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Waterfall and gorge of recession formation

When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form.

A river flows over a fault line. The cap rock is a hard resistant rock whilst the underlying rock is softer.

When the water flows over the fault line, the river erodes a plunge pool by abrasion (where the rocks carried by the river hit the bed, eroding it); by solution (where salts in the rock are dissolved by the water) and by hydaulic power (where the sheer force of the water erodes the rock).

Over time, the cap rock collapses into the plunge pool because it is not supported underneath

This happens again and again so a gorge of recession is formed.

At High Force, a waterfall on the River Tees, the cap rock is Whinstone

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A meander is a bend in a river

On the outside of the bend the river's velocity is fastest which causes the river to erode a river cliff

The erosion is by abrasion, saltation and hydraulic power

On the inside of the bend, the river's velocity is slower

This means there is more deposition so a slip-off slope is formed

There are meanders on the River Tees near Yarm

In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster

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Oxbow Lakes

An oxbow lake is a crescent-shaped body of water.

It occurs where a meander is cut off at the neck.

The velocity of the river is fastest here and as the water swings around the neck it erodes it, making it narrower over time

When a storm occurs, the water rushes through the neck

Deposition occurs closing the old meander.

The river erodes the neck by abrasion, solution and hydraulic power

There are many ox-bow lakes along the Mississippi near St Louis

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Deltas are found at the mouths of large rivers

When the water reaches the sea, it loses a lot of its energy so deposits its load

A delta occurs when the sea cannot remove the sediment as fast as the river deposits it

There are three main types of delta:

  • Fan-shaped or arcuate - There are many distributaries and it has a rounded shape (e.g. the Nile delta
  • Cuspate - has a triangular shape and has few distributaries e.g. the Tiber delta
  • Bird's foot - each part of the river juts out into the sea e.g. the Mississippi delta
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A levee is a rasied embankement that runs either side of a river

It is a depositional land form

In times of a flood, when a river is laden with sediment, it may burst its banks

Then the velocity of a river falls so it cannot carry large bolders and stones

These are deposited first and build up the levees

Sometimes the levee gets so high that the river flows higher than the flood plain

Levees occur on the River Rhine in the Netherlands

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Flood plain

A flood plain stretches either side of a river and is made by erosion and deposition.

The meanders migrate down stream eroding into the valley sides making the valley wider

The erosion processes are abrasion, hydraulic power and solution

Deposition occurs when the river which is laden with sediment, bursts its banks

The sediment is spread over the valley floor

There is a flood plain either side of the River Thames

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Case study: River Clyde

The River Clyde is about 160km long

its source is in the Southern Uplands region of Scotland and the river flows north-west through Glasgow

the mouth of the river is an estuary on the west coast of Scotland

  • The source is in the Lowther Hills where two tributaries met
  • There are two interlocking spurs at Crawford. The spurs are between 300 and 500 m high
  • The Falls of Clyde are four waterfalls near Lankark. The highest fall is Corra Linn - its about 27 metres high. There is also a gorge formed by the waterfall retreating
  • There is an oxbow lake forming near the village of Uddingston
  • The river meanders between Motherwell and Glasgow
  • The estuary is 34 km west of glasgow where the river joins the firth of Clyde and meets the Irish sea
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Causes of river flooding:

The UK floods in 2007 where caused by physical and human factors.

June had doubled the average amount of rainfall.

Previous months had also been wet - it was the wettest period from May to June since records began in 1776.

This meant the ground was saturated and could not absorb anymore water

Hull recieved 100mm of rainfall in 24 hours; this was more than the amount it usually recieved in a month

The rain was due to a series of depressions that crossed the UK from the Atlantic Ocean

Human factors made the situation worse:

  • People are increasingly building on the land which creates an impermeabke surface - for instance people in Bedford have covered drive ways with paving
  • Wetland areas in the Yorkshire Moors have also been drained increasing surface run-off
  • People have built homes in flood plains a problem in Maidenhead particularly
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Causes of river flooding

In 2007, the rivers Brahmutra and Ganges flooded Bangladesh.

Physical factors caused the flooding:

Bangladesh is a very low-lying area - most of the land is less than 1 m above sea level as it is a rivre delta.

