The River Valley
- The shape of the valley and the channel changes along the river depending on whether erosion or deposition is the dominant porcess.
- The long profile of a river shows you how the gradient changes over the three different courses (the upper course-near the source of the river, the middle course, and the lower course - at the mouth of the river). It is a summary of the shape and gradient of a river bed from source to mouth.
- The cross profile shows you what a cross section of the river looks like.
- Upper course - steep gradient, v shaped valley, steep sides, narrow shallow channel.
- Middle course - medium gradient, gently sloping valley sides, wider deeper channel
- Lower course - gentle gradient, very wide, almost flat valley, very wide, deep channel.
- Vertical and lateral erosion change the cross profile of a river. Both types happen at the same time but one is usally dominant over the other at different points in the river.
- Vertical erosion deepens the river valley and channel, making it V-shaped. It is dominant in the upper course.
- Lateral erosion widens the river valley and channel. It's dominant in the middle and lower courses.
- There are 4 processes of erosion.
- Hydraulic Action - the sheer force of the water breaks rock particles away from the bank and bed.
- Corrasion - small particles picked up by the river scrape and rub against the river beds and banks, wearing them away in a sandpapering effect.
- Attrition - eroded rocks picked up by the river collide and break into smaller pieces. Their edges are rounded and smoothed off as the rub together.
- Corrosion - mild acid in the water dissolves some types of rock such as limestone and chalk.
- The faster a river is flowing the more erosion happens.
Transportation and Deposition
- Transportation is the movement of eroded material. The material a river has eroded is transported downstream.
- There a four processes of transportation.
- Traction - large boulders roll along the river bed. They are pushed by the force of the water.
- Saltation - pebbles are bounced along the river bed by the force of the water and are picked up and then dumped as the flow of the river changes.
- Suspension - smaller particles like silt and clay are carried along by the water, giving it a brown appearance.
- Solution - Soluble minerals such as chalk and limestone dissolve in the water and are carried along in the flow, even though they cannot be seen.
- Deposition is when a river drops the eroded material that it's transporting.
- It happens when a river slows down (loses velocity).
- There are a few reasons why rivers slow down and and deposit material..
- A river will deposit material if the volume of water in a river falls, the amount of eroded material in the water increases, the water is shallower (e.g in the inside of a bend) or the river reaches it's mouth.
Meanders and Ox-Bow Lakes
- When a river erodes and deposits material, meanders can form.
- In the middle and lower course of a river large bends called meanders develop.
- The current (the flow of water) is faster on the outside of the bend because the river channel is deeper (there's less friction to slow the water down).
- So more erosion takes place on the outside of the bend, forming river cliffs.
- The current is slower on the inside of the bend because the river channel is shallower (there's more friction to slow the water down).
- Therefore eroded material is deposited on the inside of the bend, forming slip-off slopes.
- So the outside of the bend - erosion, and the inside of the bend - deposition.
- Meanders get larger over time. They can eventually turn into an Ox-Bow Lake.
- Erosion causes the outside bends to get closer until there's only a small section of land left between the bends (called the neck)
- The river breaks through this land, usually during a flood, and the river flows along the shortest course.
- Deposition eventually cuts off the meander forming an Ox-Bow Lake.
Waterfalls and Gorges
- Waterfalls and Gorges are formed by erosion.
- Waterfalls form when a river flows over an area of hard rock followed by an area of soft rock.
- The soft rock is eroded more quickly than the hard rock, creating a 'step' in the river.
- As the 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. It becomes unsupported and collapses.
- The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock through abrasion. This creates a deep plunge pool.
- Over time, more undercutting causes more collapses. The waterfall will retreat (move back up the channel) leaving behind a steep-sided gorge.
- Waterfalls are found in the upper course of a river.
Flood Plains and Levees
- The flood plain is the wide valley floor on either side of a river which occasionally gets flooded.
- When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting. This builds up the flood plain (it gets higher).
- Meanders 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. During a flood, eroded material is deposited over the whole flood plain but the heaviest material is deposited closest to the river channel because it is dropped first when the river slows down. Over time the deposited material builds up, creating levees at the sides of the channel such as along the Yellow River in China.
River Discharge and Hydrographs
- River discharge is the volume of water flowing in a river per second. It is measured in cumecs (cubic metres per second).
- Hydrographs show how the discharge at a certain point in a river changes over time.
- Peak discharge is the highest discharge in the period of time you're looking at.
- Lag time is the delay between peak rainfall and peak discharge.
- Rising limb is the increase in river discharge as rainwater flows into the river.
- Falling limb is the decrease in river discharge as the river returns to it's normal level.
- Lag time happens because most rainwater doesn't fall directly into the river channel. There's a delay as rainwater gets to the channel by flowing quickly overland (surface runoff) or by soaking into the ground (infiltration) and flowing slowly under ground.
