River Landforms

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Waterfalls

1. Water flows over two layers of rock; soft shale and hard limestone. It erodes the softer rock through abrasion and HA faster than the hard rock

2. Bed of river gets steeper- flow of water now steep enough to be a waterfall

3. Water continues to fall against a back wall, which also continues to wear away, undercutting shale leaving an overhang of hard rock

4. Enough water moving over hard rock will undercut it breaking it away

5. Hard rock then falls 100m high into a 50m deep plunge pool. The rock makes this pool much deeper through abrasion and HA

6. As more hard rock falls into the pool a gorge will form and the waterfall retreats for 11km such as the Niagra Gorge

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Pools and Riffles

Pools- Deep areas of a river with fine particles e.g silt

  • Form when surplus energy is expanded at high flow and erodes the river bed forming a pool

Riffles- Shallow areas of a river with large particles e.g gravel and and pebbles

  • Form when surplus energy is lost decreasing the river velocity and coarser sediment get deposited on the river bed
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Meanders

1. Due to an increased frictional drag caused by particles in riffles the max velocity flow swings to avoid the riffles meaning the river thalweg is concentrated on the outside bend of the river (pools).

2. This creates a side-to-side motion which produces variation across the channel starting the formation of a meander

3. Erosion is concentrated on the outside bank where the river has lots of energy moving back laterally deepening the river channel and undercutting the bank producing a steep sided river cliff

4. Deposition is concentrated on the inside bank where there is lower velocities so the river loses energy and deposits its material forming point bar deposits and a slip off slope

5. As both these processes continue the river becomes more sinuous as it moves across the flooplain

  • Meanders migrate downstream as fastest flow does not follow the precise shape of the channel and thus point of max erosion is downstream of halfway point of meander bend
  • Mississippi River has lots of meanders
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Oxbow Lakes

1. As sinuosity increases with erosion on outside and deposition on inside causes a gap within the meander to become narrower

2. During high discharge river floods onto its floodplain and breaks through the gap producing a straight channel and so the fastest flow of the river is in the middle with deposition at the edges

3. Over time deposition builds up to cut off the meander creating a oxbow lake with meanders scars

  • Mississippi River has many oxbow lakes
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Floodplains

Floodplain= Large, flat areas around  lowland river characterised by lots of deposition which comes from two places

1. As meanders weave laterally they deposit point bars on inside bend as they migrate leaving alluvium all over floodplain. Main source of deposits on floodplain

2. Flooding causing Levees- see next card

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Levees

1. When a river floods it loses its energy and deposits much heavier particles such as gravel closer to the channel due to them being heavier as well as an increase in the wetted perimeter

2. Lighter materials such as silt is carried further downstream as it is much lighter and is deposited

3. Overtime more and more sediement will be deposited in layers after a flood forming natural embankments known as levees

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Deltas

  • Depositional features extending from the mouth into the sea or lake

1. As sediment leaves the river's mouth if deposits it in three layers: topset beds, foreset beds and bottomset beds

2. As river carries its load into the sea the energy levels drop and heavier sands and silts are deposited first- topset while lighter ones are carried further out- foreset while the lighest material is carried the furthest out- bottomset

Arcuate Delta

  • Grow out from coastline with a convex outer edge. Depositon is in a triangular shape as the mouth is blocked forcing river into distributaires at angles to original. Distributaires weave back and forth depsoiting sediment which gives fan shape. Waves help smooth the fan shape 

Bird's Foot Delta

  • Happens when river dominated and less erosion. Deposition is more haphazard and delta can look like a bird's foot
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Delta Formation Factors

Rate of Deposition is greater than Erosion- Deposition rates are higher when river has lots of sediment. Erosion rates are lower when marine environment has a smaller tidal range and weaker currents

Flocculation- When freshwater mixes with saltwater, chemical reactions with salt cause clay particles to coagulate as they have a negative charge whereas salt is neutral and so their weight increases and in turn increases depositional rates

Gentle Sea Floor Gradient- Increases deposition and aids delta formation

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