2.2 Rivers: Explain processes and landforms associated with river erosion and deposition.

2.2 Rivers – Erosion, Deposition & Human Interactions

Learning objective

Explain the processes and landforms associated with river erosion and deposition, describe the key hydrological characteristics of a river system, and evaluate the opportunities and hazards that rivers present to people.

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2.2.1 Hydrological characteristics & drainage‑basin processes

• Long‑profile of a river – four characteristic stages
  • Youthful: steep gradient, high velocity, dominant vertical erosion → V‑shaped valleys, waterfalls, rapids.
  • Mature: moderate gradient, onset of lateral erosion → meanders begin, balance of erosion & deposition.
  • Old‑age: low gradient, wide floodplain, extensive deposition → levees, oxbow lakes.
  • Marine (mouth): river meets standing water → formation of deltas or estuaries.
• Discharge (Q) – volume of water passing a cross‑section per unit time (m³ s⁻¹).
  • Formula: Q = A × v where A = cross‑sectional area, v = mean velocity.
  • Controls the river’s ability to transport sediment (capacity) and to do work on the channel (stream power).
• Channel dimensions – width, depth, wetted perimeter and hydraulic radius; all increase downstream in accordance with the Bradshaw model.
• Bradshaw model (Figure 1) – shows the downstream trends of velocity, discharge, sediment load, channel width and flood‑plain width.

• Water‑cycle links – precipitation → infiltration → overland flow → channel flow → discharge to sea.
  • Runoff intensity determines peak discharge and flood risk.
  • Ground‑water contributions maintain base‑flow during dry periods.
• Competence vs. capacity
  • Competence = the maximum particle size a river can transport (depends mainly on velocity).
  • Capacity = the total volume of sediment a river can carry (depends on discharge).

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2.2.2 River erosion processes

• Hydraulic action – pressure fluctuations of moving water dislodge rock fragments from the bed and banks.
• Abrasion (corrasion) – rock fragments carried as bed load scour and wear the channel.
• Solution (corrosion) – slightly acidic water dissolves soluble minerals (e.g., limestone).
• Attrition – collisions between transported particles make them smaller and smoother, producing rounded pebbles.
• Transportation – movement of sediment as
  • Bed load (rolling, sliding, hopping – “saltation”).
  • Suspended load (particles kept in suspension by turbulence).
  • Dissolved load (ions in solution).

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2.2.3 Landforms produced by river erosion

Landform	Dominant process(es)	Typical setting
V‑shaped valley	Hydraulic action + abrasion (vertical erosion)	Upper (youthful) course, steep gradient
Interlocking spurs	Headward erosion on both sides of a valley	Transition from youthful to mature stage
Waterfall & rapids	Hydraulic action on resistant rock; differential erosion	Hard rock outcrop over softer downstream rock
Gorge / canyon	Intense vertical erosion (hydraulic action + abrasion) over long time	Steep, confined valleys in resistant rock
Headward erosion & river capture	Hydraulic action + solution extending the source upstream; breach of a drainage divide	Near watershed boundaries; often in mountainous terrain

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2.2.4 River deposition processes

Deposition occurs when a river’s carrying capacity falls, usually because of a reduction in velocity, gradient or an increase in channel width.

• Entering a wider valley or floodplain.
• Reaching standing water (lake, sea, ocean) at the river mouth.
• Decrease in slope – e.g., after a knickpoint retreats.
• Obstructions such as vegetation, bridges or engineered structures that slow the flow.

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2.2.5 Landforms produced by river deposition

Landform	Dominant process(es)	Typical setting
Point bar (inner bend of a meander)	Lateral erosion on outer bank + deposition on inner bank (lower velocity)	Meandering middle reaches
Floodplain	Over‑bank deposition during floods	Low‑gradient mature or old‑age rivers
Levee	Successive deposition of coarse material along floodplain margins	Immediately beside active floodplain
Alluvial fan	Sudden loss of competence when a steep mountain stream spreads onto a plain	Base of a mountain front
Delta (river mouth)	River meets standing water → rapid drop in velocity → deposition of suspended load	Coastal or lacustrine settings with low wave/tidal energy
Mouth bar (mid‑channel bar)	Deposition where flow diverges around an obstacle or where velocity decreases	Near river mouth or within wide channels
Oxbow lake (cut‑off meander)	Neck formation by lateral erosion → rapid breach → abandoned meander loop	Low‑gradient, highly sinuous rivers

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2.2.6 Factors controlling erosion & deposition

The intensity of a river’s ability to erode or deposit sediment is expressed by the stream‑power equation:

$$\Omega = \rho \, g \, Q \, S$$

• \(\Omega\) = stream power (W)
• \(\rho\) = density of water (≈ 1000 kg m⁻³)
• \(g\) = acceleration due to gravity (9.81 m s⁻²)
• \(Q\) = discharge (m³ s⁻¹)
• \(S\) = channel slope (dimensionless)

Higher \(\Omega\) favours vertical erosion; lower \(\Omega\) favours lateral erosion and deposition.

