Watershed: The Stream System Connected to One Primary River

A watershed (US) / catchment (UK/Commonwealth) / drainage basin (technical) is the land area where all precipitation drains to a common outlet—a single river, lake, estuary, or ocean point.

How It Works

• Divide (watershed boundary): Ridge lines separating adjacent basins
• Tributary network: Hierarchical streams (1st order → mainstem) — Horton-Strahler ordering
• Confluence points: Where tributaries join; each sub-watershed nests within larger
• Outlet (pour point): Single exit — river mouth, dam, gauge station

Nested Hierarchy

Mississippi River Basin (3.2M km²) → Ohio River Basin (528K km²) → Allegheny Basin (30K km²) → French Creek Basin (3,200 km²) → Headwater catchment (10 km²)

What Moves Through a Watershed

• Surface runoff — overland flow, channel flow
• Groundwater — baseflow, spring discharge, inter-basin transfer
• Sediment — erosion, transport, deposition
• Nutrients/contaminants: N, P, pesticides, metals, PFAS, thermal
• Organic matter: Leaves, wood, DOC

Why Watersheds Matter

• Management unit: Water quality (TMDLs), flood control, allocation, restoration
• Ecological unit: Aquatic habitat connectivity, riparian corridors, thermal regimes
• Planning unit: Land use, zoning, green infrastructure, source water protection
• Legal unit: Interstate compacts, tribal rights, international treaties (Great Lakes, Colorado, Columbia)

Key Metrics

Everything flows downhill. The watershed is the accounting boundary for that truth.