Key Idea 4: River Processes and Landforms
4.1 What are the processes active in a drainage basin?
The Drainage Basin
Flow and Stores of Water – the hydrological cycle
Precipitation
The movement of water from the atmosphere to the ground. It may be in many forms including rain,
hail and snow.
Transpiration
Water loss from plants through pores in the leaves.
Evaporation
Where water changes state from liquid into vapour.
Stem flow
Rainwater that falls onto vegetation and then trickles down branches and stems.
Infiltration
The movement of water from the ground surface into the soil.
Evapotranspiration The combined loss of water from plants by both evaporation and transpiration.
Percolation
The movement of water out of the soil and into the rocks below.
River flow
Water moving in rivers towards an ocean, sea or lake.
Overland flow
The flow of water across the ground surface.
Throughflow
The downhill flow of water through soil.
Groundwater flow The flow of water through rocks. (Groundwater also acts as a store of water below the water table).
Condensation
The process in the water cycle where water changes from a gas state to a liquid.
Interception
Water being slowed down and trapped by trees and vegetation.
Hydrographs
A hydrograph shows the relationship between a rain storm and the discharge (amount of water flowing in a river) over time.
Peak Rainfall
Falling Limb
The hour of greatest rainfall during The period of time when the rivers
a storm
discharge is falling
Peak Discharge
The time of maximum discharge
by the river
Normal Flow
The base flow of the river
Lag Time
The period of time between peak
rainfall and peak discharge
Rising Limb
The period of rising river discharge
following a period of rainfall.
A hydrograph gives you clues about how a river responds to a rain
storm and can tell you what the drainage basin is like and how likely a
river is to flood.
A river with a hydrograph that has a steep rising limb is
more likely to flood as this is showing that water is
getting to the river faster and so will be more likely to
overflow.
- Impermeable rocks (e.g. granite) and soil (e.g. clay) will not
allow water to pass through, resulting in large amounts of surface
runoff and a greater flood risk as rivers respond quickly.
- Drainage basin with a steep gradient will result in greater
overland flow.
- Heavy rain results in rapid saturation of the upper soil
layers and the excess water therefore reaches streams
quickly as surface runoff.
- Impermeable man made surfaces such as concrete and
tarmac speed up surface run off and therefore the speed of
water into a river channel.
- Areas cleared by deforestation will respond quickly to
rainfall due to the reduced interception.
- Small Drainage Basin - water will enter the river quicker.
- If the soil has already been saturated by heavy rain its
infiltration capacity will be reduced and further rain will go as
surface runoff.
- If the ground surface is frozen lag time is short as water
cannot infiltrates and passes quickly to the river as runoff.
A river with a hydrograph that has a less steep rising
limb is less likely to flood as this is showing that water
is getting to the river slower and so will be less likely to
overflow.
- Permeable rocks and soil have a high infiltration
capacity and will absorb water quickly, reducing overland
flow.
- Drainage basin with a less steep gradient allowing more
time for infiltration to occur.
- 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 through flow
and groundwater flow.
- Vegetated areas help to reduce flood risk by increasing
the time it takes for water to reach a river by encouraging
infiltration (roots opening up the soil), intercepting water by
their leaves and taking up water in their roots.
- Large Drainage Basin - water will take longer to reach the
river.
- The presence of a dam will allow flow to be controlled,
reducing flood risk and allowing rivers to gradually respond
to heavy rainfall in a controlled way.
- If the soil is dry it will be able to absorb more water during
infiltration and therefore the lag time will be longer.
To summarise…
-
Anything that slows down the speed that water gets to a river (interception or increasing infiltration or slowing the
speed of flow) will give a longer lag time.
Anything that speeds up the rate at which water gets to the river (more overland flow or faster overland flow and no
infiltration) will give a shorter lag time and make the river more likely to flood.
4.2 How do geomorphological processes contribute to the development of distinctive fluvial
landforms?
