Rivers (4) Flashcards
What is the long profile of a river?
How the gradient changes over different courses of a river
What is the cross-profile of a river?
What the cross-section of a river looks like
How does the gradient change throughout the courses of a river?
Upper: steep
Middle: medium
Lower: gentle
How does the valley and channel shape change throughout the courses of a river
Upper: V-shaped valley, steep sides. Narrow, shallow channel
Middle: Gently sloping valley sides. Wider, deeper channel
Lower: Very wide, almost flat valley. Very wide, deep channel
What type of erosion happens in the Upper course of a river? + explain
VERTICAL erosion
- deepens the river valley (and channel), making it V-shaped
- high turbulence causes the rough, angular stones to be scraped along the river bed, causing intense downwards erosion
What type of erosion happens in the Middle and Lower course?
LATERAL erosion
- widens the river valley (and channel) during the formation of meanders
Describe the aspects and landscape of a river in the Upper course
- Typically, the source of a river is in areas with a high altitude and hard, resistant rock
- Valley is V-shaped and steep sided due to vertical erosion (steep gradient)
- River channel is narrow and shallow - low discharge (amount of water flowing)
- Low velocity due to high friction between the rough channel sides
- River carries large, angular stones
Describe the aspects and landscape of a river in the Middle course
- Typically made from less-resistant rock, so the valley becomes wider due to lateral erosion
- Valley sides become gentle slopes and the gradient is less steep
- The river channel becomes wider and deeper, increasing discharge (more streams join river)
- Velocity increases as erosion makes the river bed smoother
- The sediment load is made up of smaller and more rounded rocks as erosion continues
Describe the aspects and landscapes of a river in the Lower course
- Typically made from soft rock that is easily eroded (less-resistant)
- The valley is very wide and flat
- High velocity - very little friction from the channel’s smooth sides
- Large discharge because the channel holds more water (deeper) and other rivers can join
- The sediment load is fine and well-rounded
Name the 4 processes of erosion and describe them
Hydraulic action: force of the water breaks rock particles away from the river channel
Abrasion: rocks scrape and rub against the channel, wearing it away
Attrition: rocks smash into each other and break into smaller fragments. Edges get rounded.
Solution: river water dissolves some type of rock eg. chalk and limestone
Describe freeze-thaw Weathering (a type of mechanical weathering)
- Happens when temperatures fluctuates above and below 0C
- Water gets into the cracks of a rock. When the water freezes, it expands, putting pressure on the rock
- When the water thaws, it contracts, releasing the pressure on the rock
- Repeated freezing and thawing widens the cracks, causing the rock to break up
Name and describe the 4 transportation processes
Traction: large boulders are rolled along the river bed by the force of the water
Saltation: pebble-sized particles are bounced along the river bed by the force of the water
Suspension: small particles are carried along by the water, suspended off the bed
Solution: soluble materials dissolved in the water are carried along
What is deposition and when does it happen?
When a river drops the eroded material it’s transporting
It happens when a river loses velocity
Why might the velocity of a river slow down?
- Volume of water in the river falls
- Amount of eroded material in the river increases
- Water is shallower (on the inside of a bend)
- The river reaches its mouth
Which landforms would you find in the upper course of a river?
Waterfalls
Gorges
Interlocking Spurs
How are Waterfalls formed?
(1) They form where a river flows over an area of hard rock, followed by an area of softer rock
(2) The softer rock is less resistant, so is eroded by hydraulic action and abrasion, creating a step in the river
(3) As water flows over, it erodes more and more of the rock
(4) A steep drop is eventually created, called a waterfall
(5) The hard rock is eventually undercut (becomes unsupported and collapses)
(6) Collapsed rocks are swirled around at the foot of the waterfall, eroding the softer rock by abrasion, creating a plunge pool
(7) Over time, the waterfall retreats, leaving a steep-sided gorge
How are Interlocking Spurs formed?
(1) In the upper course of a river, vertical erosion creates V-shaped valleys
(2) The rivers aren’t powerful enough to erode laterally, so they wind around the hillsides
(3) The hillsides that interlock with each other as the river winds about them are called interlocking spurs
How are Oxbow Lakes formed?
