The River Valley
- The path a river flows downhill is called its course
- Rivers have an: upper course (closest to source), middle course and a lower course (closest to mouth)
- Rivers form channels and valleys as they flow downhill
- They erode the landscape (wear it down) - then transport the material to somewhere else where it is deposited.
- The shape of the river valley/channel changes along the river depending on whether erosion or deposition (dominant process) is having the most impact.
- The long profile of a river shows you how the gradient changes through the course.
- The cross profile shows you what the cross section of the river looks like.
Course Of The River
Upper- Steep - Vertical erosion
- V-shaped valley, interlocking spurs, steep sides, narrow, shallow channel - waterfalls & gorges
Middle- Medium - Lateral Erosion
- Gently sloping valley sides, wider, deeper channel - meanders & oxbow lakes
Lower- Gentle - Lateral Erosion
- Very wide, almost flat valley, very wide, deep channel - deltas
Vertical & Lateral Erosion
- Upper Course
- Deepens the valley making it V shaped
- Middle & lower course
- Widens the valley
The 4 Processes Of Erosion
- The force of the water breaks rock particles away from the river channel
- Eroded rocks picked up by the river scrape and rub against the channel, wearing it away. Most erosion happens by abrasion
- Eroded rocks picked up by the river smash into each other and break into smaller fragments, their edges get rounded off as they rub together
- River water dissolves some types of rock e.g. chalk and limestone
The eroded material is transported downstream by 4 processes:
Traction: Large particles like boulders are rolled/pushed along the river bed by the force of the water (roll along).
Saltation: Pebble sized particles are bounced along the river bed, they are picked up and dropped as the flow of the river changes.
Suspension: Small particles like silt and clay are carried along by the water giving the river abrown appearance.
Solution: Soluble materials like limestone and chalk are dissolved in the water and carried along in the flow.
Types of Rock
- Rocks with pore spaces between the grains which allow it to absorb/store water.
- e.g. chalk
- Rocks with cracks and spaces that allow water to pass through- cannot store water.
- e.g. limestone
- A surface that cannot store water- water runs off it.
- e.g. Concrete
Deposition is when the river drops the eroded material it is transporting
It happens when the river slows down (loses velocity)
There are a few reasons why rivers slow down and deposit material:
- The volume of water in the river falls.
- The amount of eroded material in water increases (load gets too heavy).
- The water is shallower e.g. on inside of bend.
- The river reaches its mouth (end).
Meanders are formed by erosion and deposition- found in middle and lower course.
(Named example- lots of meanders in the Mississippi river USA)
1. The water in a river flows in a spiral and therefore there is variation in the speed of the water. The current is faster on the outside of the bend because the river is deeper and there is less friction to slow the water down. The current is slower on the inside of the bend because the river channel is shallower so there is more friction to slow the water down
- Water movement is slower on the inside of the bend. This causes deposition.
- Water movement is faster on the outside of the bend. This causes erosion.
2. More erosion takes place on the outside of the bends forming river cliffs
3. Eroded material is deposited on the inside of the bend, forming a point-bar. The gentle slope down to the point bar is called a slip off slope.
Ox-Bow lakes are formed from meanders.
1. Erosion causes outside bends to get closer. (A really tight meander is called a Swans Neck)
2. It erodes until there is only a small bit of land between the bends.
3. The river breaks through. (Often after a flood/storm when the river has high discharge).
4. The river flows along the shortest route. (Which is the new channel formed)
5. Deposition cuts off the meander, forming an ox-bow lake.
Waterfalls and Gorges
Waterfalls form where a river flows over an area of hard rock followed by softer rock.
1. Softer rock is eroded more than the hard rock- creating a 'step' in the river
2. This leaves a layer of hard rock which overhangs the layer of soft rock.
3. A deep drop is created and water flows over the overhang- waterfall.
4. The hard rock is eventually undercut by erosion-it becomes unsupported and collapses
5. The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion, creating a plunge pool.
6. Over time, more undercutting causes more collapses the waterfall will retreat leaving behind a steep sided gorge.
Waterfall Model Answer
* A waterfall is formed usually in the upper course of a rivers journey to the sea, where there are horizontal bands of hard and soft rock. The overlying hard rock is eroded slower than the softer rock through hydraulic action and abrasion. An overhang is formed which eventually collapses as the weight of the overlying cap rock is too great, creating the steep drop of the waterfall. The eroded material will erode the back wall and sides creating a large plunge pool through the processes of attrition. Over time these processes of undercutting and collapse is repeated many times, causing the waterfall to retreat upstream, creating a gorge. *
Niagara falls, in Canada has been formed in this manner, where the hard limestone rock overlies softer shales and sandstone on the Niagara river.