4.3 What landforms are produced?
Processes
There are 3 main processes in rivers:
- Erosion, Transportation and Deposition
These take place in different ways at different points of the rivers journey. Different amounts of energy result in different
processes occurring.
Erosion
Hydraulic action
Corrasion/abrasion
Attrition
Solution
The wearing away of the landscape
river uses the power of the water to break up the bed and banks
river uses material/rock to rub against the bed and carry along/ wear away/ rub away banks
rocks bang together and break up
river uses chemicals to dissolve the bed and banks
Transportation
Traction
Saltation
Suspension
Solution
The movement of material by a river.
The rolling of boulders along the bed.
The bouncing of rocks along the river bed.
When a river carries small pieces of rock by them floating down the river channel.
When a river dissolves sediment and carries it along the channel.
Deposition
When a river loses energy and places / puts down the material it was carrying.
A Rivers Journey
A river changes shape as it flows from its source (where a river starts) to its mouth (where a river flows into a sea
or lake). The shape of both the long profile (a slice through the river from source to mouth) and the cross profile (a
slice across the river) changes. The source of a river is often - but not always - in an upland area. Near the source, a
river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas
are usually composed of hard igneous rocks, which are ideal for forming such features.
As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river
bed and carves out steep-sided V-shaped valleys.
As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it
approaches the sea. The discharge (amount of water flowing) will increase as the river approaches the sea.
Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction. As
a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered
from the steep valley sides. The sediment in the river creates turbulence and friction.
As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more
lateral erosion making the channel wider, and smoother. As a result there is less turbulence and friction, making the flow of
water more efficient.
Long profiles
Over the long profile of the river, it is steeper
near the source, where there is a lot more
material for the river to cut through to reach its
base level. As you go further down the river it has
cut further down towards base level and is
beginning to cut laterally (sideways) as well. The
slope angle of the river decreases. By the time
you reach the lower stages of the river it almost
flows over a flat surface, as it meanders its way
towards the sea.
Landforms Created
Upper-course river features include steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.
Middle-course river features include wider, shallower valleys, meanders, and oxbow lakes.
Lower-course river features include wide flat-bottomed valleys, floodplains and deltas.
As the river moves through the upper course it cuts downwards. The gradient here
is steep and the river channel is narrow. Vertical erosion in this highland part of the
river helps to create steep-sided V-shaped valleys, interlocking spurs, rapids,
waterfalls and gorges.
Interlocking spurs on a tributary of the Yangtse
-
As the river erodes the landscape in the upper course, it winds and bends to avoid areas of hard rock. This creates
interlocking spurs, which look a bit like the interlocking parts of a zip.
When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form.
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The main characteristics of a
V - shaped valley and its river:
-
-
The valley is narrow with a narrow,
shallow river channel
The valleys have steep sides
The channel has a steep gradient with
features such as pot holes, rapids and
waterfalls
The water is mainly slow flowing as
most of the rivers energy is used to
overcome the friction of the river bed &
obstructions
The load is mainly large, angular and
rough
Waterfall Formation
Waterfalls form where the river bed has a layer of harder
rock overlying the softer rocks.
1. As water flows over the river bed the softer rock is
eroded much more quickly than the harder rock; therefore
undercutting the harder rock.
2. The hard, overhanging rock eventually collapses.
3. The collapsed rocks fall into the plunge pool
causing more erosion of the soft rocks and a deeper
plunge pool.
4. This goes on continuously causing the waterfall to
retreat upstream.
5. The waterfall leaves a steep sided gorge as it
retreats.
Meanders and Ox-Bow Lakes
Meanders - 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 basically 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.
Ox-Bow Lakes - In the lower course of the river
meanders can become so pronounced that they can form
ox-bow lakes. 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, and as this is the easiest way for the
water to go, 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 block off the old meander, creating an ox-bow
lake.
