River Landforms (OCR GCSE Geography)

River characteristics

• All rivers have a long and cross profiles
• Each river's long and cross profiles are unique but they do have some characteristics in common
• These profiles show changes in river characteristics from the source to the mouth

Long profile

• The long profile of a river shows the changes in the river gradient from the source to the mouth
• Most long profiles have a concave shape with similar characteristics:
  • The source is usually in an upland area
  • The upper course of the river includes areas which are steep with uneven surfaces
  • In the middle course the gradient decreases
  • In the lower section the gradient decreases further until it becomes almost flat

Diagram of a River's Long Profile 

Long Profile

Cross profiles

• The cross profiles of a river are cross-sections from one bank to another
• Cross profiles of the upper, middle and lower courses show the changes in the river channel 
• Upper course characteristics include:
  • Shallow
  • Steep valley sides 
  • Narrow
  • Low velocity
  • Large bedload
  • Rough channel bed
  • High levels of friction
  • Vertical erosion

Upper Course Profile of a River

Middle course characteristics:

• Deeper than upper course channel
• Gentle valley sides
• Wider than upper course channel
• Greater velocity than upper course channel
• Material in river decreases in size
• Smoother channel bed
• Lower levels of friction than upper course channel
• Lateral erosion

Middle Course Profile

Lower course characteristics

• Deeper than middle course channel
• Flat floodplains 
• Wider than middle course channel
• Greater velocity than the middle course channel (apart from as the river enters the mouth) 
• Material carried mainly sediment and alluvium
• Smooth channel bed
• Lowest friction
• Deposition is dominant

Lower Course Profile

• Erosion can be mainly vertical or lateral: 
  • Vertical erosion is dominant in the upper course of rivers. It increases the depth of the river and valley, as the river erodes downwards
  • Lateral erosion is dominant in the middle and lower course of rivers. It increases the width of the river and valley as it erodes sideways
• The upland and lowland areas of rivers have distinctive landforms
• Upland:
  • V-shaped valleys
  • Interlocking spurs
  • Waterfalls
  • Gorges

V-shaped valleys 

• Vertical erosion is dominant in the upper course of the river
• This cuts down into the river bed and deepens the river channel 
• Weathering and mass movement leads to material from the valley sides collapsing into the river forming a steep V-shaped valley

Formation of a V-Shaped Valley

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

Waterfalls and gorges

• Waterfalls form where there is a drop in the river bed 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

Diagram of the Formation of a Waterfall and Gorge

• Most landforms of deposition occur in the middle to lower course
• Features include:
  • Meanders
  • Ox-bow lakes
  • Floodplains
  • Levees

Meanders 

• Meanders are a feature of deposition and erosion
• In lowland areas lateral erosion is dominant as the gradient lessens and water begins to wander across the land leading to meanders (bends in the river)
• The more the water wanders the more the 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 meander 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

Floodplains and levees

• Floodplains are flat expanses of land 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