River Landforms: Erosion & Deposition (Edexcel GCSE Geography A)

River Landforms - Erosion

• 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

• These are landforms created through erosion

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

Waterfall formation

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

River Landforms - Interaction of Erosion & Deposition

• Some landforms are created through a combination of erosion and deposition, including:

  • Meanders

  • Ox-bow lakes

Meanders 

• In lowland areas, 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 sloping bank is a slip-off slope and the deposits are called a point bar

• 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 increase

• The erosion on outside bends can eventually lead to the formation of a meander neck

• At the 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

River Landforms - Deposition

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