River Landforms (Cambridge International IGCSE Geography)

Specification checklist

This content page covers the following section of the specification:

• 1.2.1 - The characteristics and formation of landforms: waterfalls, rapids, gorges, V-shaped valleys, meanders, oxbow lakes, levées, floodplains, deltas, interlocking spurs, potholes.

Landforms in the river upper course

Spec point — The characteristics and formation of landforms: waterfalls, rapids, gorges, V-shaped valleys, meanders, oxbow lakes, levées, floodplains, deltas, interlocking spurs, potholes.

• The changes in river channel characteristics lead to changes in the river landscape

• The upland and lowland areas of rivers have distinctive landforms

• Upland:

  • waterfalls
  • gorges
  • v-shaped valleys
  • interlocking spurs

Waterfalls and gorges

• Waterfalls form when there is a drop in the riverbed from one level to another

• This drop is often due to changes in the hardness of the rock, where hard rock overlies soft rock

• Hydraulic action and abrasion are the main erosional processes

  • The soft rock erodes quicker, undercutting the hard rock and creating a plunge pool
  • This leads to the development of an overhang of hard rock, which, eventually, over time, collapses
  • The overhang falls into the plunge pool, increasing abrasion and making the plunge pool deeper
  • The process then begins again and the waterfall retreats upstream leaving a steep-sided gorge

Waterfall formation

Rapids

• Rapids form where there are bands of alternating hard and soft rock
• The softer rock (less resistant) erodes more rapidly through abrasion and hydraulic action, leaving the harder rock jutting upwards from the river bed
• This creates an uneven riverbed

V-shaped valleys

• Vertical erosion is dominant in the upper course of the river

  • This cuts down into the riverbed and deepens the river channel

• Weathering and mass movement lead to material from the valley sides collapsing into the river, forming a steep v-shaped valley

V-shaped valley formation

Interlocking spurs

• In the upper course of the river, the channel starts to meander
• Erosion happens on the outside of the bend
• In the upland areas this forms interlocking spurs

Interlocking spurs

Potholes

• Potholes are round depressions in the riverbed

• They are formed by abrasion

• Where there are dips in the riverbed, the river flow can cause the sediment to spin

  • This erodes the dip, forming a circular hollow (pothole)

• As the size of the hollow increases, larger material becomes trapped in the pothole

  • This further increases the erosion of the pothole

River landforms in the river middle course

Spec point — The characteristics and formation of landforms: waterfalls, rapids, gorges, V-shaped valleys, meanders, oxbow lakes, levées, floodplains, deltas, interlocking spurs, potholes.

Landforms in the river's middle course

Meanders

• In the middle and lower course of a river, lateral erosion is dominant

• Meanders increase in size

• The fastest water flow (thalweg) is on the outside of the river bends, leading to erosion

  • The erosion undercuts the riverbank, forming a river cliff
  • The riverbank collapses and the edge of the meander moves further out

• The slowest flow is on the inside of the river bends, leading to deposition

  • The deposits form a slip-off slope

• Deposition on one side and erosion on the other leads to the meander migrating across the valley

Cross-section of a meander

Oxbow lakes

• With distance downstream, the size of the meanders increases
• The erosion on outside bends can eventually lead to the formation of a meander neck
• At a time of the flood, the river may cut through the neck of the meander, forming a straighter course for the water
• The flow of water at entry and exit from the meander will be slower, leading to deposition
• The meander becomes cut off from the main river channel, forming an oxbow lake

Oxbow lake formation

Exam Hint Remember when describing the formation of oxbow lakes, it is important to state that the river will break through the neck of the meander during a flood. At other times, the river does not have enough power to break through.

Landforms in the river lower course

Spec point — The characteristics and formation of landforms: waterfalls, rapids, gorges, V-shaped valleys, meanders, oxbow lakes, levées, floodplains, deltas, interlocking spurs, potholes.

Landforms in the river's lower course

Floodplains and levees

• Floodplains are flat expanses of land on either side of the river
• The migration of meanders leads to the formation of the floodplain
• High discharge may cause the river to overflow the banks
• More of the water is in contact with the land surface as the water spreads across the floodplain
• Increased friction reduces velocity and material is deposited across the floodplain, gradually increasing the floodplain height
• The heaviest material is deposited first, nearest to the river channel, forming natural embankments called levees

Levee and floodplain formation

River mouth

• A river enters a sea or lake at its mouth

• This may be in different forms, including

  • estuaries
  • deltas
  • narrow mouth

• This is dependent on:

  • the amount of sediment the river is carrying
  • the strength of the waves/tides

Estuary

• An estuary is a wide, deep mouth
• It forms the change between the river and the sea and is where freshwater and saltwater mix

Deltas

• Rivers must carry a large amount of sediment for deltas to form

• Delta formation requires a rapid drop in river velocity

• This leads to the deposition of sediment

  • As the sediment is dropped the fine particles clump together, this is known as flocculation
  • The layers of sediment then build up blocking water flow and forming distributaries

• There are a variety of delta formations, such as arcuate and bird’s foot

Types of delta

Delta formation and types of delta