AQA AS Geography Rivers: River Characteristics

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  • Created on: 22-08-13 20:10
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V-Shaped Valleys
V-shaped valleys are formed in the upper course of a river.
Water flows quickly through a narrow channel with a steep gradient.
As it does so, it cuts downwards into the land ­ vertical erosion.
Formation of a V-shape Valley
1. Vertical erosion ­ in the form of abrasion, hydraulic action and solution ­ in the river
channel results in the formation of a steep sided valley
2. Over time the sides of this valley are weakened by weathering processes and
continues vertical erosion at the base of the valley
3. Gradual mass movement of materials occurs down the valley sides, creating the
distinctive V shape
4. This material is then gradually transported away by the river when there is enough
energy to do so
As the river flows through the valley it is forced to swing from side to side around more
resistant rock outcrops ­ spurs.
As there is little energy for lateral erosion, the river continues to cut down vertically, flowing
between spurs of higher land creating interlocking spurs.

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Waterfalls
A waterfall is a sudden fall of water that occurs where there are rapid changes in gradient
in the rivers course.
They are most commonly found where there are marked changes of geology in the river
valleys ­ where hard rock meets softer rock.
The soft rock is eroded more than the hard rock causing a step in the river bed.
The water flowing over the step speeds up due to the lack of friction as it drops over the
step.…read more

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Rapids
Rapids are areas along the rivers course where water becomes more turbulent.
It is caused by a localised increase in gradient along the rivers gradient or where the river
flows over alternating bands of harder and softer rocks.
These are often linked in with pool and riffle sequences - the rapids form the riffles.
The pools are areas of deeper water whereas the riffles are areas of shallower water.…read more

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Potholes
Potholes are round to oval shaped holes in the bedrock of a river bed.
They are created where sediment accumulates within naturally occurring small depressions
on the rock surface of the river bed.
Turbulent flow swirls the stones around in the depressions, widening and deepening them
through the prolonged process of abrasion and corrasion.
As the holes get bigger, even bigger debris can become trapped in the pot hole, and this
material is again used as an abrasive tool.…read more

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Meanders
Meanders are large, sweeping curves in a river's middle and lower cours e.
They're formed by erosion and deposition.
Meanders form where alternating pools and riffles develop at equally spaced intervals
along a stretch of river.
Because the river channel is deeper in pools it's more efficient, so it has greater energy and
more erosive power.
Energy is lost as the river flows over riffles because of friction.…read more

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The combined process also creates the meanders' distinctive asymmetric cross-section.…read more

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Oxbow Lakes
Oxbow lakes are formed when the neck of the loop of a meander is broken through, often
during flooding.
It is formed when a meander becomes very curved and eventually the neck will touch the
opposite side and the river will cut through the neck, cutting it off.
Deposition dams off the loop leaving an Oxbow lake.
As the water in the Oxbow lake isn't replenished as often, it dries up easily.…read more

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Floodplains
When a river is at bankfull stage they may spill over onto relatively flat adjacent land
known as floodplains.
There is an increase in wetted perimeter and reduction in hydraulic radius which results in
more frictional contact and lower velocities than water in the main channel.
These factors combine to deposit fine sediment over the floodplain.
As rivers migrate across the floodplain over time, they may leave pointbars on the inside of
meander bends, adding to the extent of the floodplain.…read more

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Levées
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 the material builds up on the river bank, creating a levee.…read more

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