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The Middle and Lower Course of a River


What are the main processes that operate in the middle and lower course of a river?What landforms are found in the middle and lower course of a river?How are these landforms created?

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What are the main processes that operate in the middle and lower course of a river?

Erosion is still an important process.

The river is now flowing over flatter land and so the dominant direction of erosion is lateral (from side to side).

The river has a greater discharge and so has more energy to transport material. Material that is transported by a river is called its load.

Deposition is also an important process and occurs when the velocity of the river decreases or if the discharge falls due to a dry spell of weather.

Why do you think that a river in the middle and lower course has a higher discharge than near the source?


How is material transported downstream?



Saltation

Solution

Traction

Suspension

Boulders and pebbles are rolled along the river bed at times of high discharge.

Sand sized particles are bounced along the river bed by the flow of water.

Fine clay and sand particles are carried along within the water even at low discharges.

Some minerals dissolve in water such as calcium carbonate. This requires very little energy.


Do you know the difference between erosion and transport?


The River Conwy has deposited material in this section of its course. Suggest reasons why this has happened.

Deposition



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What landforms are found in the middle and lower course?

1) Meanders

2) Oxbow Lakes

3) Flood plains and Leveés

4) Deltas

DEPOSITION created these landforms

DEPOSITION and EROSION create these landforms



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Meander = a bend in a river


Water is pushed to the outer bend . This reduces friction with the bed and banks. So the river has more energy for transporting material which can erode the outside bank via abrasion.

Meanders

As the course of a river approaches its middle stages it flows over flatter land. Lateral erosion dominates as the river swings in large bends known as meanders. Meanders constantly change their shape and position.


Explain the following hypotheses:

The fastest current is always on the outer bend.

All meanders have river cliffs and slip-off slopes.

Meanders


A cross section through a meander


Meander bend on the River Conwy

A B

Explain why there is more deposition at ‘A’ rather than at ‘B’.



Oxbow lakes


Meander neck becomes smaller

new course of the river

oxbow lake

When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.

Explain the formation of an oxbow lake


Explain the formation of an oxbow lake


Floodplains


Floodplains and leveés are formed by deposition in times of river flood. The river’s load is composed of different sized particles. When a river floods it deposits the heaviest of these particles first. The larger particles, often pebble-sized, form the leveés. The sands, silts and clays are similarly sorted with the sands being deposited next, then the silts and finally the lightest clays. Every time the river floods deposition builds up the floodplain.

Floodplain formation


This is a cross section of a floodplain.

Draw a simple sketch of the diagram and annotate with the following labels:

leveés clays and siltssands

Floodplain formation


Deltas are found at the mouth of a river, where the river meets the sea. At this point the river is carrying too much load for its velocity and so deposition occurs. The top of the delta is a fairly flat surface. This is where the coarsest river load is dropped. The finer particles are carried into deeper water. The silt is dropped to form a steep slope on the edge of the delta while the clay stays in suspension until it reaches the deeper water.

Delta formation


Delta formation


The ‘Grand Rhône’

The mouth of the Rhône


The area of the Rhône’s delta is known as the Camargue. It is an example of an arcuate or fan delta. It has been created because of the vast amounts of sediment deposited over the years and the low tidal range of the Mediterranean.

The Rhône’s delta – the Camargue

Draw and label a diagram of the Rhône delta and explain its formation.


Name the river landform shown in this aerial photograph.

What else can you identify?

How can you tell that this is not the upper course of a river?

Techniques – interpreting aerial photographs


Examination question

A

B

C

Study the photograph below:

Name the 3 features labelled A, B and C (3)

Choose one of the features and explain its formation. You may use diagrams to help you.(6)


The landforms of a river

Which landforms are missing? Why?


Landforms are primarily caused by: processes of erosion processes of deposition a mixture of erosion and deposition

deltas meanders floodplains

waterfalls leveésoxbow lakes

rapidsv-shaped valleys interlocking spurs

Which of the following landforms are the result of a mixture of erosion and deposition?

The landforms of a river


How much do you know about rivers?


Key ideas

Processes of erosion, transport and deposition operate to create the landforms of the middle and lower course of a river.

There are four types of transport: traction, saltation, suspension and solution.

The main direction of erosion is lateral.

Erosion and deposition contribute to the formation of meanders and ox-bow lakes.

Deposition is the main process contributing to the formation of flood plains, leveés and deltas.

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Documents

river slide

meander slide

deposition slide

river bed

river conwy

river swings

river cliffs

river oxbow lake