Rivers

• River Courses


  • Upper Course
  • Middle Course
  • Lower Course
• Upper Course


  • Steep V-shaped valley
  • Narrow channel
  • High bed load
• Upper Course


  • V-shaped valleys
  • Interlocking spurs
• Upper Course


  • Waterfalls
  • Gorges
• Lower Course


  • Wider, deeper channel
  • More suspended sediment
  • Meanders
  • River cliffs
  • Slip-off slopes
  • Levees
• Middle Course


  • Gently sloping valley sides
  • Wider, deeper channel
  • More suspended sediment
  • Meanders
• Erosion Processes


  1. Hydraulic action

  2. Traction

  3. Saltation

  4. Abrasion

  5. Attrition

  6. Solution

  7. Suspension
• Waterfalls form when harder rock overlies softer rock, leading to undercutting and waterfall retreat
• Gorges form when waterfalls retreat over time, creating steep-sided valleys
• Interlocking spurs form in the upper course due to vertical erosion and lack of lateral erosion
• Meander Formation


  1. Faster current on outside of bend causes erosion

  2. Slower current on inside of bend causes deposition

  3. Slip-off slope forms

  4. Ox-bow lake forms when meander is cut off
• Levees form in the lower course due to deposition
• Meanders


  • Current is faster on the outside of the bend due to the river channel being deeper, causing more erosion and forming river cliffs
  • Current is slower on the inside of the bend due to the river channel being shallower, causing more deposition and forming slip-off slopes
• Formation of ox-bow lakes


  1. Erosion causes the outside bends to get closer until there's only a small bit of land in between the bends & neck (gradually erodes)

  2. River breaks through this land, usually during a flood

  3. Deposition eventually cuts off the meander, forming an ox-bow lake
• Floodplains


  • Flat areas of land that flood on either side of a river
  • When rivers flood, water slows down and loses energy, depositing the materials it was transporting
  • Deposition builds up the floodplain over time
  • Meanders widen as they migrate across floodplains and also migrate downstream
  • Deposition on the slip-off slopes of meanders also builds up the floodplain
• Levees


  • Natural embankments along the edges of a river channel
  • During a flood, eroded material is deposited over the whole floodplain, with the heaviest material deposited closest to the river channel as it gets dropped first when the river slows down and loses energy
  • Over time, the deposited material builds up creating levees along the edges of the channel
• Estuaries


  • Tidal areas where the river meets the sea
  • Found at river mouths, where the land is close to sea level and the river valley is widest
  • Water level rises and falls with the tide, flooding over the banks of the river and carrying silt and sand onto the valley floor
  • As the tide reaches its highest point, the water moves slowly and has little energy, so it deposits sediment
  • Over time, more mud builds up, creating large mudflats that are exposed at low tide
• Discharge


  Volume of river water flowing per second (measured in cumecs)
• Hydrograph


  Shows how the discharge at a certain point in a river changes over time in relation to rainfall
• Hydrograph


  • Shows peak discharge
  • Shows lag time - delay between peak rainfall and peak discharge
  • Shows rising limb - increase in river discharge as rainwater flows into the river
  • Shows falling limb - decrease in river discharge as the river returns to its normal level
• Lag time occurs because most rainwater doesn't land directly in the river channel, it flows quickly overland, soaks into the ground and then flows slowly underground to the channel
• Shorter lag time


  Greater risk of flooding
• Physical factors affecting flood risk

  • Heavy rainfall - water arrives too quickly to infiltrate, so there's a lot of surface runoff increasing discharge
  • Prolonged rainfall - can saturate the soil, so only further rainfall can't infiltrate increasing runoff into river channels
  • Impermeable geology - such as shales and clays encourage water to flow overland and into river channels, speeding up water flow and making flooding more likely
  • Steep-sided valleys - water reaches the river quicker because it can flow faster down the steep slopes, rapidly increasing discharge
  • Torrential rainstorms - can lead to sudden flash floods as river channels cannot contain the sheer volume of water flowing through
• Human factors affecting flood risk

  • Urbanisation - building on floodplains creates impermeable surfaces, transferring water quickly to drains, sewers and then into urban river channels
  • Deforestation - much of the water that falls on trees is evaporated/stored temporarily on leaves and branches, when trees are removed much more water is suddenly available and transferred rapidly to river channels
  • Agriculture - in arable farming, soil is left unused and exposed, leading to more surface runoff, and ploughing up and down steep slopes allows water to flow quickly along the furrows
• Dams


  Barriers built across rivers, usually in the upper course, to form a reservoir (artificial lake) that stores water, controls water flow and prevents floods downstream. Can also be used to generate hydroelectric power.
• Dams


  • Very expensive to build
  • Can flood existing settlements
  • Deposit material in the reservoir, making farmland downstream less fertile
• Channel straightening


  Removing meanders by building straighter, artificial channels so water leaves the area more quickly rather than building up, reducing flood risk
• Flooding may happen downstream instead of where the channel is straightened
• Faster-moving water may cause more erosion downstream
• Embankments


  Raised walls built along river banks
• Embankments


  • River can hold more water, so floods are less frequent
  • Quite expensive
  • Risk of severe flooding if water rises above embankment level and they break
• Flood relief channels


  1. Channels built to divert water around built-up areas or to divert excess water if the river level gets too high

  2. Gates on the channels mean the release of water can be controlled, reducing flood risk

  3. There will be increased discharge where the diversion channel rejoins the river, which could cause flooding in that area
• Soft engineering


  Schemes set up using knowledge of a river and its processes to reduce the effects of flooding
• Soft engineering measures

  • Flood warnings and preparation
  • Modifying buildings to minimise flood damage
• Flood warnings


  • Give people time to move possessions upstairs, put sandbags in place, or evacuate
  • Don't prevent floods
  • People may not have access to the warnings
  • Modifying buildings is expensive
  • Preparation doesn't guarantee safety from a flood and could give a false sense of security
• Flood plain zoning


  • Restrictions prevent building on parts of a flood plain that are likely to be affected by a flood
  • Fewer impermeable surfaces are created
  • No buildings to get damaged
  • Expansion of an urban area is limited if there aren't any other suitable building sites
  • Can't help in areas with existing buildings
• Planting trees


  • Increases the interception of rainwater (and lag time)
  • Discharge and flood risk decreases
  • Vegetation reduces soil erosion in the valley and provides habitats for wildlife
  • Less land is available for farming
• River restoration


  • Making the river more natural so the flood plain can flood naturally
  • Discharge is reduced, less risk of flooding downstream
  • Little maintenance is needed and there are better habitats for wildlife
  • Local flood risk can increase if nothing is done to prevent major flooding