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Explain the formation of a waterfall
- A waterfall is formed when harder rock is on top of soft rock
- Softer rock is eroded quicker than the hard rock by the force of the running water
- This is called differential erosion
- Eventually the softer rock is eroded away and the hard rock is left overhanging
- Eventualy the hard rock collapses as there is nothing supporting it
- This rock gets swirled around by the river and erodes a plunge pool
- This process is repeated over a long period of time causing the waterfall to gradually retreat upstream leaving a steep sided gorge
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Explain the formation of a meander
- Alternating series of irregularities develop on the river bed- This starts off the meander
- This causes the river to develop a winding or sinuous course
- Slower areas are found in deeper parts of the river filled with fine sediments and are called pools.
- Faster areas are found in shallower parts of the river around larger stones and are called riffles.
- Faster flow on outerbend results in erosion and formation of river cliff- more energy
- Slower flow on inside bend results in deposition and formation of slip off slopes- less energy
- Helicolidal flow further assists meander formation and transports sediment from river cliff to the slip off slope on the inside of the next bend
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Explain the formation of an Oxbow lake
- lateral erosion on outside of bends of pronounced meanders
- Narrow neck of meander gradually becomes narrower due to erosion on outer bends
- Neck is cut through by river during floods and river forms new straighter channel
- Cutt-off is sealed by deposition
- Ox-bow lake begins to silt and evaporate. Over time it will completely dry up
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Explain the formation of a levee
- When a river floods and it's water spills over onto the flood plain, friction increases, velocity decreases and the river begins to deposit it's load
- The coarsest material (sand and gravel) is deposited at the channel edge and form natural ridges or levees, which increase in height each time the river floods
- The finest alluvium is carried further
- The gradual accumulation of alluvial bed-load causes river-level to rise slowly
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Explain the formation of a river terrace
- River terraces are examples or rejuvenated landforms
- The terraces are portions of a former floodplain, formed before rejuvenation, and they flank either side of the present floodplain
- Rejuvenation occurs when:
- Base level(sea level is lowered)
- Erosional processes are initiated again and the river regrades its bed to the new case level
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