Cross Section of a Meander
Floodplains and Natural Levees
The river now has a wide floodplain. A floodplain is the area around a river
that is covered in times of flood. A floodplain is a very fertile area due to the
rich alluvium deposited by floodwaters. This makes floodplains a good place
for agriculture. A build up of alluvium on the banks of a river can create
levees, which raise the river bank.
Rivers flood on a regular basis. The area over which they flood is known as
the floodplain and this often coincides with regions where meanders form.
When rivers flood in the middle valley the cover an area of land known as the
flood plain.
When they flood velocity is slowed and deposition of any rocks being
transported is encouraged.
This deposition leaves a layer of sediment across the whole floodplain.
After several floods there are several layers of sediment (rocks) deep on the flood plain.
In addition, the largest rocks and most deposition occurs next to the river channel. This leaves a ridge of higher material
next to the river channel on both banks of the river known as a levee.
Levees are basically caused by floods. In times of low flow any deposition takes place on the river bed and this raises the
height of the river bed.
In times of flood the water leaves the channel. As it does so it loses energy and the courser and heavier material is
deposited near to the river on the banks. Finer material is carried further onto the flood plain. After many floods the river
builds a bank on either side of the river and this can lead to catastrophes. As the river has been raised above the flood plain
and the banks are often higher still, when the river floods the water can no longer drain back into the river channel and often
has to form a new one.
Deltas
Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its
material faster than the sea can remove it.
Some rivers reach the sea in deltas, which form where river mouths become choked with sediment, causing the main
river channel to split into hundreds of smaller channels or distributaries.
Deltas only form under certain conditions
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The river must be transporting a large amount of sediment
The sea must have a small tidal range and weak currents
The sea must be shallow at the river mouth
4.4 What physical and human factors contribute to cause river flood hazards?
Rivers flood more when water flows overland rather than infiltrating into the soil. Therefore conditions and activities which
increase overland flow are the main causes of flooding.
Physical causes (Natural)
Factor
Heavy rainfall
So What / Explanation
This leads to large amounts of overland flow in a short amount of time as there is too much rain all
at once to infiltrate the soil. Water gets to the river quickly and the river will overflow.
Prolonged rainfall
This means the ground is already saturated and when there is more rain, it cannot infiltrate the soil.
This causes overland flow and too much water in a river.
Impermeable rock
If water cannot soak through soil and underlying rock it means there is more overland flow. Rivers
eg clay or granite in fill quickly and can overflow their banks.
the river catchment
Steep valleys in the This means that water flows quickly over the surface and is unable to infiltrate. Water reaches the
drainage basin
river quickly and is more likely to flood.
Warm temperatures If there is a sudden temperature rise leading to lots of snow melt, rivers receive large volumes of
and snow melt
extra water quickly and cannot cope. This means they may flood.
Human causes (Man Made)
Factor
Growing population
needing more
housing
Urbanisation in a
drainage basin
Deforestation of a
river catchment
Arable (crop) farming
in large areas of the
drainage basin
So What / Explanation
Pressure to build more houses means land that is unsuitable is built on and therefore more flood
plains have houses. If there was no building there, there would not be a flood issue as the water
would be covering empty land.
Building towns and houses means there is more concrete, tarmac and drainage systems. These
speed up the rate at which water reaches the river as water cannot infiltrate through concrete and
drains feed straight into rivers. The extra speed and volume of water can lead to flooding.
When trees are removed it reduces the amount of interception, storage and transpiration. This
means there is more water in the drainage basin and river discharge increases.
Growing arable crops means that the soil is bare during the non-growing season. This reduces the
amount of interception and increases the overland flow to a river.
4.5 What effect does flooding have? For specific examples see Case Study 10 - Boscastle
LEDC – Bangladesh Floods
Effect
Telephone, water, electricity and
gas supplies were all immediately
interrupted…
Disease spread quickly as
there was no clean water
available…
All domestic and internal
flights had to be suspended
during July…
Landless labourers and small
farmers were the most severely
affected in rural areas…
900,000 houses were destroyed…
So what / extra detail
…so communication difficult and health at risk as no drinking water. Difficult to
coordinate the relief response as no telephones.