River discharge is affected by different factors - the more water that flows as surface run off, the shorter the lag time (it travels more quickly). This means discharge will increase because water gets to the channel more quickly.
Factors that Affect River Discharge
- Amount and type of rainfall - Lots of rain and short, heavy periods of rainfall mean that there is more runoff. Lag time is decreased, so discharge increases.
- Previous weather conditions - After lots of rain, soil can become saturated (it can't absorb any more water). More rainwater won't be able to infiltrate into the soil so surface run off increases. Lag time is decreased, so discharge increases.
- Land use - urban areas have drainage systems and are covered in lots of impermeable materials (like concrete) which increase run off. Lag time is decreases so discharge increases.
- Temperature - Hot, dry conditions and cold, freezing conditions both result in hard ground. This increases run off and lag time is decreased, so discharge increases.
- Rock type - Water infiltrates through pore spaces in permeable rock and flows along cracks in pervious rocks - therefore there isn't much runoff. Lag time is increased so discharge increases. However water can't infiltrate into impermeable rock. This means there is lots of runoff. Lag time is decreased so discharge increases.
- Relief (how the height of the land changes) - lots of runoff occurs on steep slopes. Lag time is decreased, so discharge increases.
- Flooding happens when the level of a river gets so high that it spills over it's banks. Sometimes a flood happens without warning - this is a flash flood.
- The river level increases when the discharge increases because a high discharge means there is more water in the channel. This means the factors that increase discharge can also cause flooding.
- Physical factors that can cause flooding are; prolonged rainfall, heavy rainfall, snowmelt and relief (how the height of the land changes).
- Rivers also flood because of human factors such as deforestation (trees intercept rainwater on their leaves, which then evaporates. Trees aslso take up water from the ground and store it. Therefore, cutting down trees increases the volume of water that reaches the river channel, which increases discharge, and makes flooding more likely) and building construction (buildings are often made from impermeable materials such as concrete, and they're surrounded by roads made of tarmac, which is also impermeable. Impermeable surfaces increase runoff and drains quickly take runoff to rivers. This increases discharge quickly, which can cause a flood).
Flooding in an MEDC
**Carlisle, England - 8th January, 2005, River Eden**
Causes- Heavy rainfall (20mm of rain in 36 hours). This saturated the soil and increased runoff to the River Eden. Carlisle is large urban area full of impermeable materials like concrete, which increased runoff. Discharge in the River Eden reached 1520 cumecs! (it's average is only 52 cumecs!)
Primary effects - 3 deaths, 3,000 people made homeless, 4 schools severly flooded, 350 businesses were shut down, 70,000 homes lost power, some roads and bridges were damaged, river polluted with rubbish and sewage.
Secondary effects - Children lost out on an education (one school was closed for months), stress related illnesses increased after the flood, 300 jobs were at risk in businesses affected by the floods.
Immediate response - People were evacuated from areas that flooded, reception centres set up to provide food and drink for evacuees, temporary accommodation was set up for the homeless
Long term response - Community groups set up to provide emotional support and to give practical help to people affected by the floods, a flood defence scheme has been set up to improve flood defences, e.g build up banks on the River Eden to prevent flooding.
Flooding in an LEDC
**South Asia (Bangladesh and India) - July and August, 2007 - River Brahmaputra and Ganges**
Causes - heavy rainfall (in one region 900mm of rain fell in July). The continous rainfall saturated the soil, increasing run off to rivers. Melting snow from glaciers in the Himalayan mountains increased discharge to the Brahmaputra river. The peak discharge of both rivers happened at the same time, which increased the discharge downstream.
Primary effects - over 2000 deaths, around 25 million people made homeless, 44 schools totally destroyed, many factories closed and lots of livestock killed, 112,000 houses destroyed in India, 10,000 km of roads destoryed, rivers polluted with rubbish and sewage.
Secondary effects - Children lost out on an education (around 4000 schools were affected by the floods), around 100,000 people caught water-borne diseases like dysentary and diarrhoea, flooded fields reduced basmati rice yields (prices rose 10%), many farmers and factory workers became unemployed.
Immediate response - Many people didn't evacuate from areas that flooded and blocked transport links slowed down any attempted evacuations, other governments and international charities ditributed food, water and medical aid. Technical equipment like rescue boats were also sent to help stranded people.
Flooding in an LEDC - continued
Long-term response - International charities have funded the rebuilding of homes and the agriculture and fishing industries, some homes have been rebuilt on stilts, less likely to be damaged by floods.
Hard engineering is man made structures built to control the flow of rivers and reduce flooding.
