River Processes and the Formation of Landforms

Erosion (4 types).

• Hydraulic action — sheer force of water; compresses air in cracks.
• Abrasion — load scrapes against bed/banks. Most powerful.
• Attrition — load wears against ITSELF; particles round and shrink.
• Solution (corrosion) — chemical dissolving of soluble rocks.

Transport (4 types).

• Traction — large stones rolled along the bed.
• Saltation — pebbles bounce along.
• Suspension — fine sediment carried in the flow.
• Solution — dissolved minerals (chemically transported).

Deposition. When the river slows (less energy), it deposits material — heaviest first (boulders), lightest last (silt, clay). Causes: meander curves, overflows onto floodplain, river meeting sea.

Energy depends on velocity and volume. Flatter gradient + more water = more energy = more erosion. Less energy = deposition.

Cambridge tip. Memorise both 4-lists. Mark scheme rewards specific names, not generic 'erosion' or 'transport'.

In the upper course, rivers are STEEP, NARROW, and FAST. Vertical erosion deepens the channel.

V-shaped valley. Vertical erosion cuts down. Sides slump under gravity (mass movement). Result: V-shaped cross-section with steep, often forested sides.

Waterfall. Forms where the river crosses a band of HARD ROCK overlying SOFT ROCK.

1. Soft rock erodes faster, creating a step.
2. Falling water and load erode a PLUNGE POOL at the base.
3. Continued erosion undercuts the hard rock, creating an OVERHANG.
4. Overhang collapses; waterfall RETREATS upstream.
5. Steep-sided GORGE left in the wake.

Examples. Niagara Falls (Canada/USA), Victoria Falls (Zambia/Zimbabwe), Angel Falls (Venezuela).

Cambridge tip. Mark scheme rewards step-by-step formation diagrams with named processes (hydraulic action, abrasion). Don't skip the overhang collapse.

In the middle course, the river is WIDER, FLATTER, and slower. Lateral (sideways) erosion replaces vertical.

Meander. A pronounced bend. Forms because:

1. Small variations in flow deflect water side to side.
2. Outer bend: water flows FAST (longer path) → MORE erosion → forms a steep RIVER CLIFF.
3. Inner bend: water flows SLOW (shorter path) → DEPOSITION → forms a gentle SLIP-OFF SLOPE (point bar).
4. Over time, meanders MIGRATE downstream and become more pronounced.

Why meanders matter. They mark the transition between cutting-down (upper) and depositing (lower) river behaviour. Often visible from above as snake-like loops.

Cambridge tip. Mark scheme rewards explicit reference to FLOW VELOCITY differences (outer fast → erosion; inner slow → deposition).

In the lower course, the river is WIDE, FLAT, and SLOW. Deposition dominates.

Ox-bow lake. A meander gets so pronounced that during flood, the river cuts across the narrow neck, abandoning the meander loop. The cut-off loop becomes a curved lake.

Floodplain. Flat land alongside the river formed by sediment (alluvium) deposited during floods. Highly fertile — crucial for agriculture.

Levee. A natural raised bank along the river. Floods deposit COARSER sediment closest to the channel (where energy drops fastest), creating a low ridge.

Delta. Where a river meets a sea / lake, it deposits its load as flow slows. Builds up over centuries into a fan-shaped or arcuate landform. Famous deltas: Nile, Ganges, Mississippi.

Estuary vs delta. ESTUARY: river mouth widens with strong tides; sediment swept away. DELTA: gentle tides; sediment accumulates.

Cambridge tip. Mark scheme expects all four lower-course landforms with mechanism. Match each to deposition logic.