1225 Schools were destroyed.
Another 25000 were closed with
5000 used as emergency shelters
for homeless people…
Nearly 500,000 tonnes of rice has
been ruined in the fields…
…this means children miss out on education and will impact Bangladesh’s long
term development as they will have poorer qualifications and will not be able to
get as good a job. Money that could have been spent on improving the education
of the country will be spent rebuilding old schools.
…which means there will be food shortages in the country which could lead to
starvation and famine. Many people will have lost their only income and will
become reliant on aid to survive, costing the government and charities more
money.
…so the death toll increased from illness such as cholera. Few hospitals and
fewer medicines to treat the sick also added to the death toll and impact.
…this made it very difficult to bring in supplies to help the survivors.
…they lost their homes and crops for the year and have no compensation /
insurance. They will start from scratch but have lost their income for the year as
their crop was destroyed.
…2 million people had to be accommodated in temporary shelters after the flood.
REVISION CHALLENGE: WHY ARE THE EFFECTS SO MUCH WORSE IN BANGLADESH THAN AN
MEDC LIKE THE UK (EG BOSCASTLE)?
Complete the tables by adding in the explanation for each factor
1. Problems in LEDCs which make the effects of floods worse and management and recovery
harder:
Housing is often poor quality
so…
Lack of sanitation / sewerage
means clean water supplies
harder to get so…
There is often a lack of
preparation and training for a
disaster so…
There is little money in the
country for flood defences
so…
Roads and communications
(the infrastructure) are easily
damaged so…
2. Reasons why MEDCs are better prepared for flooding and so the effects are less severe:
Homes and possessions are
insured against flood damage
so…
Good water supplies and
sewerage systems mean
clean water is still available
when pipes are damaged so…
Roads and communications
(infrastructure) are good and
less likely to be damaged
beyond repair so…
Less houses are built in areas
at risk of flooding due to
planning regulations so…
Governments have more
money to invest in flood
defences and early warning
systems so…
Rescue teams are equipped
and prepared for disasters
so…
4.6 How can river flooding be managed?
Flood Prevention
Actually stops areas of land flooding
Flood Protection
Minimises the effects when a river floods
Flood Defence Types:
Hard Engineering Methods
Soft Engineering Methods
Artificial structures which are designed to control the flow of a river. Generally give better
immediate results but are expensive and may be unsustainable or make further issues.
Reduce flood risks by not interfering with the flow of the river but by working alongside the
rivers natural processes. Generally less dramatic impacts but are cheaper and more
sustainable.
The main aims of hard engineering solutions are:
To increase a river’s capacity (size)…
…so that it can carry more water and doesn’t flood.
To increase the velocity (speed) at which the water …so that the flood risk passes as water does not build up at one point.
flows through a point on a river
To reduce the discharge (amount of water) in river …so there will be less water and so the river can cope and not over
flow.
To stop flood water reaching houses and other
…so that flood water does not causes damage and cost for repairs.
property…
Flood Management – Hard Engineering Strategies (methods which alter the river to prevent flooding)
Aim of the
Strategy
Examples of
Description / How it works
how to do this
Widen and / or - This creates extra capacity in the river to reduce the chance of the river overflowing.
deepen the
- Digging out the bottom of the river (dredging) or making the channel wider gives the river extra capacity so that it is less likely to flood.
channel
- This needs to managed constantly as sediment will build up and make the river shallow again.
To increase a
river’s capacity
- These add extra height on a river’s banks so that the channel can hold more water before it overflows so it will flood less often.
(size) so it can
- These are raised banks which add extra height to the river banks.
Build
hold more
- This means the river can hold more water so would burst its banks less often.
embankments
water
- They basically make a river deeper so it can hold more water.
or levees
- They can be made to look natural using earth but are often concrete and ugly.
