Features of the upper course

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Rapids & Braided Channels

Rapids

Turbulent flow that are caused by the sudden change in gradient causing faster flowing water

Braided Channels

  • The river is forced to split into several channels seperated by islands.
  • Occurs when a river has variable discharge-large loads of sand and gravel
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Formation of meanders

  • River flows over alternating bars of sediment and the water is forced to weave around them 
  • The river will take the path of least resistence
  • Creates shallow sections and deeper sections
  • Maximum velocity occurs towards the outside of the bank and results in erosion by undercutting creating a river cliff
  • Water is slowest on the inside of the meander meaning it has less energy therefore depostion occurs, forming a point bar
  • This continues over time causing the bend to become sharper and sharper 
  • The cross section is asymmetrical-deeper near the river cliff and shallow near the point bar
  • Once the meander is developed there is a corkscrew type movement called helicoidal flow-water moves from the outside to the inside of the bend
  • Meander migrates laterally and downstream
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Oxbow Lakes

  • Feature of erosion and deposition
  • Horse shoe shaped lake seperate from the river
  • Water is stagnant and lake gradually silts up and becomes more crescent shaped
  • Stretch of marsh called meander scar
  • Formed by increasing sinuosity of a meander
  • Most erosion on the outer bank
  • Deposition on the inner bank
  • Neck of the meander becomes narrower
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Incised Meanders

  • If a rejuvenated river occupies a valley with well developed meanders, renewed energy results in them becoming incised or deepend
  • Erode vertically
  • Incision is slow and lateral
  • Erosion occurs and on ingrown meander might be produced 
  • Calley becomes asymetrical with steep cliffs on outer bends and gentle slip off slope on the inner bends
  • Rapid incision where dowcutting or vertical erosion dominated 
  • Valley is more symetrical with steep sides and a gorge like appearance, these are entrenched meanders
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As a river moves downstream it...

  • Widens

  • Deepens

  • Particles get smaller

  • Particles are rounder
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Features of the middle course

  • Meander-bends in the river
  • Ox Bow Lake-when the river has enough energy to cut off the meander
  • Rejuvenation-sudden increse in energy
  • Incised meander-More symetrical
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Delta's

  • Feature of deposition
  • At the mouth of a river
  • Deposition occurs when the velocity and sediment carrying capacity decrease
  • Bedload and suspended material are dumped
  • Flocculation occurs as fresh water mixes with sea water and clay
  • Only form when the rate of depostion exceeds the rate of sediment removal
  • To form they need:large sediment load of the river, small coastal tidal range and weak currents, limited wave action and little transportation of sediment
  • Larger and heavier particles are deposited first which form the topset beds
  • medium sized particles travel further before they are deposited as steep angled wedges of sediment forming foreset beds
  • Finest particles travel furthest into the lake before deposition and form topset beds
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Floodplains

  • Created from erosion and deposition
  • Flat areas of land on eith side of the river
  • Alluvium-river deposited clay and silts
  • Over time it becomes wider and deeper
  • Width is determined by the amount of meander migration and lateral erosion
  • Lateral erosion is most powerful downstream of the apex of the meander bend
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Levees

  • During a flood thick, coarse sediment is deposited at the channel edges
  • Thin and fine material deposited over outer parts of the floodplain
  • Natural levees buiold up by floods
  • More floods increase the size of levees
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