River Landforms

River Landforms; including landforms of erosion and deposition for AQA A AS Geography-Rivers

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  • Created by: Amy
  • Created on: 17-05-12 17:45

Braided Channels

  • landform of deposition
  • river carries excessive amount of sediment
  • when the load of the river is increased, or the velocity of the river decreases deposition of this material occurs in the main channel as the river does not have the capacity to transport the load.
  • This causes the formation of sediment accumulations in the main channel of the river
  • The river splits and swings around these accumulations before rejoining the main channel.
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Floodplain

  • landform of deposition
  • lowlying area on either side of the rivers banks- frequently flooded and can be a water store during flood conditions
  • when a river floods and bursts its banks the wetted perimeter is increased as more land is in contact with the water, this increases friction.
  • friction reduces the river velocity causing deposition to occur as the river no longer has the energy to carry its load.
  • this leads to the deposition of fertile alluvium deposits on the floodplain
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Levees

  • landform of deposition
  • a naturally occuring embankment the restricts river flooding
  • when a river floods and bursts its banks deposition occurs on the floodplain (as previously explained).
  • the largest and heaviest particles such as sand and gravel are deposited closest to the river as they cannot be transported as far in conditions of low velocity.
  • Eventually, after successive flooding these large particles build up and form an embankment (levee).
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Delta

  • landform of deposition
  • when a river reaches a water store such as the sea, its velocity is lowered as its energy is absorbed by the larger body of water.
  • This causes deposition in the main channel
  • Deposition continues until the accumulation reaches above sea level, causing a partial blockage of the river mouth.
  • This means that the river has to split into several distributaries in order to continue its course into the sea.
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Potholes

  • landforms of erosion
  • round to oval shaped holes in the bedrock of the river
  • these form when small pieces of sediment become trapped in naturally occurring depressions in the riverbed.
  • due to the helicoidal flow of the river these particles are swirled round in the hole and abrasion occurs.
  • Abrasion=when particles erode the rock by banging into it and hitting weaknesses in the rock-causing it to break up.
  • abrasion causes the hole to become deeper and wider
  • this means that larger particles can now be trapped in the hole, and increase the the abrasive power of the water.
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Rapids

  • landform of erosion
  • areas of high water turbulance
  • these occur where there is a localised increase in gradient, or more commonly over alternating bands of more and less resistant (hard and soft) rock.
  • the water is able to cause vertical erosion in areas over softer rock and does so by hydraulic action and abrasion
  • this forms deeper areas of the river called pools- where the water is of higher velocity and turbulance
  • harder rock is eroded more slowly and therefore forms shallower areas of the river called riffles- this is where the river is exposed to greater friction and therefore has a lower velocity.
  • This causes helicoidal flow as the water swings backwards and forwards around the riffles to make use of the deeper water of higher velocity.
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Waterfalls

  • landform of erosion (and rejuvenation)
  • a change in the gradient of the river
  • Casestudy: High Force, Yorkshire
  • At high force there are alternating bands of more resistant (granite) and less resistant (limestone) rock.
  • the water erodes both rock types by hydraulic action and abrasion.
  • the top layer of limestone is quickly eroded eposing the granite, which forms a shelf of cap rock that the river flows over.
  • The limestone is also eroded by the acidic peaty river
  • The river flows downwards into a plunge pool (a deep pool at the base of the waterfall).
  • The force of the water causes the underlaying limestone rock to be exploited, and eventually the granite is undercut and collapses into the plunge pool.
  • This causes the waterfall to retreat upstream
  • The eroded material is used for further abrasion in the plune pool, leading to potholes
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Meander

  • landform of erosion and deposition
  • a sinuous bend in a river
  • helicoidal flow caused by riffles and pools means that the water swings from side to side leading to lateral erosion.
  • The thalweg (fastest flow) is on the outer bank of the river, where the river is deeper and the fastest velocity.
  • this means that erosion takes place on the outer bank, the processes of hydraulic action and abrasion form a river cliff (a steep sided bank).
  • At the inner bend, the water is shallower and as a result is exposed to greater friction and therefore the water is of a lower velocity.
  • This leads to deposition forming a point bar or slip off slope- gravel accumulation.
  • Erosion of the outer bank continues to provide material for deposition at the inner bank.
  • this means that the cross profile of the river at this point is asymmetrical- gentle sloping at the inner bank, and steep sided at the outer bank.
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Oxbow Lakes

  • these occur when the neck of the river becomes narrow and eventually cuts through so that the two ends are joined, returning the river to its orginal course.
  • This means that the river stops flowing around the meander bend.
  • This increases deposition at the bend due to much lower velocities.
  • This disused part of the meander becomes cut off from the main channel forming an oxbow lake- an area of stagnant water, which is gradually silted up by continued deposition.
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Comments

James Tilley

abit basic........but useful :D

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