(1) Erosion causes the outside bends in the meander to get closer to each other
(2) Eventually, there will only be a small area of land between the bends (the neck)
(3) The river breaks through this land, typically during a flood
(4) The river will flow along the shortest course, ie. through the neck
(5) Deposition eventually cuts off the meander as it is slower here (less discharge)
(6) The Oxbow lake is formed
How are Meanders formed?
(1) The current is faster on the outside of the bend as the channel is deeper (less friction)
(2) More erosion (hydraulic action + abrasion) happen at the bend, forming river cliffs
(3) The current is slower on the inside of the bend as the channel is shallower (more friction)
(4) More deposition happens on the inside of the bend, forming slip-off slopes
Name some factors that Storm Hydrographs/Flooding risks affected by
- Geology: water can’t infiltrate through impermeable rock
- Drainage basin type: circular basins have a shorter lag time and higher discharge than narrow ones
- Antecedent conditions: previously wet conditions can increase runoff as soils are saturated
- Urbanisation: water can’t infiltrate through concrete/tarmac so there’s more runoff. Drains quickly take runoff to rivers, increasing discharge because water reaches the channel at the same time
- Deforestation: trees take up water from the ground and store it, reducing runoff. Loss of trees increases runoff and discharge
Name the labels on a storm hydrograph
Peak discharge: highest discharge in the certain period of time
Lag time: delay between peak rainfall and peak discharge
Rising limb: increase in river discharge as rainwater flows into the river
Falling limb: decrease in river discharge as the river returns to its normal level
(Base flow)
(Peak rainfall)
Why does lag time even happen?
Most rainwater doesn’t land directly into the river - there’s a delay as rainwater gets to the channel
Why is the risk of flooding increasing in the UK (2 main factors)
Increased Storm Frequency
Land Use Change
Why does Increased Storm Frequency lead to a higher risk of flooding in the UK?
(1) Frequency and intensity of storms is increasing, possibly due to global climate change
(2) More periods of wet weather makes the ground more saturated, making flooding more likely
Why does Land Use Change increase the risk of flooding in the UK?
(1) As the population grows, the pressure to expand urban areas grows too
(2) Leads to an increase in impermeable surfaces (eg concrete and tarmac) - more runoff
(3) Removing vegetation means that water that would have been stored in the soil flows into the river
State some examples as to how Flooding can threaten people
- People can be killed/injured by floodwater
- Roads, bridges and rail lines can be destroyed
- Floodwater is often contaminated (with sewage) which can lead to a lack of clean drinking water
- People can be made homeless
- Businesses may be forced to shut down due to flood damage, leading to a loss of livelihood
Examples of how Flooding can threaten the Environment
- Farmland can be ruined by silt and sediment deposited after a flood
- Floodwater contaminated with sewage can pollute rivers, damaging wildlife habitats
- Force of floodwater can uproot trees
- River banks are eroded, causing huge changes to the river landscape
Name and explain the 2 types of Engineering used to reduce the effects of Flooding
Soft engineering: schemes set up using the knowledge of a river and its processes
Hard engineering: man-made structures used to control river flow
Name the 6 types of flood defences
Flood Walls
Embankments
Floodgates
Flood barriers (demountable)
Flood plain retention
River restoration
What are Flood Walls
Artificial barriers built along river banks, increasing their height, allowing the channel to hold more water
However, they are very expensive and can be unsightly
What are Embankments
High banks built along the river banks. They stop the river flowing into built-up areas.
They can be made from earth or other natural materials.
They are quite expensive and there is a risk of them breaking - leading to flooding
What are Floodgates? (eg Thames Barrier)
Structures built on river estuaries to stop flooding from storm surges or high tides.
They can be shut when there is a surge forecast to prevent flooding
However, they are very expensive and need to maintained regularly
What are Flood Barriers (demountable)?
Structures that provide temporary protection against flooding. The barriers are only put up when a flood forecast is put out, so there’s a risk they may not even be put up in time.
They are quite expensive, but do not ruin the aesthetics of an area
What is Flood Plain Retention?
Involves maintaining the flood plain by not building on it. It helps slow floodwaters down by absorbing and storing the water.
No money has to be spent, but it restricts development and can’t be used in urban areas
What is River Restoration?
Involves making the river more natural so that the flood plain can flood naturally. There’s less risk of flooding downstream as discharge is reduced.
Little maintenance required, however it can increase local flooding risk
