2What 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?Learning objectives
3What 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?
5How is material transported downstream? Boulders and pebbles are rolled along the river bed at times of high discharge.TractionSand sized particles are bounced along the river bed by the flow of water.SaltationFine clay and sand particles are carried along within the water even at low discharges.SuspensionSome minerals dissolve in water such as calcium carbonate. This requires very little energy.Solution
7How is material transported downstream? The drag and drop activity follows the animation
8Do you know the difference between erosion and transport?
9DepositionThe River Conwy has deposited material in this section of its course. Suggest reasons why this has happened.
10What 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?Learning objectives
11Flood plains and Leveés What landforms are found in the middle and lower course?MeandersOxbow LakesFlood plains and LeveésDeltasDEPOSITION and EROSION create these landformsDEPOSITION created these landforms
12What 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?Learning objectives
14MeandersAs 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.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.
15Meanders Explain the following hypotheses: The fastest current is always on the outer bend.All meanders have river cliffs and slip-off slopes.
20Explain the formation of an 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.new course of the riverMeander neck becomes smalleroxbow lake
23Floodplain formationFloodplains 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.
24Floodplain formation This is a cross section of a floodplain. Draw a simple sketch of the diagram and annotate with the following labels:leveésclays and siltssands
25Delta formationDeltas 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.
28The Rhône’s delta – the Camargue 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.Draw and label a diagram of the Rhône delta and explain its formation.
29Techniques – interpreting aerial photographs 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?
30Examination question Study the photograph below: B A C Features – a key word that often comes up in examination questions.Answers expected here are A = slipoff slope B = Floodplain C = rivercliff.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)
31The landforms of a river Which landforms are missing? Why?
32The landforms of a river Landforms are primarily caused by:processes of erosionprocesses of depositiona mixture of erosion and depositionWhich of the following landforms are the result of a mixture of erosion and deposition?deltasmeandersfloodplainswaterfallsleveésoxbow lakesrapidsv-shaped valleysinterlocking spurs
34Key ideasProcesses 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.