Numerical example

Consider two reaches of the same river:

• Youthful reach: \(Q = 30\; \text{m}^3\text{s}^{-1}\), \(S = 0.015\). \(\Omega = 1000 \times 9.81 \times 30 \times 0.015 \approx 4.4 \times 10^{3}\; \text{W}\).
• Mature reach: \(Q = 150\; \text{m}^3\text{s}^{-1}\), \(S = 0.003\). \(\Omega = 1000 \times 9.81 \times 150 \times 0.003 \approx 4.4 \times 10^{3}\; \text{W}\).

Although discharge is five times larger downstream, the much lower slope reduces stream power to a value comparable with the youthful reach. Consequently, vertical erosion diminishes and lateral processes (meandering, deposition) become dominant.

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2.2.7 River capture & headward erosion

• Headward erosion – upstream extension of a river valley caused by hydraulic action, abrasion and solution.
• River capture (stream piracy) – when headward erosion of one river breaches a drainage divide and diverts the flow of a neighbouring river.

DSE (named detailed specific example): The River Thames was captured by the River Avon during the Pleistocene, diverting water that formerly flowed eastwards into the Thames basin. This event enlarged the Thames catchment and altered the regional drainage pattern.

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2.2.8 River pollution (human impact)

• Types of pollution
  • Chemical – industrial effluents, heavy metals, pesticides.
  • Biological – pathogenic bacteria, algal blooms caused by nutrient enrichment.
  • Sediment‑loading – excessive erosion from agriculture or construction, increasing turbidity.
• Causes
  • Urban runoff, untreated sewage, mining, deforestation, over‑grazing.
• Consequences
  • Loss of aquatic biodiversity, reduced drinking‑water quality, health hazards, decreased suitability for irrigation.
• Management strategies
  • Legislation (e.g., EU Water Framework Directive), wastewater treatment plants, riparian buffer zones, sustainable land‑use practices.

DSE – Pollution: The Yangtze River in China carries an estimated 1.5 million tonnes of plastic waste each year, leading to severe ecological damage in the riverine and marine environments. Recent government initiatives include stricter discharge standards and large‑scale river‑clean‑up campaigns.

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2.2.9 Opportunities & hazards for people

Opportunity	Explanation / Example
Water supply & irrigation	Fresh water for domestic, agricultural and industrial use (e.g., Nile Basin irrigation schemes).
Hydropower	Steep gradients and high discharge generate electricity (e.g., Three Gorges Dam, China).
Transport & navigation	Broad, low‑gradient rivers support inland shipping (e.g., Amazon River).
Recreation & tourism	White‑water rafting, fishing, river cruises – economic benefits for local communities.
Fertile floodplains	Regular over‑bank flooding deposits nutrient‑rich silt (e.g., Ganges basin).

Hazard	Explanation / Example
Flooding	Rapid rise in discharge can inundate settlements; DSE – 2010 Pakistan floods on the Indus caused > 20 million people displaced.
Erosion of banks & infrastructure	Lateral erosion can undermine bridges, roads and houses on riverbanks.
River capture	Diverts water away from existing irrigation schemes or reduces downstream flow.
Pollution & sedimentation	Industrial discharge, agricultural runoff and excessive sediment load degrade water quality and habitats; DSE – Yangtze plastic pollution.
Salinisation at deltas	Reduced freshwater flow (e.g., upstream dams) allows seawater intrusion, harming agriculture.

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2.2.10 Summary table – processes, landforms & key conditions

Process	Typical resulting landform(s)	Key controlling conditions
Hydraulic action	Waterfall, rapids	High velocity, steep gradient, resistant rock
Abrasion	V‑shaped valley, gorge	Abundant coarse bed load, turbulent flow
Solution	Cave systems, karst valleys	Soluble rock (limestone), acidic water
Attrition	Rounded pebbles on alluvial fan, beach cobbles	Long transport distance, frequent particle collisions
Deposition (velocity decrease)	Point bar, floodplain, levee	Widening channel, reduced slope, lower discharge
Deposition at mouth	Delta, mouth bar	River meets standing water, low gradient, high sediment load
Meander cut‑off	Oxbow lake	High sinuosity, rapid lateral erosion, neck formation
Headward erosion & capture	New drainage pattern, enlarged basin	Steep upstream gradient, weak divide, high stream power
Pollution (chemical/biological)	Degraded water quality, loss of biodiversity	Industrial/agricultural discharge, insufficient treatment

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2.2.11 Exam‑style questions

1. Describe how hydraulic action and abrasion work together to deepen a river valley in its youthful stage.
2. Explain why a river forms a delta when it reaches the sea, using the concepts of sediment load, velocity and standing water.
3. Using the stream‑power equation, discuss how a decrease in gradient influences the balance between erosion and deposition downstream.
4. Evaluate two opportunities and two hazards that a river presents to a nearby community, giving specific examples (include at least one DSE for flooding and one for pollution).