Fluvial Processes

Abrasion- fluvial erosion

  • Also called corrasion.
  • Small boulders and stones may scratch and scrape their way down a river during transport, thereby wearing down the river banks and bed.
  • Most erosion happens by abrasion.
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Hydraulic Action- fluvial erosion

  • When the sheer force of fast flowing water hits the river banks and bed and forces water into cracks.
  • The force of the water and repeated changes in pressure weaken the channel and gradually, the banks and bed wear away.
  • Responsible for vertical erosion in the upper course and lateral erosion in the lower course.
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Attrition- fluvial erosion

  • Attrition affects a rivers load.
  • When stones first enter a river, they will be jagged and angular. As they are transported downstream, the stones collide with each other and the river banks and bed.
  • This gradually erodes the stones jagged edges so that they are smooth and rounded.
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Solution (Corrosion)- fluvial erosion

  • Solution refers to the dissolving of rocks such as chalk and limestone.
  • Rivers travelling over these rocks will erode them in this way.
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Solution- transportation

  • Soluble materials (e.g. minerals) dissolve in the water and are carried along.
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Suspension- transportation

  • Small particles like silt and clay are carried along by the water.
  • The load is fine, light material.
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Saltation- transportation

  • Small pebbles and stones are bounced along the river bed by the force of the water
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Traction- transportation

  • Large particles like boulders and rocks (bedload) are pushed along the river bed by the force of the water.

What is the difference between bedload and load?

  • The bedload is the load on the bed of the river.
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Deposition

  • Deposition is when a river drops the load (sediment) it is transporting.
  • It happens when a river slows down (loses velocity)
  • Large boulders are the first to be deposited as they need more energy to be transported. 
  • The finest particles are deposited last as they require little energy.
  • This is why mountain streams have boulders aong their bed, whereas close to the river's mouth, there is only fine silt.

 

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Deposition

What causes velocity to fall?

  • The river reaches its mouth.
  • The river is less steep.
  • The amount of eroded material (load) increases (as this increases the amount of friction).
  • The volume of water in the river falls (can be caused by weather- low rainfall = low flow).
  • The river is shallower (e.g. on the inside of a bend)
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