Rivers

• River course

  The path of a river as it flows downhill
• River sections

  • Upper course
  • Middle course
  • Lower course
• River processes

  1. Erode the landscape

  2. Transport material

  3. Deposit material
• River shape


  • Changes along the river depending on whether erosion or deposition is taking place
• River long profile


  Shows how the gradient (steepness) changes over time
• Upper course features

  • Steep
  • V-Shaped valley with steep sides
  • Narrow and shallow
• Middle course features

  • Gently sloping
  • Sloping Valley sides
  • Wider and deeper channel
• Lower course features

  • Steep
  • V-Shaped valley with steep sides
  • Narrow and shallow
• Vertical erosion

  Deepens the river valley (and channel) making it V-Shaped. It's dominant in the upper course. High turbulence causes rough, angular particles to be scraped along the bed causing downwards (vertical) erosion
• Lateral erosion

  Widens the river valley (and channel) during the formation of meanders. It's dominant in the middle and lower courses
• Waterfalls

  • Form where a river flows over an area of hard rock followed by an area of softer rock
  • The softer rock is eroded by hydraulic action & abrasion more than the harder rock, creating a step in the river
  • As water goes over the step it erodes more and more of the softer rock
  • A steep drop is eventually created, which is called a waterfall
  • The hard rock is eventually undercut by erosion. It becomes unsupported and collapses
  • The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion. This creates a deep plunge pool
  • Over time, more undercutting causes more collapses. The waterfall will retreat (move back) leaving behind a steep-sided gorge
• Interlocking spurs

  • In the upper course of a river most of the erosion is vertical. This creates steep valley sides
  • The rivers aren't powerful enough to erode them laterally (sideways) – they have to wind around the high hillsides that stick out into their paths
  • The hillsides that interlock with each other (like a zip) as the river winds around them are called interlocking spurs
• Meanders

  1. Rivers develop large bends called meanders in their middle and lower courses

  2. The current is faster on the outside of the bend because the river channel is deeper (less friction to slow it down)

  3. So more erosion takes place on the outside of the bend, forming river cliffs

  4. The current is slower on the inside of the bend because the river channel is shallower (there's more friction to slow it down)

  5. So eroded material is deposited on the inside of the bend, forming slip-off slopes

  6. The fastest flow of a river through a meander is called a thalweg
• Oxbow lakes


  Meanders get larger over time –they eventually turn into an oxbow lake
• Estuary


  • Where the river meets the sea
  • The river here is tidal and when the sea retreats the volume of the water in the estuary is less reduced
  • When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife
  • The land of an estuary is not very fertile so it is not good for agriculture, however it does create good habitats for wildlife
• Floodplains


  1. An area of land which is covered in water when a river bursts its banks

  2. Floodplains form due to both erosion and deposition

  3. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river

  4. During a flood, material being carried by the river is deposited (as the river loses its speed and energy to transport material)

  5. Over time, the height of the floodplain increases as material is deposited on either side of the river

  6. Floodplains are often agricultural land, as the area is very fertile because it's made up of alluvium (deposited silt from a river flood)
• Deltas

  1. Found at the mouth of large rivers

  2. A delta is formed when the river deposits its material faster than the sea can remove it

  3. As the river approaches the sea it loses energy and deposits material

  4. The sediment may be sorted as the heaviest material is deposited first

  5. Over time, more and more sediment is added

  6. If the tides are strong enough the sediment will be washed away. If not, it will build up a land mass (delta) at the mouth of the river
• Levees


  • Natural embankments along the edges of a river
  • During a flood, eroded material is deposited over the whole flood plain
  • The heaviest material is deposited closest to the river channel, because it gets dropped first when the river slows down
  • Overtime, the deposited material builds up, creating levees along the edges of the channel
• River discharge

  The volume of water that flows in a river per second. Measured in cubic metres per second
• Hydrograph


  Shows how the discharge at a certain point in a river changes over time in relation to rainfall
• Hydrograph features

  • Peak discharge
  • Peak rainfall
  • Lag time
  • Rising limb
  • Falling limb
  • Base flow
• Lag time


  The time taken between peak rainfall and peak discharge
• Factors influencing lag time

  • Size of drainage basin
  • Vegetation
  • Valley side steepness
  • Soil type
• Human & physical factors affecting flood risk

  • Rainfall
  • Rock type
  • Relief
  • Land use
  • Trees
• Hard engineering


  • Using man-made structures to prevent or control natural processes from taking place
  • Usually very expensive
• Hard engineering methods

  • Dams and reservoirs
  • River straightening and dredging
  • Embankments
  • Flood relief channels
  • Floodplain zoning
  • Flood warnings
• Soft engineering

  • Does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding
• Dams and Reservoirs


  The dam traps water, which builds up behind it, forming a reservoir.

  Water can be released in a controlled way.
• Dams and Reservoirs - Pros and Cons

  Pros

  
  

  • Can be used to produce electricity by passing the water through a

  turbine within the dam.

  • Reservoirs can attract tourists.

  
  

  Cons

  
  

  • Very expensive.

  • Dams trap sediment which means the reservoir can hold less water.

  • Habitats are flooded often leading to rotting vegetation.

  This releases methane which is a greenhouse gas.

  • Settlements are lost leading to the displacement of people.
• River Straightening and Dredging


  Straightening the river speeds up the water so high volumes of water can pass through an area quickly. Dredging makes the river deeper so it can hold more water.
• River Straightening + Dredging - Pros and Cons

  Pros

  
  

  • More water can be held in the channel.

  • It can be used to reduce flood risk in built-up areas.

  
  

  Cons

  
  

  • Dredging needs to be done frequently.

  • Speeding up the river increases flood risk downstream.
• Embankments

  Raising the banks of a river means that it can hold more water.
• Embankment - Pros and Cons

  Pros

  • Cheap with a one-off cost

  • Allows for flood water to be contained within the river.

  Cons

  • Looks unnatural.

  • Water speeds up and can increase flood risk downstream
• Flood relief channels

  The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.
• Flood Relief Channels - Pros and Cons

  Pros

  • Removes excess water from the river channel to reduce flooding.

  Cons

  • Expensive to build.

  • If water levels continue to rise, the relief channel may also flood.
• Floodplain Zoning

  Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.
• Floodplain Zoning - Pros and Cons

  Pros

  • More expensive buildings and land uses are further away from the river, so have a reduced flood risk.

  • Less damage is caused, leading to fewer insurance claims.

  Cons

  • Not always possible to change existing land uses.

  • Planners have to decide what type of flood to plan for.
• Flood Warnings


  The Environment Agency warns people in advance
• Flood Warning - Pros and Cons

  • Warnings give people time to evacuate

  • Warnings don’t stop a flood from happening.