HKDSE Geography/M2a/Fluvial Processes

HKDSE Geography

development mutually exclusive?

The three fluvial processes are erosion, transportation and deposition.

  • Erosion is the process in which materials are removed by an agent.
  • Transportation is the process in which eroded materials are carried away.
  • Deposition is the process in which materials are 'dumped'.


There are three directions and four processes. Remember the names and what they mean (but don't memorise the definitions).

Three DirectionsEdit

Headward erosion Vertical erosion/Downcutting Lateral erosion
Primary location Source River bed of upper course River bank of middle and lower courses
Result The river is lengthened. The river is deepened. The river is widened.
Description Loose rock debris is brought down by overland flow running downslope. The source is extended backward and the backwall retreats upslope, accelerated by mass movement. The riverbed is eroded. The two riverbanks are eroded when the river swings from side to side.

Four ProcessesEdit

Four processes:

  • Hydraulic action: Water exerts pressure on the river bank and undercuts it. Cracks are widened.
  • Corrosion/Solution (Solvent action): Water dissolves soluble minerals from the bed or bank, e.g. calcium in limestone. It is a form of chemical erosion.
  • Abrasion (Corrasion): River load acts as a tool to wear away the bed or bank, e.g. large, angular boulders downcut the bed.
  • Attrition: Load knocks against each other, the bed or the bank. They break down into smaller pieces and get smaller and smaller.
Attrition vs. Abrasion

Attrition is the wearing down of the load. Abrasion is the wearing down of the bed or bank.

When load knocks against the bed or bank, abrasion occurs on the bed or bank, and attrition occurs in the load.


These factors affect erosion:

  • Nature of load:
    • Load weight ↑ → Erosion ↑
    • Load angularity ↑ → Erosion ↑
  • Amount of load: Load amount ↑ → Erosion ↑
  • Discharge and velocity:
    • Discharge ↑/Velocity ↑ → Energy ↑ → Erosion ↑
  • Geology of rocks: Joints ↑ → Erosion ↑
  • Additional factors for corrosion:
    • Water acidity ↑ (pH value ↓) → Erosion ↑
    • Hot and humid climate → Erosion ↑
    • Rocks composed of soluble minerals → Erosion ↑


The materials carried by transportation are load.

Four ProcessesEdit

Name Load Description Weight
Solution Dissolved load Soluble minerals are dissolved and carried downstream Lightest
Suspension Solid load (suspended load) Small particles are carried downstream in water without touching the bank Light
Saltation Solid load (suspended load) Load is moved in a series of hops and bounces along the bed Heavy
Traction Solid load (bed load) Load rolls and slides on the bed Heaviest


  • Energy ↑ → Velocity ↑ → More, heavier load
  • Fluvial Erosion ↑ → Load ↑
  • Soil Erosion ↑ → Load ↑
  • Deposition ↑ → Load ↓


One ProcessEdit

One process occurs in deposition: Sorting. The heaviest load is deposited first. Cobbles are deposited, then pebbles, then sand, then silt, and finally clay. Thus deposited materials are deposited in layers.


  • Input of water ↓ / Output of water ↑
    • Drought, dry season, high evaporation rate → Amount of water (discharge) ↓ → river energy ↓ → Deposition ↓
  • Velocity ↓
    • Inland flow of seawater → Velocity ↓ → Deposition ↑
    • Load ↑ → Internal friction ↑ → Velocity ↓ → Deposition ↑
      • Examples: Siltation, landslide
    • Channel friction ↑ → Deposition ↑
      • Examples: entering a channel with long wetted perimeter
    • Gradient ↓ → Velocity ↓ → Deposition ↑
      • e.g. waterfall, entering a flood plain
    • Water enters a large body of water
      • shallow lake, shallow sea
    • At inner banks (will be discussed later)

Impact of Velocity on Fluvial Process OccurrenceEdit

Downstream VariationEdit