Cylone Yemyin brought heavy rain to the area and it rained for about 20 days

The monsoon rains in the Himalayas were also very heavy

Human factors made the situation worse:

There had been deforestation in Tibet and Nepal, this means the water is not stored in trees and flows into rivers

there had been urbanisation in India, ecspecially in Delhi meaning the rainwater flowed into rivers rather than the ground

Embankments built to stop the flooding forced it down stream cauing more flooding instead 

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Case study: Boscastle floods

The Boscastle floods, 16th August, 2004


  • Rain - 60mm in two hours, remnants of hurricane Alex, tall convectional clouds, hot day
  • Valley Shape - Steep sided valleys of R.Valency and R.Jordan, the valleys funnel the water into Boscastle
  • Geology - impermeable rocks + urbanisation
  • Human - built on a flood plain at the end of two narrow valleys


  • Effected 1000 residents however there were no deaths and only one minor casualty
  • 7 helicopters airlifted 100 people (including 6 fire fighters)
  • 29 of the 31 Cornwall County Fire Brigade stations were involved for 7 days
  • 58 properties were flooded, 25 businesses (40 further in other villages)
  • 84 wrecked cars were found
  • Eventual cost with management: £15million
    To prevent it happenning again a new culvert is being built on the River Jordan 
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Effects of Bangladesh floods

There were short term consequences;

Nearly a third of the country was flooded

Over 400 people were killed and 7.5million people fled their homes

The floods cost over $0.5 billion

The crops in the fields were destroyed and the farm animals killed

In the long term, the government has had to spend money on rebuilding rather than educating, furthering poverty

As the crops were damaged people went hungry and suffered from malnutrition

There was very little clean drinking water so over 50,000 people got diarrhoea meaning they couldn't work and earnt even less money

People's seed were destroyed so couldn't plant crops the next year

However the fields were fertilised by the alluvium from the river

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Responses to Bangladesh flood

The area recieved aid from organisations such as UNICEF and Care International.

The Bangladesh government has tried to reinforce flood defences around Dhaka but this can increase flooding elsewhere

There is a debate whether to protect rural or urban areas

The government is trying to build more cyclone shelters as over 400 people were killed in 2007

However it is a very poor country with a GDP per person of less than $1,400 so money is rare

To warn people of the floods, volunteers cylce around with megaphones urging movement to safety

The government wants to improve the system

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Responses to Boscastle floods


Braiding was encouraged as deposition on the river bed slows down the flow and removes material

Debris Screen - wire mesh to collect the load (500 tonnes collected in a test)

Trees felled -  Traps debris

Raised and widened car park, re-enforcing the grass as a permeable surface

Widened and deepened channel

Bridge removed as it created a constriction

A culvert was introduced ensuring the Jordan flowed into the Valency more easily

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Rich vs. Poor impacts of flooding

In 2007 there were floods in the UK and in Bangladesh

The Bangladesh floods caused $0.5billion of damage whilst in the UK it was $2billion

This is because people in Bangladesh are not as rich and have fewer possessions to lose

In Bangladesh 400 people died compared to only 13 in the UK

This is because the UK has a smaller population density so fewer people were in danger

The UK is a rich country and most could get away in their cars on the motorways, whilst in Bangladesh there are not enough Cyclone shelters and some don't have radios or TV

In the UK people can easily check the weather using the Environment Agency Flood Warning website

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Hard and soft engineering

Hard engineering involves the use of concrete whilst soft engineering involves working with nature.

Hard strategies are expensive and involve engineering the river - cutting off meanders and building dams.

However soft strategies are cheaper; they involve putting in meanders and reed beds encouraging the river to flow down its natural course

Soft engineering is relatively sustainable and works with nature.

It is cheap because it is easy to construct and maintain. It involves understanding the river and how it will flow in the future.

Reed beds absorb water so reduce flooding and putting in meanders gives the river more space so it won't flood a built up area

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Transferring water

Wales has a surplus of water with a rainfall level of over 1500mm per year. This is relief water as the land in Wales is mountainous. Snowdon is over 1,000 metres high

It has many large rivers such as the river Usk. It has a population density of less than 50 people per km squared so there is relatively little demand for water

The steep sided valleys in Wales are easily dammed meaning the water can be stored. 

The situation is very different in Greater London, which is an area of deficit. The area has less than 60mm of rain per year. The major river is the Thames but this is salty when passing through London so cannot be used for drinking.

The population for the whole of Wales is only 3 million whislt Greater London has a population of 7million. Some parts of London have a population density of 5,000 people per km squared so there is a very high demand for water

The Thames basin is less than 30 m above sea level so it is not suitable for large resevoirs meaning the water is very expensive to store and collect unlike in Wales

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George G Heal


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