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Landforms of fluvial erosion and deposition.
Potholes Rapids Waterfalls Braiding Meanders Deltas Floodplains Levees
How Deltas are created

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.  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. A large percentage of the world's population live on these landforms, due to their high agricultural productivity and proximity to ocean resources. Great examples can be found at the Ganges delta in Bangladesh, the Nile Delta in Egypt and the Delta of the Lena river which flows through the Central Siberian plateau.

The Ganges river delta

The process of formation is as follows;

1.     As rivers near the sea they contain high sediment loads and the rivers start to spread out laterally. Ocean water entering these streams can reduce velocities as well.

2.     This lateral movement of water reduces hydraulic radius and increases wetted perimeter.

3.     This causes sediment to be deposited, as does flocculation where clay sediments join together, gain in mass and sink.

4.     This sedimentation builds up over time and can create small islands which split the channel, similar to braided streams.

5.     This can happen again and again until the river consists of a number of smaller streams separated by islands.

6.     If sediment is coarse grained arcuate deltas form.  If it is fine grained then birds foot deltas can be created (e.g. Mississippi).

7. Sediment is often deposited on the sea ward side, which builds the delta outwards into the sea.  These areas are called fore beds, and are highly unstable.  They often collapse and cause mass movements within the sea and clouds of sediment known as turbidity currents.

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The location of High Force
High Force Geological diagram
High Force

Waterfall – High Force on the River Tees

Waterfalls are one of the most spectacular land features created by rivers and area created by vertical erosion over bands of varying rock resistances.  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.  In this example at High Force on the River Tees in the North of England tough igneous intrusion rock Whin Sill (granite) overlies weaker Limestone which has bedding planes and joints. 

1.     The brown peaty waters of the River Tees flow over both rocks and at some times of the year the river does flood.

2.     As the water flows over the rocks it will erode both by hydraulic action, the sheer force of the water flowing over both.

3.     Stones carried in the water will also bash against the bed and abrade it.

4.     The limestone will also be attacked by the acidic peaty waters of the Tees.

5.     The net result is that the weaker Limestone is eroded at a faster rate than the Whin sill leaving a steep gradient/cliff that the water falls down. This is exacerbated by the fact that the limestone is full of joints and bedding planes.

6.     As erosion continues rocks and water further erode the plunge pool, with water swirling backwards and rocks bashing into the limestone.

7.     The harder Whin Sill is eventually undercut and some collapses into the plunge pool, with the waterfall moving upstream leaving a gorge. 

8.     The Whin Sill that has fallen into the plunge pool can then be used as abrasive tools to further erode the limestone. It may even create potholes.

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Meanders and Ox Bow lakes – e.g. River Till near Wooler

Meanders!

Meandering rivers result in widening of the river valley and the production of Ox-bow lakes.  They 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. 

They form by the following process;

  1. The sequence starts with a pool riffle sequence. 

    First, even in straight channels the river will deposit sediment in alternating bars that will eventually create riffles.  This will occur when there are low flows in the river, and the low hydraulic radius (channel efficiency) is enough to encourage deposition.  Once created, the riffle will continue to lower the hydraulic radius for the area and water is seen to flow even more inefficiently over it.

    Second, water needs to find a way around these areas of higher frictional contact so it flows around them.  This creates the variations in flow and introduces a side to side motion to the water; this is effectively the start of meander development.  Between these shallow riffle areas deeper areas called pools are eroded, mainly at times of higher discharge, so a series of pools and riffles develop over time. 

    It is thought that during times of flood a corkscrew motion of water develops between the pools called Helicoidal flow, which moves material from the outside of one bank of a pool and moves it to the inside bend of the next pool. Next , at times of higher flow the water will swing around one side of a riffle and this will undercut the opposite bank through erosion processes because of centripetal force (it is swung outwards).

     

  2. As the water enters the meander it is faster on its outside edge because it is deeper (it has a larger hydraulic radius) so there is less friction acting upon the water. 
  3. On the inside bend of the meander it is shallower (it has a smaller hydraulic radius) so more friction from the rivers bed and bedload act upon the water, slowing it down.
  4. On the outside of the bend a steep river cliff or bluff forms where the processes of Hydraulic action and abrasion get to work eroding the outside of the bend.
  5. As the velocity slows on the inside of the bend critical deposition velocity is often reached and deposition occurs. This creates a point bar or slip off slope.
  6. This Helicoidal flow of the river causes the river to move laterally across the flood plain.
  7. If the meander neck gets really narrow it can eventually cut through, where the 2 outside edges of the meander edge closer together until the meander bed is cut through during a period of high flow.
  8. The result is an abandoned ox bow lake, which will slowly fill with sediment and aquatic plants.  Reeds will invade and create soil and add vegetative matter, and these processes will fill in the meander.
  9. A good example of this is on the River Till, which the River Breamish flows into.


Diagram A - cross profile of a meander

 

Through this constant side ways erosion and deposition across the floodplain valley sides can be eroded and the meandering nature of the river certainly helps to contribute to the development of floodplains. See a model meander formation  answer, which you can annotate.

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Floodplains and levees the River Till, North of Wooller.

These landforms are very closely linked to meanders and ox-bow lakes and are combined erosion and deposition landforms.  they are popular for farming and (unfortunately) building for towns and settlements as they are flat features that can have back swamps.  They are created by;

  1. The river in the middle reaches erodes laterally, from side to side, because of the fastest velocities being on the outside edge of meanders.
  2. This means that the river erodes the valley sides and widens the valley floor to create a floodplain.
  3. 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. 
  4. 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.
  5. Flooding occurs repeatedly over time and these deposition processes occur again and again.
  6. The net result is a multilayered floodplain with large banks of sediment called levees next to the river channel. 
  7. The Mississippi River is a good example of a river with Levees while the River Till North of Wooller has a classic floodplain.

 

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Braided riversFranz Joseph River, South Island, New Zealand

Braiding on the Franz Joseph River

Braided rivers are basically rivers that have multiple channels and islands of sediment in between those channels.  We find braided rivers in deltas, in areas where the river channel banks are made of easily erodible material or in areas of high sediment load where discharge varies (e.g. glacial melt water rivers).

  1. The Franz Joseph River drains the melt waters of the Franz Joseph Glacier on the West coast of New Zealand’s South Island. 
  2. The discharge is highly variable because there is more melt water during daytime melting than there is at night (when the temperature is colder and freezing occurs), and also because of seasonal differences in temperature.
  3. When the discharge is high the river can carry a lot of the glacially eroded till away from the glacier downstream.  However, when the river spreads out laterally or when the discharge falls sediment is deposited as the river has a lower Hydraulic radius.
  4. This sediment can build up into islands that split the river into multiple channels.
  5. These islands can become colonised and stabilised by vegetation, but more often than not they get eroded and the shape of the river channel is constantly shifting.

 

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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 on the river bed.  Turbulent flow swirls the stones (called grinders) around in the depressions, widening and deepening them through the prolonged process of abrasion.  As the holes gets bigger even bigger debris can become trapped in the pothole, and this material is again used as an abrasive tool.

Bourkes luck potholes

Rapids

Rapids are areas along the rivers course where water becomes more turbulent (often creating white water which canoeists and rafters love so much!) .  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. 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.  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.


   
 
Find out more

Try the Venn diagram activity below

Make flashcards of all of the landforms
 
 

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