Unit 1.3B_4 River Landforms

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Unit 1.3B_4River Landforms
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Now you know a bout the processes that enable the
river to change

• What does the river look like?

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In the Upper Course

• You see V-shaped valleys, interlocking spurs,
  waterfalls, gorges and rapids.
• This because the erosion takes place downwards
  only - Vertical erosion
• This mainly by abrasion and hydraulic action,
  making valley deeper
• Most of the transport is by traction and
  saltation.
• As the main work is vertical erosion, the water
  winds it way around obstacles, weaving from side
  to side the point on the valley side ahead is
  worn away, increasing the cut into the valley
  side hense V-shaped valley with interlocking
  spur

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RAPIDS
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In summary
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Fast flowing water has more energy
River cliff forms
Slip-off slope
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Slow, shallow water and deposition on the inside
of the bend
Deep, fast water and erosion on the outside of
the bend
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• As the outer banks of a meander continue to be
  eroded through processes such as hydraulic action
  the neck of the meander becomes narrow and
  narrower.

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Oxbow lakes

• Eventually due to the narrowing of the neck, the
  two outer bends meet and the river cuts through
  the neck of the meander. The water now takes its
  shortest route rather than flowing around the
  bend.
• Deposition gradually seals off the old meander
  bend forming a new straighter river channel.
• Due to deposition the old meander bend is left
  isolated from the main channel as an ox-bow lake.
• Over time this feature may fill up with sediment
  and may gradually dry up (except for periods of
  heavy rain). When the water dries up, the feature
  left behind is known as a meander scar.

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Levees (thanks to Mark Bailey)
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What are they?
Often several metres higher than the rivers
floodplain, they are high natural embankments of
silt situated close to the river channel.
Where are they found?
They are most commonly found in the lower course
of a river where there is a floodplain, along
rivers that flood periodically, have a slow flow
and carry large amounts of load.
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How are they formed?
The river before the flood at regular height.
The river whilst flooding
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There is an increase in friction between the
water leaving the channel and the floodplain when
a river overflows. The water is shallower on the
river banks and valley floor, thus this is why
deposition of load occurs. After repeated
flooding causing the deposition of silt onto the
banks, this eventually settles which over time
builds up to form a levee. The coarser material
is deposited first, and is positioned closer to
the river.
After repeated flooding, the river bed and levees
are raised due to the constant process of the
deposition of silt. Over time, this eventually
causes the river to rise above level of flood
plain. This is called an aggraded bed.
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The end of the river

• Eventually the river flows into a lake or a sea
  or an ocean.
• If the water it flows into has little current,
  any remaining silt is dropped as it enters and
  there is no current to remove it and a delta
  forms.

Mississippi Delta
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But in the UK

• There are plenty of currents to wash away any
  silt that is dropped and so our rivers have
  mouths instead.
• I google-mapped this one. Where is it?