The drainage basin
Area drained by a river. Open system with water moving in and out.
Key terms.
- Drainage basin — area of land drained by a river and its tributaries.
- Watershed — boundary (often a ridge) separating one drainage basin from another.
- Tributary — smaller river joining a larger one.
- Confluence — point where two rivers meet.
- Source — start of a river.
- Mouth — where a river enters the sea / lake.
Open system. A drainage basin is an OPEN SYSTEM — water (and energy, sediment) flows in and out.
Inputs. PRECIPITATION (rain, snow, hail). Also solar energy driving evaporation.
Outputs. RIVER DISCHARGE to the sea. EVAPOTRANSPIRATION (evaporation + transpiration from plants).
Flows / transfers. SURFACE RUNOFF (overland flow), INFILTRATION (water entering soil), PERCOLATION (deeper into rock), THROUGHFLOW (lateral through soil).
Stores. SURFACE storage (puddles, lakes), SOIL moisture, GROUNDWATER, vegetation INTERCEPTION, channel storage.
Cambridge tip. Mark scheme expects students to know all four categories (inputs, outputs, flows, stores). Don't conflate them.
- Drainage basin = open system.
- Inputs (precip), outputs (discharge, ET), flows, stores.
- Watershed = boundary; basin = area.
See the full worked example for river systems and hydrological processes →