbours).
- Flow accumulation — count how many upstream cells drain through each cell, revealing the stream network.
- Watershed delineation — for a chosen pour point, trace all cells that drain to it.
Concrete example
In QGIS using SAGA or GRASS, or with r.watershed, you take a 30 m SRTM DEM, fill it, derive flow direction and accumulation, threshold accumulation to extract channels, then snap a pour point to the stream network and run watershed delineation. The output is a polygon of the contributing area, from which you can compute drainage area in km², stream length, and basin slope.
Common pitfall
A pour point placed even one cell off the true channel can grab a wildly wrong (tiny or huge) catchment. Always snap the outlet to the high-accumulation stream cell before delineating. Also note that DEM resolution and the sink-filling step strongly affect the result, especially in flat terrain.