- When they fail the flood usually more damaging as the force of the water is greater.
- Meanders (bends) are removed to make the river straighter and shorter. This speeds up the flow of water through an area.
To increase the
Straighten the - This speeds up the time it takes for water to pass through an area at risk of flooding so less water can build up.
velocity
channel
- The water is taken downstream so may put other locations at risk from flooding.
(speed) at
- Straightened river channels look unnatural and are unpopular.
which the
water flows
Line the
- The smoother banks and river bed causes less friction so water flows through the area at risk of flooding quicker
through a point channel with - This means that there is not a build-up water and reduces the risk or flooding
on a river
concrete
- Concrete channels look unnatural and are unpopular
- A reservoir is formed behind the dam which can catch floodwater and allow it to be released gradually.
- These barriers are used to trap and store water, which can be released later.
- This means the amount of water downstream can be controlled so that it is at safe levels and does not cause flooding.
- A reservoir is formed behind the dam which can catch floodwater and allow it to be released gradually.
Build a dam
- They are expensive to build, are unnatural but bring other advantages.
To reduce the
- Water can be used to generate electricity in a clean way.
discharge
- Lots of land is flooded to create the reservoir.
(amount of
- The reservoir can be used for leisure activities and supply water.
water) in river
- Floodwater may go over the top or the dam burst which will cause far more damage.
- Extra channels or areas are created that can store water when the flood risk is high. The water is returned later once the risk has passed.
Build storage
- These need lots of space, are very expensive and mean land cannot be used for other purposes.
reservoirs or
- Overflow channels which take surplus water out of a river in times of flood.
relief channels
- Expensive to build and are only used in extreme cases. Look ugly in a natural landscape.
To stop flood
- These are built around settlements to protect them and keep water out
water reaching
Build flood
- When a river bursts its banks the water is contained behind the wall and property is left undamaged.
houses and
walls
- They look artificial and are expensive but are effective.
other property
Flood Management – Soft Engineering Strategies (methods which work with the river and may be
more natural)
Technique and How it Works
Flood Prediction and Warning
Monitor the river to predict and
warn people if it is likely to flood
Many rivers are monitored by the
Environment
Flood Proofing
Change building design to
withstand the flood risk and lessen
the damage
In LEDCs many homes are built on
stilts so flood waters pass
underneath and do not damage
In MEDCs homeowners
encouraged to have rugs instead if
carpets and have sockets above
the flood level for safety
Flood Abatement
These reduce the amount of water
that reaches a river by changing the
drainage basin (eg planting trees in
the catchment to increase
interception)
Flood Plain Zoning
A strategy where land is used for
different purposes depending on
the risk of flooding. High risk =
recreation such as parks / open
spaces. Low risk = essential
services like hospitals / high value
business land
Washlands
Areas of the flood plain are left
undeveloped and during times of
flood they are sacrificed and
allowed to flood.
Advantages
Allows people to evacuate homes and
save possessions
reduce the risk of lives and the cost of
replacing possessions
Increases knowledge and education of
local people so they can prepare and
mean responses need to be lessened
Disadvantages
Not always able to give much warning
so people still at risk
Homes still damaged so there is still a
repair bill
Not all rivers can be monitored
Can save a lot of money from repairs
and damages that are caused
Allows people to return home sooner
after the flood waters have passed so it
is less inconvenient / upsetting and
cheaper as less temporary
accommodation
Expensive and not available to
everyone
Can be a long term solution
Make area look more attractive and
good for local wildlife
Forests can be used for leisure and
make money for the local area
Large areas of land are taken out of
use for other activities like farming so
not always popular
Flood damage costs less to repair and
there is less disruption to lives if there
is a flood
Areas which can be built on may not be
the most suitable for that purpose
because of access or size.
This gives a safe place for
floodwater to go and is an
It means certain areas of the flood
inexpensive option for management. plain cannot be used for other things
It also leaves the natural
and upsets some locals.
environment unspoilt