River Landforms (OCR GCSE Geography B)

River Landforms - Erosion

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 

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

River Landforms - Deposition

• 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