Rivers
The drainage basin, river processes of erosion, transport and deposition, the landforms they create, and flooding and its management.
📌 Key points
- A drainage basin is the area of land drained by a river and its tributaries; its edge is the watershed; a tributary joins at a confluence; the river ends at its mouth.
- Erosion processes: hydraulic action (force of water), abrasion (load scraping the bed and banks), attrition (load knocking together and becoming smaller and rounder) and solution (rocks such as limestone dissolving).
- Transport processes: traction (large boulders rolled), saltation (pebbles bounced), suspension (fine particles carried in the water) and solution (dissolved minerals).
- Deposition happens when a river loses energy, for example when velocity falls on the inside of a bend, in shallow water or where a river meets the sea.
- Upper course landforms: V-shaped valleys, interlocking spurs, rapids, and waterfalls with gorges, formed where hard rock lies over softer rock.
- A waterfall retreats because soft rock is undercut, the hard rock overhang collapses into the plunge pool, and a gorge is left behind.
- In a meander, the fastest flow is on the outside bend, causing erosion and a river cliff; slower flow on the inside causes deposition and a slip-off slope; an ox-bow lake forms when the neck is cut through, usually in a flood.
- Lower course landforms include the floodplain, levees (raised banks of sediment deposited in floods), estuaries and deltas.
- Discharge = cross-sectional area × velocity, measured in cumecs (m3/s); on a storm hydrograph, lag time is the time between peak rainfall and peak discharge.
- Floods are made worse by heavy rain, impermeable or saturated ground, steep slopes, deforestation and urbanisation; they are managed by dams, levees, channel straightening, afforestation, floodplain zoning and warning systems.
📖 Key terms
- Drainage basin
- The area of land drained by a river and its tributaries.
- Watershed
- The boundary of a drainage basin, usually high ground.
- Confluence
- The point where two rivers join.
- Hydraulic action
- Erosion by the force of moving water, forcing air into cracks in the bed and banks.
- Abrasion
- Erosion when the load is scraped and rubbed against the bed and banks.
- Attrition
- Load particles knocking into each other and becoming smaller and rounder.
- Discharge
- The volume of water passing a point in a given time, measured in m3/s.
- Lag time
- The time between peak rainfall and peak discharge.
✏️ Worked example
A river channel is 6 m wide with a mean depth of 0.5 m. The mean velocity is 0.8 m/s. Calculate the discharge.
- Discharge = cross-sectional area × velocity.
- Cross-sectional area = width × mean depth = 6 × 0.5 = 3 m2.
- Discharge = 3 m2 × 0.8 m/s = 2.4.
- Give the units: m2 × m/s = m3/s (cumecs).
- Answer: 2.4 m3/s (2.4 cumecs).
When explaining the formation of a landform, use a clear sequence of processes and name them (e.g. hydraulic action, abrasion). A labelled diagram can earn marks too.
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