Rivers

• Drainage basin


  A major part of the hydrological system, drainage basins drain all the water which lands on the Earth's surface
• Drainage basin


  • An open system
  • Unique in shape and size, with different rock types, relief and land use
• Drainage basin features


  • Watershed
  • Source
  • Confluence
  • Tributary
  • Mouth
• Drainage density


  The numbers of tributaries in a drainage basin
• Drainage basins


  • Lots of tributaries have a high drainage density
  • Few tributaries have a low drainage density
• Erosion processes


  • Hydraulic action
  • Abrasion
  • Attrition
  • Corrosion (solution)
• Vertical erosion


  Dominant in the upper course of rivers, increases the depth of the river and valley
• Lateral erosion


  Dominant in the middle and lower course of rivers, increases the width of the river and valley
• Transportation processes


  • Traction
  • Saltation
  • Suspension
  • Solution
• Traction


  The action of pulling something over a surface
• Saltation


  Leaping or jumping
• Deposition


  When a river does not have enough energy to carry materials it drops them
• Causes of reduced energy for deposition

  • 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
• Bedload


  The heaviest material deposited first
• Alluvium


  The lighter materials, gravel, sand and silt, carried further downstream
• The dissolved materials are carried out to sea
• As a result of erosion, transportation and deposition the character of a river changes as it moves down stream
• These changes are summarised in the Bradshaw model
• Long profile


  • Shows the changes in the river gradient from the source to the mouth
  • Most have a concave shape
  • The source is usually in an upland area
  • The upper course is steep with uneven surfaces
  • The middle course gradient decreases
  • The lower section gradient decreases further until it becomes almost flat
• Upper course characteristics

  • 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
• Waterfall


  Forms where there is a drop in the river bed from one level to another, often due to changes in the hardness of the rock
• Waterfall formation


  1. Hydraulic action and abrasion erode the soft rock quicker, undercutting the hard rock and creating a plunge pool

  2. This leads to the development of an overhang of hard rock which eventually collapses

  3. The overhang falls into the plunge pool increasing abrasion and making the plunge pool deeper

  4. The process then begins again and the waterfall retreats upstream leaving a steep sided gorge
• 1. shaped valley


  Formed by vertical erosion cutting down into the river bed and deepening the river channel, with weathering and mass movement collapsing material from the valley sides
• Interlocking spurs


  Formed in the upper course of the river as it starts to meander, with erosion on the outside of the bends
• Pothole


  Round depressions in the riverbed formed by abrasion, where sediment spinning in dips in the riverbed erodes the dip into a circular hollow
• Meander


  Formed in lowland areas where lateral erosion is dominant, with the fastest flow on the outside of bends eroding the bank and the slowest flow on the inside depositing material
• Oxbow lake formation


  The size of meanders increases downstream, until the river cuts through the neck of a meander forming a straighter course, leaving the old meander cut off as an oxbow lake
• Floodplain


  Flat expanses of land either side of the river, formed by the migration of meanders
• Levee


  Natural embankments formed by the deposition of the heaviest material nearest to the river channel as the river overflows its banks
• Delta


  Formed when streams flow into standing bodies of water, with a rapid drop in stream velocity leading to deposition
• Types of delta

  • Arcuate
  • Bird's foot
• The main river hazards are flooding and erosion
• Flooding


  Occurs when the water in the river reaches bankfull discharge and then overspills across the floodplain
• Causes of flooding

  • Heavy, torrential rain leading to high levels of overland flow
  • Prolonged steady rain saturating the ground and leading to high levels of overland flow
  • Landslides
  • Snow and ice melt
  • Storm surges pushing water up the river channel
• Human activities increasing flood risk

  • Urbanisation
  • Deforestation
  • Building of bridges and dams
  • Human induced climate change
  • Agriculture
• Hazards of flooding and erosion

  • Spread of water related diseases
  • Deaths and injuries as floodplains are often densely populated
  • Damage or destruction of bridges and transport routes
  • Loss of farmland, housing and transport routes
  • Destruction of crops
  • Increased insurance costs
  • Lower house prices
• Opportunities provided by rivers

  • Silt deposited during flooding is rich in minerals and nutrients, making it ideal for growing crops
  • Source of food
  • Flat floodplains make construction and building of transport networks easier
  • Water can be used to irrigate farmland
  • Leisure and tourism
  • Generating electricity
  • Transporting goods and people
• Flood hydrograph


  Used to predict the reaction of a river discharge to a rainfall event, showing changes in discharge over a short period of time