There are 2 main methods of hard engineering;
- Dams and reservoirs - Dams (huge walls) are built across the rivers, usually in the upper course. A reservoir (artificial lake) is formed behind the dam. Benefits - reservoirs store water, especially during periods of prolonged or heavy rain, which reduces flooding, the water in reservoir is used as drinking water and can be used to generate hydroelectric power. Disadvantages - dams are very expensive to buid, creating a reservoir can flood existing settlements, eroded material is deposited in the reservoir and not along the river's natural course so farmland downstream can be less fertile.
- Channel straightening - The river's course is straightened. The meanders are cut out by building artificial straight channels. Benefits - water moves out of the area more quickly because it doesn't travel as far, reducing the risk of flooding. Disadvantages - flooding may happen downstream of the straightened channel instead, as flood water is carried there faster. There's more erosion downstream because the water's flowing faster.
Soft engineering is schemes set up using knowledge of a river and its processes to reduce the effects of flooding.
There are 4 main mathods of soft engineering;
- Flood warnings - The Environment Agency warns people about possible flooding through TV, radio, newspapers and the internet. Benefits - the impact of flooding is reduced. Warnings give people time to move possessions upstairs, put sandbags in position and to evacuate. Disadvantages - warnings don't stop a flood from happening, living in a place that gets lots of warnings could make it difficult to get insurance, people may not hear or have access to warnings.
- Preperation - Buildings are modified to reduce the amount of damage a flood could cause. People make plans for what to do in a flood (they keep important documents and items like torches and blankets in a handy place). Benefits - the impact of flooding is reduced (buildings are less damaged and people know what to do if a flood happens), people are less likely to worry if they're prepared. Disadvantages - preparation doesn't guarantee safety from a flood, it could give people a false sense of security, it's expensive to modify homes and businesses.
- Flood plain zoning - Restrictions prevent building on parts of a flood plain that are likely to be affected by a flood. Benefits - the risk of flooding is reduced (impermeable surfaces aren't created such as buildings and roads), the impact of flooding is reduced as there aren't any houses or roads to be damaged. Disadvantages - the expansion of an urban area is limited if there aren't any other suitable building sites, it's no help in areas that have already been built on.
- 'Do nothing' - No money is spent on new engineering methods or maintaning current ones. Flooding is a natural process and people should accept the risks of living in an area that's likely to flood. Benefits - the river floods, eroded material is deposited on the flood plain, making farmland more fertile, it doesn't cost any money. Disadvantages - the risk of flooding and the impacts of flooding are not reduced, a flood will probably cause a lot of damage.
Managing the UK's Water Supply
- The demand for water is different across the UK.
- The north and the west have high rainfall, which means they have a good supply of water.
- The south east and the midlands have high population densities, which means there is a high demand for water.
- The south east and midlands are areas of water deficit (there's a greater demand than supply)
- The north and west are areas of water surplus (there's a greater supply than demand)
- One way to deal with the supply and demand problem is to transport water from areas of surplus to areas of deficit e.g. Birmingham is supplied with water from the middle of Wales.
- Issues with water transfer are; the dams and aqueducts (bridges used to transport water) that are needed are expensive, it could affect the wildlife that lives in the rivers (e.g. fish migration patterns could be disrupted by dam building), there might be political issues (people may not want to give their water to another country).
- Another way to increase water supplies in deficit areas is to build more reservoirs to store more water. However building a reservoir can involve flooding settlements and relocating people.
- Or by fixing leaky pipes. Less water would be lost during transfer.
- The supply and demand problem could also be dealt with by reducing the demand for water. People can reduce the amount of water that they use (taking showers instead of baths, using hosepipes less, etc). Water companies want to install water metres as they charge people for the exact amount of water they use so people will be more careful.
A UK Reservoir
**Case Study - Rutland Water - the East Midlands**
- The dam was built and the reservoir created during the 1970s.
- The reservoir covers 12km squared and is filled with water from the River Welland and the River Nene.
- Rutland water was designed to supply the East Midlands with more water - enough to cope with the rapid poulation growth in places like Peterborough.
- Economic Impacts - The reservoir boosts the local economy (it's a popular tourist attraction because of the wildlife and recreation facilities), around 6km squared of land was flooded to create the reservoir including farmland so some farmers lost their livelihoods.
- Social Impacts - Lots of recreational activities take place on and around the reservoir (such as sailing, cycling, windsurfing,etc), many jobs have been created to build and maintain the reservoir and to run the nature reserve and recreational activities, schools use the reservoir for educational visits, two villages were demolished to make way for the reservoir.
- Environmental Impacts - Rutland water is a Site of Special Scientific Interest (SSSI) so the wildlife is protected, hundreds of species of birds live around the reservoir, a variety of habitats are found around the reservoir which means there is more biodiversity, Ospreys (fish-eating birds that were extinct in Britain) are being reintroduced in the Rutland Osprey Project, a large area was flooded to create the reservoir which destryed some habitats.