2.2.4 River Landforms

River Landforms

River Landforms

River landscape characteristics

• 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 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

Lowland features:

• Meanders

• Ox-bow lakes

• Floodplains

• Levees

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 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 increase

• 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