River Landforms (Cambridge (CIE) IGCSE Geography)

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 when 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

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 lead to material from the valley sides collapsing into the river, forming a steep 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

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

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

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

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

Deltas

• Deltas are formed when streams flow into standing bodies of water

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

• Delta formation requires a rapid drop in river velocity

  • Flocculation, when salt particles stick together, increases deposition

  • Bioconstruction, when vegetation slows the river, increases deposition

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

Types of delta