River Processes

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  • Created by: yott33
  • Created on: 30-12-15 16:45

Vertical and Lateral Erosion

Vertical Erosion

  • Makes a river deeper
  • Occurs in upper stages

Lateral Erosion

  • Makes a river wider
  • Happens in middle and lower courses
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Types of Erosion

Hydraulic Action: Occurs where the pressure of the water breaks rock particles away from the bed and banks.

Abrasion: Eroded pieces of rock in the water scrape and rub against the bed and banks, removing material. Most erosion of river beds and banks happen by abrasion.

Attrition:Eroded rocks collide into each other and break into smaller fragments. Attrition doesn't erode the bed and banks - it only makes the pieces of rock smaller and more rounded.

Solution:Occurs where rocks dissolve into the water and are carried away. Carbone dioxide dissolves in the water to form a weak acid that reacts with limestone and chalk, breaking them down.

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Types of Transportation

Solution: dissolved substances carried along in the water e.g. limestone that's dissolved in slightly acidic water

Suspension: very fine material such as clay and silt particles carried along in the water

Saltation:the force of the water causes larger particles such as pebbles or gravel to bounce along the river bed

Traction:very large particles such as boulders are pushed along the river bed by the force of the water

Material transported by traction or saltation is called the river's bedload.

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Deposition

Deposition occurs when the river loses energy. When it slows down, it loses energy and drops some of its load.

Ways in which speed and energy can be reduced:

  • Reduced rainfall causing lower discharge
  • Increased evaporation or abstraction causing lower discharge
  • Friction e.g. shallow areas of the river where the water is close to the banks the river's speed reduces, reducing its energy
  • When the river is forced to slow down e.g. before a narrow section in the channel
  • Energy is lost when the river meets the sea - the sea absorbs the energy.
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Capacity of a River

The capacity is the total load (measured in volume, weight or mass) that a river can transport at a given point.

The load of a river can be divided into different categories according to particle size:

  • Clay
  • Silt
  • Sand
  • Gravel
  • Pebbles
  • Cobbles
  • Boulders

The competence describes the maximum particle size that a river is capable of transporting at a given point

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Hjulstrom Curve

The Hjulstrom Curve shows the relationshiop between river velocity and competence. The competenece of a river is affected by the amount of energy it has - greater velocity = greater energy

The Hjulstrom Curve shows how erosion, deposition and transportation vary with river velocity. 

  • The critical erosion velocity curve shows the minimum velocity needed for the river to erode and transport particles of different sizes
  • The mean settling curve shows the velocities at which particles of different sizes are deposited i.e. the compentence of the river at different velocities.
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