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
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
Exam Tip
Remember valley shape and river shape are not the same thing. If you are asked to describe valley shape you should focus on the gradient and shape of the land either side of the river channel.
- 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