Edexcel IGCSE Geography 复习笔记 1.2.3 River Landscapes

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 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:
  • 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 river bank forming a river cliff
  • The river bank 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 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 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

Floodplain and Levee Formation