The Shape of River Valleys Changes As the River Flows Downstream
What are the Key Ideas for this Topic?
- The Shape of river valleys changes as the river flows downstream
- River Features & Landforms
- Storm Hydrographs
- Flooding in MEDC’s and LEDC’s
- Hard and Soft Strategies for River Management and Reservoirs
The Shape of river valleys and River Processes
A rivers long profile changes over its course.
Every river has:
An Upper Course
Middle Course
Course / Gradient / Valley Shape / Channel Shape
Upper / Steep / V-Shaped valley with steep sides / Narrow and Shallow
Middle / Medium / Gently sloping valley sides / Wider and Deeper
Lower / Gentle / Very wide and almost flat valley / Very Wide and Very Deep
Lower Course
Long Profile- shows how the gradient of a river changes along its length.
Cross Profile- shows what a cross section of a river looks like at a specific point.
Why does the upper course have a different cross profile from the lower course?
- In the upper course of a river valley, vertical erosion is dominant.
- This makes the valley shallower than in the lower course.
- In the lower course of the river valley, lateral erosion is denominate.
- This makes the valleys much wider than the upper course.
Erosion
Erosion-the process of the river bed, sediment and valley being gradually worn away. There are four types of erosion:
Hydraulic Action: The force of the water breaks down particles away from the river channel.
Abrasion: Eroded rocks picked up by the river scrape and rub against the channel, wearing it away. Most erosion happens by abrasion.
Attrition: Eroded rocks picked up by the river smash into each other and break into smaller fragments. Their edges become rounded as they rub together.
Solution: River water dissolves some types of rock, e.g. chalk and limestone.
Transportation
Transportation - the process of a rivers eroded material moving downstream. This also happens in four different ways:
Traction: Large particles like boulders are pushed 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 like silt and clay are carried along by water. This may give the water a brown appearance.
Solution: Soluble materials dissolve in the water and are carried with the waters discharge.
Deposition
Deposition- when a river drops the eroded material that is has eroded and then transported. It happens when a rivers VELOCITY (how fast the river is flowing).
Velocity may decrease because:
The volume of water decreases
The amount of sediment in the water increases
The water is shallower
The river has reached the mouth
River Features and Landforms
Formation of a waterfall:
- Waterfalls form where a river flows over an area of hard rock followed by an area of hard rock.
- The softer rock is eroded quicker than hard rock, creating a step in the river.
- As water passes over step it erodes more and more of the softer rock.
- A steep drop is eventually created, which is called a waterfall.
- Meanders occur in the mid course and lower course of the river, where it is beginning to cut laterally as it gets closer to base level.
- Meanders are bends in the river, where the faster water on the outside of the bend has cut into the bank, eroding it and creating a river cliff.
- At the same time the slow moving water on the inside of the bend deposits its load, building up a shallow slip-off slope.
- Meanders migrate downstream as they cut through the valley sides.
This creates a line of parallel cliffs along the sides of the valley.
- In the lower course of the river meanders can become so pronounced that they can form ox-bow lake.
- In the lower course the rapid lateral erosion cuts into the neck of the meander, narrowing it considerably.
- Eventually the force of the river breaks through the neck of the meander.
- The old meander is left without any significant amount of water flowing through it.
- Quickly the river deposits material along the side of its new course, which completely blocks off the old meander, creating an ox-bow lake.
Levees- Natural embankments along the edges of a river channel.
- When a river floods 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 river channel.
Storm Hydrographs and discharge
Discharge -the amount of water in a river at any given point and time. Discharge is measured in cubic metres per second.
Hydrograph - a graph showing changes in river discharge over time in response to a rainfall event.Lag time - the time taken between peak rainfall and peak discharge.
Rising Limb - shows the increase in discharge on a hydrograph.
Falling Limb - shows the return of discharge to normal.
Peak Discharge - maximum discharge.
When describing hydrographs you must consider:
- Lag time
- Rising limb
- Precipitation intensity
- Peak flow and returning back to base flow
- Impermeable rocks will not allow water to pass through, resulting in large amounts of surface runoff and a greater flood risk as rivers respond quickly results in a short lag time.
- permeable rocks and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow - results in a longer lag time
- heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff (short lag time)
- slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow (longer lag time)
- impermeable man made surfaces such as concrete and tarmac are impermeable therefore rivers in urban drainage basins tend to have short lag times due to higher amounts of surface runoff and drainage systems taking water to rivers quickly.
- vegetated areas help to reduce flood risk by increasing the time it takes for water to reach a river (longer lag time) by encouraging infiltration
Flooding: LEDC’S VS MEDC’S
General Causes of Flooding
A steep sided channel can cause flooding by creating a surface run-off
A heavy consistent amount of rainfall e.g. Rainstorm, Monsoon e.c.t.
Deforestation or a deficiency of trees, vegetation and woodland (increasing surface run off)
Flooding can be caused when two rivers meet e.g. in Boscastle (Rivers Jordan & Valency)
Urbanisation is also a cause of flooding, as the less rural areas tend to have more concrete, an impermeable substance
MEDC Case Study:Carlisle,2005
CAUSES:
Heavy rainfall. 200mm of rain fell in 36 hours. The rainfall saturated the land causing the water table to be high, therefore causing surface run-off into the River Eden.
Carlise is urbanised, so there is lots of impermeable materials such as concrete, increasing surface run-off.
Primary Effects (immediate effects)
3 deaths
3000 people left homeless
4 schools severely flooded
350 businesses were shut down
Rivers were polluted with rubbish and sewage
Damaged infrastructure
Secondary Effects (delayed effects)
Children lost out on education
Stress-related illnesses due to floods
3000 jobs affected as businesses were ruined
RESPONSES:
Primary Responses (Immediate Responses)Evacuation
Reception centres were opened around Carlisle to provide food and drink for evacuees
Temporary accommodation was set up
Secondary Responses (Delayed Responses)
Community groups were set up to provide emotional support for people affected by floods
A flood defence scheme has been set up to improve flood defences.
