River Landforms

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  • Created by: yott33
  • Created on: 02-01-16 16:48

Waterfalls

Waterfalls are caused by fluvial erosion.

  • Waterfalls form where a band of hard rock meets soft rock.
  • The soft rock is eroded faster than the hard rock, causing a 'step' in the river bed
  • The water flowing over the step speeds up due to a lack of friction as it drops over the step. This increase in speed gives the water greater erosive power.
  • The harder rock is undercut, and gradually starts to collapse. A plunge pool is formed through abrasion at the foot of the waterfall.
  • The waterfall gradually continues to retreat as more undercutting causes more collapse.
  • A steep-sided gorge is formed due to this process
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Potholes and Rapids

Potholes

Potholes are small circular hollows in the river bed.

  • Formed by abrasion as turbulence swirls a river's bedload round in a circular motion, causing it to rub and scrape out holes

Rapids

Rapids are relatively steep sections or river with turbulent flow where there are several sections of hard rock.

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Meanders

Meanders are large, sweeping curves in a river's middle and lower stages formed by erosion and deposition

  • Formed where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is 5-6 times the width of the river bed
  • Because the river channel is deeper in pools it's more efficient - it has greater energy and more erosive power
  • Energy is lost as the river flows over a riffle due to friction
  • The spacing and distance between riffles and pools cause the river's flow to become unevenmaximum flow is concentrated on one side of the river
  • Turbulence increases in and around pools as the water speeds up, so the flow of water begins to twist and coil
  • This causes the helicoidal flow to occur, which spiral from bank to bank between pools
  • The helicoidal flow causes more erosion and deepening of the pools
  • The helicoidal flow causes eroded material to be deposited on the inside of the next bend, where the river loses energy.
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Braiding and Flood Plains

Braiding

Braiding occurs when rivers are carrying a vast amount of eroded sediment.

  • If velocity drops or if the sediment load becomes too much for the river to carry, sediment is deposited in the channel
  • This causes the river to divide into many small, winding channels that eventually rejoin to form a single channel

Flood Plains

Flood plains occur where the river has overflowed its banks and floods the flat land either side of the river.

  • This means there is an increase in the wetted perimeter and a reduction in hydraulic radius
  • Friction is increased, reducing the velocity of the river and causing fine silt and sand to be deposited across the flood plain
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Levees and Deltas

Levees

Levees are natural, raised embankments formed as a river overflows its banks.

  • During a flood, material is deposited across the whole flood plain as the river loses velocity and energy due to increased friction
  • The heaviest material e.g. sand and gravel is dropped first, closest to the river channel
  • Over time this material builds up on the river bank, creating a levee.

Deltas

A delta occurs where the river has to braid into several distributaries in order to reach the sea

  • When a river reaches the sea or a lake, th energy of the river is absorbed by the slower moving water of the sea or lake
  • This causes the river to deposit its load - deposits build up on the sea bed until the alluvium (deposited sediment) rises above sea level, partially blocking the river mouth.
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