1.3 River channel processes and landforms

• Upland Channel Characteristics

  Steep gradients

  Rough Bed

  Narrow, shallow channel

  High friction

  High turbulence, but low speed

  Vertical erosion predominates
• Landforms of Upland Channels

  V-Shaped Valley

  Interlocking Spurs

  Waterfalls and Gorges

  Rapids

  Potholes
• Lowland Channel Characteristics


  Gentle gradient

  Smooth bed

  Wide, deep channel

  Low friction

  Low turbulence but high speeds

  Lateral erosion predominates.
• Landforms of Lowland Channels

  Wide floodplain

  Levees

  Delta or Estuary

  Ox-bow lakes
• Long Profile


  The gradient of a river, from its source to its mouth
• Abrasion


  River uses load ot wear away its bed and banks.

  
  

  Abrasion creates potholes in the uplands where small pebbles become trapped in a hollow and swirled around, digging a pothole.
• Attrition


  Particles of sediment bump into one another and wear each other away.

  
  

  River sediment becomes smaller and more rounded further downstream since it is carried in the river longer.
• Hydraulic Action


  The force of the water breaks rock particles away from the river channel.
• Cavitation


  The force of exploding air.

  
  

  When air gets trapped in rock gaps and compressed by water, it can explode which expands the hole.
• Corrosion


  Water is often slightly acidic, which dissolves rocks like chalk and limestone.
• Traction

  Larger particles are rolled along the bed of the river by the force of the flowing water.
• Saltation


  Smaller particles of bedload hop along the bed of the river.

  
  

  They are picked up by a faster eddy, where they move along the river bed until they are deposited.
• Suspension


  Accounts for most of the load particularly in lowland river.

  
  

  Rivers often look muddy and brown due to suspended silt, sand and clay in the water.
• Solution


  Dissolved load comes from dissolved rock like chalk and limestone.
• Bankfull Stage


  When the river is at maximum capacity.
• Competence


  Maximum size of particle the river is capable of transporting at the bankfull stage.
• When does deposition occur?

  During a period of low discharge - often due to lack of rain.

  
  

  On the inside of a meander bend.

  
  

  When a river bursts its banks due to a reduction in the hydraulic radius.

  
  

  When load is increased like after deforestation.

  
  

  When a river enters the still water of a lake of the sea.
• Flocculation


  When sediment in a river meets sea water and the charged salt ions attract the oppositely charged clay and silt particles causing them to bind to one another, forming large clumps of sediment that is then deposited.
• Hjulstrom Curve


  Shows the relationship between river velocity and competence.

  
• Key points of the Hjulstrom Curve

  The velocity needed to keep particles moving is always lower than the velocity needed to start them moving.

  
  

  Sand is the easiest material to erode and can be picked up at low velocities.

  
  

  Fine clays will stay in suspension even if water stops moving.

  
  

  When a river slows down, coarse material is deposited first and finer material later. It is why levees form.
• Laminar Flow:

  • The flow of water in parallel streams.
  • No cross currents or swirls.
  • One directional, orderly flow.
• Turbulent Flow:

  • Disorderly flow.
  • Changes in velocity.
  • Can be caused by friction, especially when river channels are irregular.
  • Eddies (Swirling reverse currents) common in this.
• Helicoidal Flow:

  • Corkscrew movement of water (spiral).
  • Occurs in bends in the river channel (Meanders).
  • Responsible for the erosion and deposition in a meander.
• Thalweg:

  • Line of fastest flow in a river.
  • Deepest point within the river channel.
• Straight Channel:

  • Mainly in upper course of river.
  • Single, straight channel.
  • Vertical erosion is prevalent.
  • Thalweg moves from side to side.
• Meandering Channel:

  • Middle and lower course of the river.
  • Series of bends and curves from side to side.
  • Deposition on the inside of the bend.
• Potholes (UC):

  • Turbulent flow swirls pebbles around a depression in river bed.
  • Pebbles enlarge depression and pothole by abrasion.
  • Usually small features.
  • Evidence that vertical erosion dominates in upland rivers.
  • Blyde River, South Africa is extreme example.
• Rapids (UC):

  • Form at steep gradients.
  • Riverbed is often rocky and resistant to erosion.
  • Caused by bands of hard rock within a river bed.
  • Hiwasee River
• Waterfalls / Gorges (UC):

  • Horizontal rock lies on a layer of soft rock.
  • Soft rock is eroded more quickly.
  • Plunge pool develops once soft rock at bottom is eroded.
  • Plunge pool undercuts overhanging hard rock layer until it is unsupported.
  • Process continues and waterfall can retreat.
• Floodplain (LC):

  • Flat land next to river which is liable to flood after a storm.
  • Often badly drained and feature marshes and ox-bow lakes.
  • Water may rise beyond floodplain, but is usually contained by levees.
  • Levees are natural embankments.
  • Made of alluvium (River Silt)
• Bluffs (LC):

  • Bluffs are the point where the floodplain ends and the valley walls begin.
  • Can be caused by erosion from a flooded river.
• Levees (LC):

  • Depositional feature.
  • River burst banks, current slows and deposition occurs.
  • Current slows down further away from river it gets.
  • Bigger material is deposited first and smaller material further away from the river. (Hjulstrom Curve)
  • Can be reinforced or totally artificial. (New Orleans, USA)
• Deltas:

  • Depositional
  • River meets still body of water like sea or lake.
  • Loss of velocity means sediment may be deposited by the river.
  • Flocculation occurs as clay particles are attracted to charged salt ions.
  • Clay particles bind until too heavy to be suspended and is then deposited.
  • Deposition blocks main channels.
  • Channel splits into smaller distributaries.
  • Deltas are attractive places to live due to fertility of alluvium.
  • They are dangerous as they are prone to flooding by river or sea.
  • Ganges Delta in India and Bangladesh.
• Types of Delta:

  • Arcuate Deltas
  • Fan-shaped with smooth seaward edge.
  • Form when tidal range is low and strong movement of sediment along the coast e.g. Longshore drift.
  • Niger Delta, West Africa.
  • Cuspate Deltas
  • Shaped like an arrowhead.
  • Low tidal range and two offshore currents that operate in opposite directions at different times in the year.
  • Ebro Delta, Barcelona.
  • Bird's Foot Delta
  • Formed where tidal range is low and where the river currents are strong.
  • No offshore currents to shape, so each distributary creates land as it flows acting as conveyor belts.
  • Mississippi Delta in USA.
• Pools and Riffles:

  • Deeper sections where erosion predominates (Pools)
  • Shallower sections where deposition occurs (Riffles)
  • Suggested to be caused by helicoidal flow.
  • Initial slightly deeper areas where river was able to flow faster.
  • Initial shallower areas with slowed flow down and caused deposition.
  • Effects amplified over time.
• Meanders:

  • Meander wavelength is usually 8-10 times river width.
  • Deep water and river cliff are found on outside of bend.
  • Depositional features like slip-off slope and point bar are found on inside where flow is slowest.
  • Caused as Thalweg moves side to side, slowing as it reaches the edges.
  • As it slows, sediment may be deposited which then continues to slow the flow and could create a meander.
• Oxbow Lakes:

  • Old meandering turns of a river that have been cut off.
  • Lateral erosion makes the neck of the meander really narrow.
  • When a flood event occurs, the neck may be cut off as the excess water finds the path of least resistance.
  • Meander is then cut off, forming an oxbow lake.