LEDC Case Study: Bangladesh, July/August 2007
CAUSES:
Heavy rainfall. 900mm of rain fell in July. The rainfall saturated the land causing the water table to be high, therefore causing surface run-off.
Melting snow from the Himalayan mountains increased river discharge
EFFECTS:Primary Effects (immediate effects)
2000 deaths
2500000 people left homeless
44 schools severely flooded and destroyed
Factories flooded and livestock killed
Rivers were polluted with rubbish and sewage
10000 km of roads were destroyed
Secondary Effects (delayed effects)
Children lost out on education, 4000 schools affected
Unemployment
Rice fields destroyed, so rice production became scarce
100,000 people caught water-borne diseases such as diarrhoea and dysentery
RESPONSES:
Primary Responses (Immediate Responses)
People didn’t evacuate
Governments and charity groups sent out food, water and medical aid
Secondary Responses (Delayed Responses)
International charities funded rebuilding homes agriculture and industry
Homes were built on stilts so less likely to be affected by an future floods.
River Management
Flood Control
Flooding can be prevented using a range of different strategies. These strategies are split up into two groups, “hard engineering” and “soft engineering”.
Hard engineering: Hard engineering strategies are usually more expensive and less sustainable, however tend to last longer, work more efficiently and work immediately.
Soft Engineering: Soft engineering strategies are usually less expensive and more sustainable. They are not as good at preventing floods, but are cheap.
Hard Engineering:
- Dams and Reservoirs. Pros: generate hydroelectric power, Reduce flooding and store water Cons: Expensive to build, Water pollution
- Raise Levees. Pros: prevent rivers from bursting banks, existing land uses can be maintained Cons: Needs lots of maintenance
- Straighten rivers. Pros: Relatively cheap, Re-attracts wildlife Cons: flood plains will turn to “wetland”, Needs lots of maintenance
- Afforestation (plant trees in drainage basin) Pros: reduces amount of water that reaches river Cons: Forests generate little income
- Houses on flood plains are made “flood proof” Pros: Stops owners from worrying Cons: Doesn’t stop a flood from happening
In the UK, the supply and demand for water differs.
On average the annual rain in the UK shows that Scotland, the north of England, the west of England and Wales have the largest amount of precipitation on average each year.
On average the population density shows how many people are in each area. Scotland, Wales and the north of England have low population densities.
This means basically these areas are water surplus.
This means that that on average, the high population areas have a water deficit, and the low population areas have water surplus.
The UK manages its water supply and demand problems. It does this by:
Transferring water from areas of surplus to areas of deficit. (For example Wales transports water to Birmingham
Fixing leaky pipes means less water is lost by transfer.
Build more reservoirs to store more water.
Reduce amount of water used.
Encouragement of water meters..
UK Reservoir Case Study: Rutland Water, Northumberland
IMPACTS:
Economic Impacts (Industry and money related impacts)
Boosts the local economy as it is a tourist attraction because of the wildlife and recreation facilities
6km2 of land was flooded, some farmers lost their livelihood(N)
Social Impacts
Recreational facilities take place on and around reservoir
Jobs have been created to build and maintain the reservoir
Schools use the reservoir for educational visits
Two villages were demolished to create space for the reservoir(N)
Environmental Impacts
Is a site of special scientific interest, where wildlife is protected
Hundreds of species of birds live around the reservoir
A variety of habitats are found around the reservoir
As land was flooded, habitats were destroyed(N)
The supply of water from the reservoir has to be sustainable. This means the water taken out for the use of people has to be able to be replaced by rivers that supply it.
Key Words
Precipitation- Any type of water falling from the clouds, rain, hail, snow, etc
Drainage Basin- The area of land that is drained by a river and all its tributaries.
River Discharge- the volume of water in a channel passing a particular point in a particular time, usually cubic metres per second or cumecs.
Erosion- the wearing of rock or soil in the river i.e. a waterfall, water taking away the soft rock and leaving the hard rock. / Surface Runoff- water leaving the drainage basin.
Hydrograph- a graph which combines a bar chart of precipitation in a particular event with a line graph of discharge for a particular river channel in the catchment area of that event and thus the reaction of the channel to the precipitation.
Deposition- when a river drops the eroded material that is has eroded and then transported.
Transportation - the process of a rivers eroded material moving downstream.
Summary Questions
- What is the difference between the long profile and the cross profile of a river? (2 Marks)
- Explain why the upper course of a river valley has a different cross profile from the lower course. (4 Marks)
- Name and describe two other processes by which material is transported in Rivers. (4 Marks)
- Deposition occurs when rivers slow down. Why do Rivers Slow down? (4 Marks)
- Explain the formation of a River cliff. (3 Marks)
- Explain the formation of an ox-bow lake. (6 Marks)
- Describe, using a diagram the formation of Waterfalls. (6 Marks)
- What is a flood plain? (1 Mark)
- What are levees? and explain how they are formed (3 Marks)
- Explain what the following terms mean: i) Peak discharge ii) Lag time (2 Marks)
- Study Fig 1. On Pg. 42, why are the two storm hydrographs different shapes? (6 Marks)
- Describe and compare the primary and secondary effects of flooding in an MEDC and in an LEDC. (6 Marks)
- What is meant by hard and soft engineering strategies? (2 Marks)
- Give two ways in which the supply of water in the UK can be managed. (2 Marks)
- Describe the economic, social and environmental impact of a studied Reservoir (8 Marks)