River Landscapes (Edexcel GCSE Geography B)

River Long Profile & Cross Profiles

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

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

River Processes

• 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) 

• 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

• 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 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 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 leads 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

• 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

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

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

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

Storm Hydrographs

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

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