Drainage basins
Big idea: A drainage basin (also called a catchment) is like a bathtub — all the water that falls in it eventually goes down the same drain (the river mouth).
Drainage basin vs watershed vs catchment
Key terms: Catchment = drainage basin = the whole AREA where water drains to one river.
Watershed = the BOUNDARY LINE (high ground) that separates one drainage basin from another.
Don't get confused!: In the UK/IB: watershed = the boundary line (ridge). In the USA: "watershed" often means the whole area (same as catchment).
For IB exams, use: catchment/drainage basin = area, watershed = boundary.
How it works
Imagine pouring water on a sloped roof. All the water flows down to the same gutter. The roof is like a catchment — all water ends up in the same place.
- Rain falls anywhere in the drainage basin
- Water flows downhill (runoff, rivers, groundwater)
- ALL water eventually reaches the same river mouth or lake
- High ground (hills, ridges) forms the watershed boundary
- Rain falling on the OTHER side of the ridge flows to a DIFFERENT river
Components of a drainage basin
- Watershed — the boundary separating drainage basins (ridge/high ground)
- Source — where the river begins (usually in highlands)
- Tributaries — smaller streams that feed into the main river
- Confluence — where two rivers meet
- Mouth — where the river reaches the sea or a lake
- Floodplain — flat land beside the river that floods periodically
The drainage basin as a system
Open system: At the local scale (within a catchment), the water cycle is an OPEN system — water flows IN (precipitation) and OUT (evapotranspiration, runoff to sea). This is different from the GLOBAL water cycle, which is CLOSED for matter.
Inputs
- Precipitation (rain, snow)
- Solar energy
Outputs
- River discharge to sea
- Evapotranspiration
- Water abstraction by humans
Global scale = CLOSED system (no water enters or leaves Earth). Local scale (drainage basin) = OPEN system (water flows in and out).
Why drainage basins matter
- Everything upstream affects everything downstream
- Pollution in one area spreads through the whole basin
- Deforestation upstream → more flooding downstream
- Dams change flow, sediment, and ecosystems for the whole river
- Water management must consider the WHOLE catchment
Exam skill: When explaining human impacts on a drainage basin, trace the effect downstream. Example: "Fertiliser runoff enters tributaries → flows to main river → causes eutrophication downstream → reduces oxygen → fish die at the river mouth."
IB-style question — deforestation feedback loop [2]
In a tropical drainage basin, farmers clear trees for cropland. Describe the positive feedback loop by which this clearance can progressively reduce water available in the region. [2]
How to answer it, step by step
- **Start the loop**<br>• Fewer trees → less evapotranspiration → less water vapour in the local air.<br>• Less vapour → less local rainfall.
- **Close the loop back to the start**<br>• Less rainfall → drier soils → more trees die or fail to grow.<br>• Even fewer trees → the cycle repeats and intensifies.
Final answer
For both marks the loop must come full circle (end effect feeds the start) — a one-way chain scores only 1.