River erosion. Image credit Internationalarchives.org

River erosion. Image credit Internationalarchives.org

ZIMSEC O Level Geography Notes: River Erosion

  • River erosion involves the wearing away of rock and soil found along the river bed and banks.
  • It also involves the breaking down of the rock particles being carried downstream by the river.
  • There are four main processes of erosion.
  • These are corrasion, attrition, hydraulic action and solution (also known as corrosion).
The processes of erosion. Image credit slideshare.net

The processes of erosion. Image credit slideshare.net

Corrasion

  • Is the wearing down of the sides and bed of the river by the load as it is being transported by the river.
  • Corrasion occurs when a river picks up material and rubs its bed and bank wear them away by abrasion like sandpaper.
  • Corrasion therefore happens when the river’s sides and bed are scrapped off by the material being transported by the river.
  • This process is most pronounced during flooding.
  • This is the major means of erosion by which a river extends both vertically and horizontally.
  • If there are hollows in the river bed, pebbles can get trapped in these and whirled by turbulent eddies (in circular motion) to form potholes.
  • When pebbles are trapped in existing potholes these are deepened further by the whirling pebbles.
  • Corrasion wears away the channel’s river bed and add more material to the river’s load thus amplifying the processes as more load means more corrasion.

Attrition

  • Is a process by which the river’s own load is broken down from larger particles into smaller ones.
  • This happens because the river’s load which is made up of different sized particles which collide and knock into each other causing them to break into smaller fragments.
  • As the load progresses downstream it gets smaller and smaller.
  • Also angular rocks become increasingly rounded.

Hydraulic Action

  • refers to the sheer force and turbulence of the moving water which can be able to remove loose material such as gravel, sand and silt.
  • This force can also weaken solid rocks by surging into cracks in the rock.
  • This processes can be aided when there is air in the cracks which is compressed causing eventual bank collapse.
  • Cavitation is a form of hydraulic action caused by bubbles of air collapsing and the resultant shock waves hit and weaken the banks of the river.
  • Hydraulic action by itself is very effective if the river does not have some load to produce corrosive erosion/abrasive erosion.
  • Hydraulic action is the weakest and least effective form of erosion.

Solution or Corrosion

  • The water in the river dissolves some soluble rocks such as rock salt and sometimes limestone.
  • This is most effective in areas where the stream bed and banks are composed of soluble rock for example in limestone regions.
  • This method of erosion takes place all the time and is independent of a river’s velocity or discharge.
  • It is similar to the chemical weathering process of solution.
  • The river’s corrosive ability is aided, however, if there are acids within it.

Forms of Erosion

  • The above four processes make up a river’s erosion processes.
  • River erosion takes place in three ways:
  • Headward erosion, lateral erosion and vertical erosion.

Headward Erosion

Headward Erosion. Image credit e-xamit.ie

Headward Erosion. Image credit e-xamit.ie

  • Is the processes by which a river  increases its length upstream.
  • This is achieved by a river cutting back at its source.
  • Rain wash and soil creep are other processes by which a river extends its channel up the slope.

Lateral Erosion

forms_of_erosion-min

Forms of Erosion, Up arrow shows headward erosion, the two arrows lateral erosion/widening or channel and the downpointin arrow vertical erosion. Image Credit WikiCommons

  • The processes by which the river’s sides are worn away and the channel being extended in width.
  • This is more pronounced along the bends (outside banks) of meanders.

Vertical Erosion

  • This is a process by which a river deepens its channel.

To access more topics go to the Geography Notes page.

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