River Landscapes (Edexcel GCSE Geography: B (1GB0))

River characteristics

• All rivers have long and cross profiles
• Each river's long and cross profiles are unique but they do have common characteristics 
• 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 where the river is eroding vertically 
  • In the middle course the gradient decreases and most of the erosion is lateral 
  • In the lower section the gradient decreases further until it becomes almost flat

Diagram of a typical 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

• 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

• 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

• Erosion is the wearing away of surfaces 
• There are four erosion processes which change the shape of the river channel:
  
  • Hydraulic action
  • Abrasion 
  • Attrition
  • Corrosion (solution) 

Processes of erosion

• 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

• There are four processes of transportation:
  • Traction 
  • Saltation
  • Suspension
  • Solution

Transportation processes

• Deposition is when a river does not have enough energy to carry its material and it drops it
• The causes include:
  • Reduced discharge due to a lack of precipitation or abstraction upstream
  • Decreased gradient
  • Slower flow on the inside of a river bend or where the river is shallower 
  • When the river enters a sea/ocean or lake as the gradient is more gentle and the river flow is affected by tides
• The heaviest material is deposited first, this is known as the bedload
• Larger rocks are only moved short distances before being deposited 
  • This happens at times of high discharge and in the upper section of the river
• The lighter materials, gravel, sand and silt are known as alluvium and they are carried further downstream
• The dissolved materials are carried out to sea

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

Meander cross-section

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

Ox-bow 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

Deltas

• Deltas form at the mouth of rivers - for example the Nile Delta
• As the river becomes tidal the speed of the river decreases
• The decrease in velocity leads to more sediment being deposited which increases friction and further increases deposition
• Over time the build up of sediment rises above the level of water and a new area of land is formed - this is a delta
• The river splits and forms streams going through the new area of land - these are distributaries

Flood prediction

• Prediction of flooding means that steps can be taken to manage flooding
• Hydrographs can be used to understand the risk of flooding because it shows the changes in discharge which result from a rainfall event
• A flood hydrograph shows the changes in river discharge after a storm event
• The graph shows a short period of time, usually 24 hours
• The flood hydrograph has a number of features:
  • Base flow
  • Peak rainfall
  • Rising limb
  • Peak discharge
  • Lag time
  • Recessional limb or falling limb

Flood hydrograph in an urban area 

Increased flood risk

• Factors which increase surface run off or overland flow lead to:
  • Short lag time
  • Steep rising limb
  • High discharge
• As a result the river may not have the capacity to contain the water and so flood risk is higher

Low flood risk

• Factors which cause lower surface run off or overland lead to;
  • Longer lag time
  • Gentle rising limb
  • Lower discharge
• As a result the river is more able to cope with the water entering the channel and the flood risk is lower

 Physical Factors which Increase the Risk of Flooding