UK River Landscapes

River processes: erosion, transport and deposition

Erosion processes

• Hydraulic action: force of water loosens particles from the bed and banks.
• Abrasion (corrasion): sediment carried by the river scrapes and wears away the bed and banks.
• Attrition: sediment particles collide and break into smaller, rounder pieces.
• Corrosion (solution): river water dissolves soluble rock (limestone, chalk).

River transport (4 modes)

• Traction: large boulders rolled along the river bed.
• Saltation: medium particles (pebbles, gravel) bounce along in a leap-frog motion.
• Suspension: fine silt and clay carried within the water flow (makes rivers look cloudy).
• Solution: dissolved minerals carried invisibly in the water.

Competence: maximum particle size a river can carry (depends on velocity — competence ∝ v⁶ — Hjulström curve). Capacity: total load a river can carry (increases with discharge).

Deposition

Occurs when velocity decreases: on the inside of meander bends, when a river enters a lake/sea, during a flood (loses energy on the floodplain).

The long profile and changing processes

River landforms in detail

Upper course: V-shaped valleys and waterfalls

• V-shaped valley: river cuts downward (vertical erosion); weathered valley sides collapse; material transported away → steep sides, narrow flat channel. The River Tees in its upper course above High Force, County Durham.
• Waterfall: river crosses a band of harder rock overlying softer rock. Softer rock eroded faster → plunge pool → hydraulic action + abrasion undercut the hard rock → overhang collapses → waterfall retreats upstream forming a gorge. High Force (River Tees): 21 m drop over Whin Sill dolerite — UK's largest by flow. Hardraw Force (Wensleydale).

Middle course: meanders and associated landforms

• Meander formation: slight bends concentrate flow to the outside → erosion of outer bank (river cliff) by hydraulic action + abrasion; deceleration on inside → deposition of slip-off slope (point bar).
• Oxbow lake: meander neck becomes increasingly narrow; during flood, river cuts across the neck; meander is abandoned and silts up to form a horseshoe-shaped oxbow lake (e.g. along River Tees near Yarm).

Lower course: floodplains and levees

• Floodplain: flat valley floor formed by lateral erosion widening the valley and repeated deposition of alluvium (fine sediment) during floods. Most fertile agricultural land.
• Levée: naturally raised embankment along the riverbank. During flooding, river spills over banks; velocity drops instantly → heaviest sediment deposited first, building up a raised ridge. E.g. River Severn levées near Worcester.
• Delta: where a river meets a lake/sea with weak tidal action; sediment deposited faster than it is removed → delta (arcuate, bird-foot, cuspate types). UK rivers rarely form true deltas due to tidal energy.

Flood risk factors

Physical factors

• Prolonged/intense rainfall → soil saturation → rapid runoff → flashy hydrograph.
• Impermeable rocks (clay, granite) → less infiltration → more surface runoff.
• Steep slopes → rapid runoff.
• Saturated antecedent soil moisture (soil already wet before the storm).

Human factors

• Urbanisation: impermeable surfaces (roads, roofs) + drains → water reaches river faster → peak discharge higher and earlier.
• Deforestation: fewer roots intercepting rainfall → increased runoff; less evapotranspiration.
• Levée construction: can make flooding worse downstream by preventing natural overbank flow.

Storm hydrographs

A storm hydrograph shows river discharge over time following a storm event.

• Lag time: delay between peak rainfall and peak discharge (longer = less flood risk; shorter = more flash flood risk).
• Peak discharge: maximum flow rate.
• Rising limb: rapid increase in discharge; steep = faster runoff.
• Falling limb (recession limb): slower decrease as groundwater continues to feed the river.

Factors reducing lag time (increasing flood risk): urbanisation, impermeable geology, deforestation, steep slopes, saturated antecedent conditions.

Case study: River Severn flooding, 2007 (UK)

• July 2007: exceptional rainfall (90 mm in 14 hours at Tewkesbury) → River Severn + Avon burst banks.
• 350,000 homes without water; 42,000 without electricity; 11 deaths nationally; £3 bn damage.
• Tewkesbury, Upton-upon-Severn, Gloucester worst affected.
• Mythe Water Treatment Works (Tewkesbury) flooded → 350,000 without clean water for 17 days.

Flood management responses

• Hard engineering: Bewdley flood barrier (temporary demountable barriers, 2 km long); raised river walls; Shrewsbury flood alleviation scheme (walls + embankments).
• Soft engineering: upstream tree planting (Natural Flood Management); restoring meanders on tributaries; floodplain reconnection (Tern Valley scheme).
• Managed retreat: purchasing and relocating properties in highest-risk zones.
• Early warning: Environment Agency flood alerts; flood-watch cameras; Flood Forecasting Centre.

Evaluation: hard engineering protects specific towns (Bewdley, Shrewsbury) but is expensive (£30 m for Bewdley) and can push floods downstream. Natural Flood Management is cheaper and more sustainable but cannot handle extreme events alone — a portfolio approach is needed.

Edexcel B exam tip

River questions typically include a figure (hydrograph, OS map, photograph). When interpreting a hydrograph: describe lag time, peak discharge, rising/falling limb shape → link to human/physical factors → suggest management response.