Formation of a waterfall
A flood plain is formed by deposition - found in the lower course of the river.
What is a flood plain?
It is the wide valley floor on either side of a river which occasionally gets flooded.
How is it formed?
- When a river floods onto the flood plain, the water slows down and deposits the material that it's transporting - this builds up the flood plain making it higher.
- Meanders migrate (move) across the flood plain making it wider.
- The deposition that happens on the slip-off slopes of meanders also builds up the flood plain.
Levees are natural embankments along the edges of a river channel- found in the lower course- caused by deposition.
- 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.
- Over time the deposited material builds up creating levees along the edges of the channel.
Hydrographs/ River Discharge
River discharge= Volume of water flowing in a river per second.
Hydrographs show how the discharge at a certain point in a river changes over time.
Storm hydrographs show the changes in river discharge around the time of a storm.
Peak discharge= the highest discharge in the period you are looking at.
Lag time= the delay between peak rainfall and peak discharge.
Rising limb= the increase in river discharge as rainwater flows into the river.
Falling limb= the decrease in river discharge as it returns to normal level.
Lag time happens because most rainwater doesn't land directly in the river channel- it's delayed as rainwater gets to the channe. It gets there by flowing quickly over land (surface runoff) or by soaking into the ground (infiltration) and flowing slowly underground.
Factors Which Affect River Discharge
The more water that flows as run off, the shorter the lag time, this means the discharge will increase as more water gets to the channel in a shorter space of time.
Amount and type of rainfall:
- Lots of rain and short, heavy periods of rainfall = more surface run-off. Lag time decreased so discharge increased.
Previous weather conditions:
- After lots of rain soil can become more saturated = rainwater won't be able to infiltrate into the soil so surface run-off will increase. Lag time decreased so discharge increased.
- Urban areas have drainage systems and they're covered with impermeable materials like concrete = more surface run-off. Lag time decreased so discharge increases.
Factors Which Affect River Discharge (2)
- Hot, dry conditions and cold freezing conditions results in hard ground = increases surface run-off. (Lag time decreased so discharge increases)
- Water infiltrates through pore spaces in permeable rocks and flows along the cracks in permeable rocks = not much run off because water is stored in rocks. (Lag time increases and discharge decreases)
- Water can't infiltrate through impermeable rock = a lot of surface run-off. (Lag time decreased and discharge increased)
- Lots of surface run-off occurs on steep slopes = (Lag time decreased, discharge increased)
Flashy floods - quick to flood
They happen in:
- Urban areas - lots of impermeable surfaces which water runs off (cannot infiltrate).
- Places of deforestation- less trees to intercept rainfall.
- Places with steep slopes- shorter lag time.
- A thunderstorm- high amount of discharge.
- Saturated soil- rainfall cannot infiltrate into soil therefore runs off.
- Impermeable rock- water runs off.
- Rapid snow melt- rapid increase of discharge.
Slow to Flood
Slow to flood rivers happen in:
- Rural areas- lots of trees to intercept rainfall and slow down discharge.
- Deep soils- water can infiltrate into soils.
- Gentle slopes- slows down the flow of discharge.
- Dry soil- water can be absorbed.
- Light rain- discharge flow is slow.
- Permeable rock- water can be stored.
Case Study- Mozambique LEDC - Causes
Mozambique, 2000, LEDC.
- Deforestation- trees cut down to use for firewood and farmland due to an increasing population. Less trees -> less interception -> more flooding.
- Rainfall- 1163mm fell compared to the usual average of 177mm. Caused by Cyclone Eline.
- Dam Opened- Kariba opened their dam to stop it bursting but mozambique were not warned so 8m high waves rushed down the limpopo river.
