Rivers

• Most rivers have their source in mountains and their mouth in the coast
• Tributaries - smaller streams that join a larger river at a confluence
• A long profile shows how the gradient of a river changes in it's different courses
• A cross profile shows the cross section of a river valley
• Upper course:

  • Steep gradient

  • Steep, V-shaped Valley • Narrow, shallow channel

  • Erosional landforms e.g waterfalls • Mainly larger sediment transported by traction and saltation
• Middle course:

  • Gentle gradient

  • Wider valley, floodplain

  • Wider and deeper channel

  • Erosional and depositional landforms e.g meanders • Mixture of sediment and transportation processes
• Lower course:

  • Very gentle gradient

  • Very wide and flat valley

  • Very wide and deep channel

  • Mainly depositional landforms, e.g levees • Mainly smaller sediment transported by solution and suspension
• Vertical erosion - downwards

  Lateral erosion - sideways
• Hydraulic action - water forces air into cracks in the rocks, breaking and eroding them
• Abrasion - rocks in the river grind against the bed and banks
• Attrition - rocks in the river grind against eachother; growing smaller, smoother, and more rounded
• Solution - soft rocks are dissolved by slightly acidic river water
• Load - material transported by the river
• Solution - load is dissolved in the river
• Suspension - small sediment particles are suspended in the river
• Traction - large rocks are rolled along the riverbed
• Saltation - small pebbles are bounced along the riverbed
• Erosional landforms:

  •Interlocking spurs

  •Waterfalls

  •Gorges
• Erosional and depositional landforms:

  • Meanders

  • Ox-bow lakes
• Depositional landforms:

  •Floodplains

  •Levees

  •Estuaries
• Interlocking spurs - erosion on the outer bend (River cliff forms) and deposition on the inner bend (slip-off slope forms), combined with the river bending to avoid hard rock in a V-shped valley
• Waterfalls:

  1. Band of hard rock above soft rock
  2. Soft rock erodes quicker and undercuts it, leaving an overhang
  3. Overhang collapses under gravity into plunge pool, which is eroded to widen and deeper channel
  4. Cycle repeats as the waterfall retreats back, leaving a Gorge
• Meanders - erosion on outer bend and deposition on inner bend causes the river channel to have many sharp curves
• Thalweg - the line of fastest current which hits the outside bend, eroding it
• Ox-bow lakes:

  1. Meanders migrate across floodplain, creating a thin meander neck
  2. During flooding, the river cuts through the neck to take a more direct path
  3. As the river flows through the new channel, deposition seals off the old meander which creates an ox-bow lake
• Floodplains - wide flat areas on either side of a river. Created by migrating meanders and floods that deposit layers of alluvium
• Alluvium - sediment deposited by a river
• Levees - alluvium is deposited either side of river channel, increasing depth/height of river and river capacity
• Estuaries - where the river meets the sea
• River capacity - the maximum amount of water a river can hold before it bursts it's banks
• River discharge - the volume of water flowing through a river at any given point in time
• Physical flood risk factors:

  •Impermeable rock -> water can't infiltrate -> increased surface runoff -> increased river discharge

  •Steep relief -> fast flow -> water can't infiltrate -> increased surface runoff -> increased river discharge
• Human flood risk factors:

  •Urbanisation -> water can't infiltrate into concrete -> increased surface runoff into storm drains -> increased river discharge

  •Deforestation -> less interception/transpiration -> soil becomes saturated -> decreased infiltration -> increased surface runoff -> increased river discharge
• Hydrograph - shows how a river reacts to rainfall
• Lag time - time between peak rainfall and peak discharge
• Shorter lag time - greater risk of flooding
• Hard engineering:

  •Dams and Reservoirs

  •Channel straightening
• Soft engineering:

  •Afforestation

  •River restoration
• Dams and Reservoirs:

  •Holds back water from the river and stores it

  •Effective and provides water stores

  •Expensive, and can cause floods upstream
• Channel straightening:

  • Cutting through meanders artificially

  • Lowers flood risk and speeds up water flow

  • Can cause flooding downstream, can damage habitats