Landforms Of Fluvial Erosion And Deposition

  • Created by: Helena26
  • Created on: 02-02-13 23:52
What are potholes?
Potholes are round to oval shaped holes in the bedrock of a river bed.
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How are potholes created?
They are created where sediment accumulates within naturally occurring small depressions on the rock surface on the river bed.
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How are potholes widened and deepened?
Turbulent flow swirls the stones around in the depressions, widening and deepening them through the prolonged process of abrasion.
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What happens as the holes get bigger?
As the holes gets bigger, even bigger debris can become trapped in the pothole, and this material is again used as an abrasive tool.
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What are rapids?
Rapids are areas along the rivers course where water becomes more turbulent (often creating white water).
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How is it caused?
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.
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What are they often linked in with?
These are often linked in with pool and riffle sequences (the rapids form the riffles).
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What are pools?
The pools are areas of deeper water whereas the riffles are areas of shallower water. The pool is an area of greater erosion as the water is deeper and therefore flows faster, whereas the riffles encourage deposition because they are shallower.
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What is thought within these features?
It is thought that it is within these features that meanders start to develop from straight channels, as the water is forced to move from side to side around deposited obstacles.
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What are waterfalls?
Waterfalls are one of the most spectacular land features created by rivers and area created by vertical erosion over bands of varying rock resistances.
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Where are the normally found?
They are typical of the upper valley, but can be found in the rivers lower courses where the process of REJUVENATION creates enough potential energy for vertical erosion to recommence closer to the rivers mouth.
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Waterfall is a sudden step in what?
A waterfall is a sudden step in a rivers long profile.
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What is it a result of?
It is often the result of a tougher more resistant band of rock cutting across the valley.
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How is a step formed?
Unable to erode the rock at the same rate as the neighbouring rocks, a step is formed and a waterfall results.
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What happens overtime?
Over time, the river cuts backwards into the resistant rock causing the waterfall to retreat backwards forming a narrow, steep-sided gorge.
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What are gorges?
Gorges are also deep, narrow and steep-sided opening between upland areas, usually containing a river.
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When are gorges formed?
Gorges are formed when very powerful rivers cut across very resistant rock.
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How are gorges formed?
Coastal limestone planes crossed by a river, which brings rain water and melted snow down from the mountains. The fault or crack is breaking, due to geological uplift and pressure. A gorge is beginning to be formed.
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The river in its bed causes what?
The river in its bed causes immense erosion. Immense erosion by the river and rising of the stone floor continues, widening the gorge.
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What are meanders?
Menders are sudden bends in the course of a river.
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What do meandering rivers result in?
Meandering rivers result in widening of the river valley and the production of Ox-bow lakes
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When do meanders form?
Meanders form when areas of alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river.
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Where are they normally?
hey are typical of the middle and lower course of a river where vertical erosion is replaced by a sideways form of erosion called LATERAL erosion, plus deposition within the floodplain.
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What is the distance between pools usually and why is this?
The distance between pools is usually 5-6 times the width of the river bed. Because water is deeper in pools, the river is more efficient when passing over them.
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The energy and erosive power is then?
The energy and erosive power is therefore increased when passing over these areas. On the other hand, the river is less efficient when passing over riffles as there is more friction causing the river to lose energy.
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This combination of the river gaining and losing efficiency causes what?
This combination of the river gaining and losing efficiency at different intervals causes the river’s flow to become uneven, and maximum flow is concentrated on one side of the river. As the water speeds up, turbulence increases in and around pools.
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What are helicoidal currents?
This leads to corkscrew-like currents in the river called helicoidal flow. These helicoidal currents spiral from bank to bank, causing more lateral erosion (abrasion and hydraulic action) and deepening of the pools – river cliff.
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What does this lead to?
This leads to the increased amount of eroded material being deposited on the inside of the next bend where the river loses energy – slip off slope.
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What does the combination of erosion and deposition do?
Combination of erosion and deposition exaggerates bend until large meanders are formed. These combined processes also give the meander’s asymmetric cross-section.
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Why to oxbow lakes form?
Oxbow lakes form because in meanders erosion is concentrated on the outer bend while deposition is predominant on the inner bend leading to the formation of point bars.
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What do alternative zones of erosion and deposition cause?
The alternating zones of erosion and deposition cause meanders to migrate both across and down the valley. In addition meanders become more exaggerated and sinous.
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What happens as opposite bends erode towards each other?
As opposite bends erode towards each other, the neck of a meander will get progressively narrower until, during a period of high discharge, the river will cut through forming an oxbow lake.
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What is braiding?
Braiding is when a river subdivides into smaller streams.
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What happens in between these channels?
In between these channels small islands of deposited sediment will form.
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Where does braiding occur?
Braiding occurs where there is a sudden decrease in energy causing deposition of large amounts of sediments making the main channel to subdivide.
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What is floodplains?
A floodplain is a flat area of land bordering a river that is subjected to periodic flooding.
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What are floodplains made up of?
It is made up of silts and sands which have been deposited over many years.
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What happens with the velocity?
As a river floods its velocity is dramatically decreased causing energy to be reduced and a bulk of sediment to be floating in a thin sheet of water on the floodplain. Once the water has evaporated a fresh layer of alluvium is left behind.
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What are leeves?
Levees are natural embankments of sediment formed when the river floods.
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What are leeves popular for?
They are popular for farming and (unfortunately) building for towns and settlements as they are flat features that can have back swamps.
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How are leeves created?
The river in the middle reaches erodes laterally, from side to side, because of the fastest velocities being on the outside edge of meanders.
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What does this mean?
This means that the river erodes the valley sides and widens the valley floor to create a floodplain.
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Why does the wetted perimeter increase?
In addition, every time the river floods sediment is deposited as a layer on the floodplain because the river’s wetted perimeter increases so its hydraulic radius decreases.
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Where is the largest size sediment deposited?
Where the river is deepest, next to the river channel, the largest size sediment is deposited (as seen on the Hjulstrom curve) and towards the edge of the floodplain where the river is shallowest the smallest sediment is deposited.
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What is the net result?
Flooding occurs repeatedly over time and these deposition processes occur again and again. The net result is a multilayered floodplain with large banks of sediment called levees next to the river channel.
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What are deltas?
Deltas are depositional landforms that are created from the loading of sediment onto the land as the rivers capacity to carry that sediment is reduced.
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When are deltas formed?
Delta is formed when a river enters a sea or lake loosing velocity and energy causing a large amount of sediment to be deposited as a delta
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Why do deltas change rapidly?
They are dynamic areas that change rapidly due to continual recreation of land or the erosion of unstable island and land during storm and flood events.
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Why do a large percentage of the world's population live on these?
A large percentage of the world's population live on these landforms, due to their high agricultural productivity and proximity to ocean resources.
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Other cards in this set

Card 2


How are potholes created?


They are created where sediment accumulates within naturally occurring small depressions on the rock surface on the river bed.

Card 3


How are potholes widened and deepened?


Preview of the front of card 3

Card 4


What happens as the holes get bigger?


Preview of the front of card 4

Card 5


What are rapids?


Preview of the front of card 5
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