processes and landforms

• Riffles


  areas of shallow water along the course of river, often spaced between deeper areas, called pools
• Helicoidal flow


  movement of water in a corkscrew motion as the river moves downstream, forming meanders
• Thalweg


  the line of fastest flow (and deepest point in the valley) in a river channel
• River cliff


  the steep outer bank of a river meander
• Ox-bow lake


  an old meander that was cut off from the river
• Meanders

  1. pool-riffle-pool sequences and helicoidal flow cause the Thalweg to move from side to side, making the channel bendy
  2. helicoidal flow transports sediment from the river cliff, and deposits it on the slip-off slope
  3. river cliff is eroded by hydraulic action, abrasion and cavitation
• Ox-bow lake

  1. helicoidal flow continues to erode the river cliff and deepens the pool, then deposits the sediment on the slip-off slope and riffles
  2. this continuous process makes the meander migrate and contract at the neck
  3. the neck becomes narrower until the river cuts directly through, forming a channel cut-off and incipient ox-bow
  4. incipient ox-bow is gradually cut off through deposition to form an ox-bow lake
  5. the lake has a short lifespan due to deposition and evaporation, which leaves meander scars on the floodplain that mark the old channel
• Describe the difference in Thalweg for a straight and sinuous river

  Straight - the thalweg is in the middle of the channel

  Sinuous - the thalweg crosses to the outside of each meander

  A) straight

  B) sinuous

  
• Where are meanders found?


  Middle course of a river
• What is a pool-riffle-pool sequence?


  A pool-riffle-pool sequence is the alternating depths of water going down a river. A pool is an area of deep water, whereas a riffle is a shallower area.
• Why do riffles cause turbulent flow?


  A riffle is a shallower area of water where the gradient of the river increases, so larger sediment is deposited, causing turbulent flow which often becomes helicoidal flow
• abrasion - a river uses its load of sediment to wear away its bed and banks
• attrition - particles of sediment in the load of a river bump into each other, wearing each other away
• hydraulic action - the direct force of the flowing river water breaks material from the bed and banks
• solution - natural river water is slightly acidic, so it dissolves some types of rocks, such as chalk and limestone
• Label the diagram

  A) abrasion

  B) attrition

  C) hydraulic action

  D) solution

  
• traction - larger particles of the bedload are rolled along by the force of the flowing water
• saltation - smaller particles of bed load hop along the bed of the river
• suspension - small grains of sand or silt are carried along with the water
• solution - the dissolved load is carried with the water downstream
• cavitation - a form of hydraulic action, where a vortex of water traps air bubbles into cracks in the river banks. The bubbles implode which weakens the rock, creating a small hole.
• potholing - a form of abrasion that follows cavitation, small pebbles and rocks get stuck in the hole, and are spun around by the vortex, grinding the hole bigger
• capacity - the maximum amount of load a river can carry in total
• competence - the largest piece of load a river can carry
• channel cross sectional area = channel depth x channel width
• wetted perimeter - the length of the bed and banks in direct contact with the water in the river channel
• hydraulic radius = channel cross-sectional area/wetted perimeter
• Hjulstrom Curve:

  • uses logarithmic scales
  • velocity needed to transport material is less than velocity needed to pick it up
  • sand is easiest to pick up as it is already loose, whereas clay is cohesive so it requires more energy to break the bonds
  • fine clays stay in suspension without velocity, so the settling velocity reaches 0 at sand
  • when velocity decreases, coarser material is dropped first
• Limitations of Hjulstrom Curve:

  • there's a mix of size, as one type of sediment isn't moved all at once
  • methods of erosion can differ, and velocity isn't needed for all types
  • density, as porous rocks are lighter so need less energy than other rocks their size
  • velocities may vary within a river
• Label the diagram

  A) erosion

  B) transport

  C) deposition

  D) clay

  E) silt

  F) sand

  G) pebbles

  H) boulders

  
• laminar flow - water flows in sheets (laminae), parallel to the river bed
• turbulent flow - chaotic flow that involves eddying and vertical movement
• eddying - water flows in the opposite direction to the normal flow
• Braided channels:

  • choking the main channel with the deposition of a considerable amount of river load, forcing the river around the bars
  • happens due to discharge variation (and velocity variation)
  • large coarse load is deposited quickly, increasing braiding
  • eroded banks so the river can expand
  • vegetation on bars are called eyots
• Waterfall formation:

  1. softer rock is easy to erode, but the hard rock is resistant, so over time a ledge develops
  2. water rushes over the ledge and erodes a plunge pool by abrasion and hydraulic action in the process of cavitation
  3. ledge collapses into the plunge pool, where the debris speeds up erosion
  4. process repeats and the waterfall retreats upstream, forming a gorge
• Rapids:

  • alternating areas of softer and harder rock on a slope, so the softer rock is eroded faster, leaving angles of hard rock
  • debris from the softer rock increases the erosion through cavitation, potholing and abrasion
• Floodplains are cut out by the erosive force of the migrating unregulated river channel, then covered by deposited alluvium from floods. They are areas of flat land contained by bluffs, which are areas of higher reliefs on the sides. Many landforms are found here, such as levees, meanders, ox-bow lakes, and meander scars.
• yazoo stream - a geologic and hydrologic term for any tributary stream that runs parallel to a larger river, in its floodplain, for a considerable distance, before joining it
• Levees:

  1. river floods so velocity drops
  2. competence decreases so larger load is deposited
  3. as water flows further, more load is deposited
  4. lightest load is deposited furthest
  5. the piles of alluvium raise the banks of the river, forming gentle slopes down the floodplain
• 3 types of delta:

  • arcuate
  • cuspate
  • birdfoot