- Increased Urbanisation- More impermeable surfaces.
Case Study- Mozambique LEDC - Effects
- 3,000 people lost their homes.
- People stranded in trees etc.
- Infrastructure (roads & railways) were swept away - resulting in aid unable to get in.
- Cost £175 million to rebuild everything.
- Maleria spreaded due to flooded ground causing mosquitos to breed.
- 20,000 cattle missing - resulting in them dieing and rotting in water & also farmers losing money.
Case Study- Mozambique LEDC - Responses
Setting up refugee camps run by various organisations.
- Building up stocks of water, food & tents - prepared for another flood.
- Improving education- to make people aware of how to prepare for floods.
- Build large dams to store water and let it out slowly- no flash floods as water is controlled
- Warning system established- people can be prepared and gather supplies.
- Making agreements with neighbouring countries about water management and warnings- so countries can be warned and can help eachother.
- Allow the river to flood in some places- where there are less people living, it's controlled and down stream will be less effected.
- Flooded zones introduced- dangerous zones can be avoided. Refugees can be given aid sooner if they know where to go to find help.
Case Study- Cockermouth, Cumbria - MEDC - Causes
Cockermouth, Cumbria, north west of England, 2009.
- It was the wettest 24 hours ever recorded - 314mm of rainfall fell into the Lake district.
- Due to heavy rainfall.
- River Greta burst its banks.
- Cockermouth is at the confluence of two rivers and suffered the worst flooding.
Case Study- Cockermouth, Cumbria - MEDC - Effects
- 39 schools closed.
- 1,200 people left without electricity.
- Telephone lines down - communication disrupted. Disadvantage to businesses.
- Bridges collapsed - difficult to get aid in.
- Many businesses ruined.
- Trees uprooed and dumped where the water had left them.
- Fields and crops destroyed by debris - food not being produced.
Case Study- Cockermouth, Cumbria - MEDC - Response
- Grant given to clear debris (£6800).
- Councils collected tvs etc from roadsides.
- Removal of gravel from one field cost £90,000
- Hiring of diggers to remove gravel cost £20,000
- Farmers had to take detours to travel around to reach stock.
- Tourists stayed away.
Hard and Soft Engineering
Hard engineering= man-made structures built to control the flow of rivers and reduce flooding
Soft engineering= schemes set up using knowledge of a river and its processes to reduce affects of flooding
Dams and reservoirs:
- Dams are built across river in the upper course - a reservoir is formed behind.
- Reservoirs store water especially during prolonged periods of heavy rain.
- Water is used for drinking and for Hydro-electric power.
- Very expensive.
- Can flood existing settlements.
- Eroded material is deposited in the reservoir not along rivers course so farms further down stream become less fertile.
- Rivers course is straightened- meanders cut out.
- Water moves out of area more quickly because doesn't travel as far.
- Flooding may happen downstream as water is carried there faster.
- More erosion downstream as water is faster.
Concrete Walls (canalization)
- Walls not going to erode.
- Water moves through swiftly and effectively.
- Water may move through too quick and cause flooding down stream.
- The environment agency warns people early on TV, radio, newspapers, internet.
- Impact of flooding reduced as people can move possessions upstairs, put out sandbags and be ready to evacuate
- Doesn't stop flood happening, some people may not hear about warnings
- Could become difficult to get insurance.
- Buildings modified to reduce the damage a flood could cause - plans can be made e.g. keeping essential things like blankets and torches at hand.
- Impact flooding reduced- buildings modified, people know what to do, worry less.
- Doesn't guarantee safety from a flood, could give false sense of security.
- Expensive to modify homes.
- Doesn't stop flooding.
Flood plain zoning
- Restrictions prevent building on flood plains.
- Risk is reduced- less impermeable surfaces, less impact as there are no houses or roads to be damaged.
- Important businesses and buildings don't flood.
- Lack of living space (ledcs).
- No help in areas that have already been built on.
- No money spent on new engineering methods or maintaining existing ones.
- Flooding is a natural process and people should accept the risks of where they live
- The river floods, eroded material is deposited, farmland more likely to be fertile
- Risk of flooding and impacts aren't reduced.
- Flood will cause a lot of damage.