Rivers

• Drainage basin

  area of land drained by a river and its tributaries - hills and mountains
• 3 river stages


  Upper course, middle course, lower course
• The upper course of a river

  1. Starts as many tributaries
  2. v shaped valley with steep sides
  3. steep gradient
• The middle course of a river

  1. tributaries form a deeper channel
  2. valley is wider with a flat floor
  3. gentle gradient
  4. River is wider and deeper
  5. Water has more energy - more erosion widens river
• The lower course of a river

  1. Largest volume of water
  2. Valley is very wide with a flat floor
  3. River is wide and deep with a large sediment load
  4. Levees appear
  5. Very gentle gradient
• velocity of water

  Friction occurs between the water and the river bed which slows the water down

  
  

  In the upper course, the speed of water is slower as there is more friction

  
  

  As the channel gets bigger, less water is in contact with the riverbed, which means the velocity of water increases.
• What are the processes of erosion?

  Upper course = mostly erosion landforms e.g. waterfalls, interlocking spurs, v-shaped valleys

  
  

  Middle course = mostly erosion and deposition landforms e.g. meanders, gorges and transportation

  
  

  Lower course = mostly depositional landforms e.g. levees, ox-bow lakes, river estuary
• the four processes of erosion

  1. Abrasion
  2. Attrition
  3. Hydraulic action
  4. Corrosion (solution)
• Abrasion


  When the load carried by the river repeatedly hits the bed or banks, so the river wears away the channel gradually
• attrition


  Rocks hitting against each other, wearing each other down and so becoming round and smaller
• hydraulic action


  Water under pressure causes cracks to force apart and widen in rocks along the banks of the river

  Overtime, this causes the rock to fracture and collapse into the river, expanding the river channel
• Corrosion (solution)


  The river can gradually dissolve chemical compounds in rocks that it flows over

  e.g. limestone can dissolve gradually into the river if river water is slightly acidic.
• what is the process of transportation in a river?

  1. Traction
  2. Saltation
  3. Suspension
  4. Solution
• Traction


  Large rocks are rolled along the river bed
• Saltation


  Pebbles and small rocks which are too heavy to be suspended bounce along the river channel
• suspension


  Particles in small rocks are light enough to float within the water
• solution


  Chemicals are dissolved in to the river water
• Deposition


  Dropping off the river load when the water in a river decreases in speed

  
  

  The heaviest materials get deposited first in the upper course

  
  

  The sediments travel as far as they can in the lower course before being deposited
• ​Distinctive river landforms

  landforms:

  A) source

  B) interlocking spurs

  C) v - shaped valley

  D) levees

  E) ox - bow lakes

  F) gorge

  G) tributary

  H) waterfall

  I) meander

  J) sea

  
• interlocking spurs

  Found in the upper course of the river, where water does not have a lot of energy so the river re-route and curls around them
• waterfalls

  Occurs when river flows over rocks with different resistances to erosion
• formation of waterfalls

  1. soft rock erodes around the hard rock over time, creating a step
  2. Soft rock continues to erode undercutting the hard rock
  3. Hard-rock is left suspended in the air as an overhang
  4. gravity collapses the overhang creating a plunge pool
  5. Rocks fall into the plunge pool, which acts as a tool for erosion to deep in the pool
  6. Step 1 - 4 repeated
  7. The continual process of the overhang collapsing causes the waterfall to retreat upstream over time
  8. The plunge pool continues to deepen, and the hard rock continues to be undercut to create an overhang causing a waterfall
• Gorges

  Formed from waterfalls as a waterfall retreats upstream, and leaves behind a steep valley carved into the rock with the river running along the base
• Meanders

  Bends in the shape of the river

  Found in the middle course

  Creation is a gradual process which depends on the velocity of water
• Formation of meander

  Water travels fastest on the outside of the bend, which means lateral erosion takes place

  On the opposite side water slowly and changes direction, so the water loses energy and deposits sediment

  Erosion wears away a cliff on the outer edge of a bend and deposition creates a slip of slope on the inside of the bend.
• Thalweg

  Line of fastest flow within the river swings from side to side

  causing erosion on the outside, and deposition on the inside
• Oxbow lakes

  Similar to a meander

  formed due to a combination of erosion and deposition
• how are oxbow lakes formed?

  First of river forms of meander

  Erosion bends the river, so the meanders travel towards each other

  The neck of the meander will eventually break creating a straight river

  The old meanders become separated from the main river as the material gets deposited at the top, creating a separate oxbow lake

  Water now takes the fastest flow
• 3 types of river depositional landforms

  levees + Floodplains

  Estuaries
• Flood plains

  wide flat area of marshy land on either side of a river

  made of alluvium (sediment)

  found in the middle and lower course

  made from silts which make land fertile

  This is why lots of farming takes place
• levees

  banks of the river in the lower course which are raised

  Is formed by flooding over many years, and a ridge of sediment is deposited naturally to build up the levee.

  Higher than the height of water, but aren't wide

  When the river floods it deposits, silt creating a very flat floodplain
• Estuaries

  At the mouth of the river

  Traditional zones between river and coastal environment and are affected by wave action as well as river processes

  A large amount of deposition

  Creates mudflats and saltmarshes
• flooding

  Occurs when an excess amount of water fills the river in a short period

  If the river channel isn't large enough to contain the water, the river over spills into surrounding land
• Human factors that increase our location's flood risk

  Urbanisation = increasing the amount of impermeable surfaces decreases the time taken for water to flow into the river increases the risk of flash flooding

  
  

  Deforestation = trees intercept the rain, so it takes longer for rain to travel through the leaves into the river, cutting down trees will speed up the time, taking for rainwater to flow into the river, increasing flash flooding.

  
  

  The capacity of the river = if the river becomes filled with rubbish and debris, the channel size will decrease meaning the river will carry less water
• physical factors that increase the flood risk

  The rate and volume of precipitation (rain) = if there is a storm and large amount of rainfall is the amount of water running into the river is increased

  
  

  Geology = If the area has many impermeable rocks, the rainwater can't run into the soil underground, instead it runs straight into the river

  
  

  Topography = the shape of the land will determine how quickly rainwater flows into the river. Steep hills are more likely to have flash floods than gradual gradients.
• storm hydrographs

  Represent the variation in the river discharge (volume of water passing through the river channel at a specific point) within a short period

  Useful in showing how precipitation affects a drainage basin
• features of a storm hydrograph

  Peak precipitation = maximum rainfall that occurs shown as a bar chart at the start of the graph

  Rising limb = increase of river discharge not necessarily straight after precipitation

  Peak flow = maximum discharge, delayed after maximum precipitation has occurred

  Lag time = as storm, and precipitation levels decrease, river discharge will in turn decrease over time

  Full limb = As the storms precipitation levels decrease, river discharge will decrease over time

  Base flow = Eventually, discharge returns to normal level
• Types of flooding 


  Flashy or subdued

  Flash flood = a flood with little warning, the large volume of water, suddenly overwhelms the river

  Subdued flood = could take up to a week of consistent rain. There isn't a large quantity of water falling per day; causing longer-term flooding
• Flooding risks


  Damaged properties and families can lose their belongings

  Farmers might lose their crops or cattle might drown

  Industry and factories could become flooded, which means workers would lose their jobs, so unemployment will rise
• soft engineering

  Uses more natural materials to restore the rivers, natural state

  Reduce the damage caused by flooding in a more environmentally friendly way