Rivers – The Essentials
Long Profile
A long profile can tell you how a river and its valley
changes from source to mouth.
Cross Profile
A cross profile is an outline of how a river or valley would
look like if you cut a section of it vertically e.g. across a line
from A to B.
Why do they change? Think about the effects of changes in
gradient, processes of erosion, deposition, velocity etc.
What might they look like?
Processes of river erosion
Abrasion/Corrasion
This is the process by which the bed and banks are worn down by the
river’s load. The river throws these particles against the bed and
banks, sometimes at high velocity.
Hydraulic Action
This process involves the force of water against the bed and banks.
Corrosion
This is the chemical action of river water. The acids in the water slowly
dissolve the bed and the banks.
Attrition
Material (the load) carried by the river bump into each other and so
are smoothed and broken down into smaller particles.
Processes of transportation
Transportation
Rivers pick up and carry material as they flow downstream. A river may
transport material in four different ways:
Solution - minerals are dissolved in the water
Suspension - fine light material is carried along in the water
Saltation - small pebbles and stones are bounced along the river bed
Traction - large boulders and rocks are rolled along the river bed
Deposition
What is it?
Deposition is where the river dumps or leaves behind material
that it has been carrying.
Why does material get deposited?
 The largest material gets deposited first as this is the
heaviest to carry
 Material will also get deposited if there is a fall in the velocity
(speed) or amount of water (discharge) in a river.
 When there us a change in gradient e.g. lower section of a
river long profile is much flatter
 When a river enters a lake or sea
Upper course river features
Waterfall formation
Waterfalls are formed
where a river flows
over a band of hard
rock next to a softer
rock. The softer rock is
eroded by hydraulic
action and corrasion.
The power of the water
creates a deep plunge
pool below the
waterfall and
undercuts the softer
rock until the waterfall
eventually collapses.
This causes the
waterfall to retreat
upstream forming a
gorge.
Middle and lower course features
Meanders
A meander is a bend or curve in
the river.
The formation of meanders is due
to both deposition and erosion.
The high velocity of the water
erodes and undercuts the river
bank on the outside of the bend
where water flow has most
energy (deeper, so less friction),
forming a river cliff.
On the inside of the bend, where
the river flow is slower (shallower,
so more friction), material is
deposited forming a slip-off
slope (river beach)
Outside of
meander
Neck
Oxbow lake formation
Erosion on the outside of the bend of a meander and deposition on the inside
make the meander wider and the neck gets narrower. Often during a flood, the
river breaks through, creating a new straight course and deposition (silt) seals off
the old meander to form an oxbow lake.
Over time, the water in the oxbow lake will evaporate (summer months) and
disappear. Mud and silt will also get washed into the oxbow lake when it rains, so it
will fill up and eventually vegetation will grow where water used to be. There will be
a slight scar in the landscape left behind.
leveés
sands
clays and silts
The formation of levees
When a river overflows, the velocity decreases (water flow on flood plain –
shallower/more friction) and it begins to deposit its load, the largest material
first (as it has more energy nearer the channel – deeper/less friction/higher
velocity). Smaller materials like sands, clays and silts get deposited further
away from the river channel and onto the flood plain as they are easier to
transport (the river does not have the energy to transport large material –
velocity is reduced in shallower water).
After repeated floods, high banks called levees are formed at the sides of the
river.
Formation of flood plains
A flood plain is the flat area of land (hillsides mark its boundary) where the
river meanders across, either side of a river. During periods of high rainfall,
a river will overflow its banks and deposit sediment (very small material e.g.
silt) as the water evaporates and recedes. Over the years, this cycle will
help build a layer of silt on the flood plain. This material is very fertile and
excellent for growing crops.
Where does rainwater go?
Think of the drainage basin hydrological cycle (above) when answering a question
about hydrographs.
Hydrographs
Basin lag time
Peak flow
Discharge (m3/s)
3
2
mm
Through flow
4
1
Overland
flow
3
2
0
Base flow
12
24
36
48
30
72
Hours from start of rain storm
What is a hydrograph and why are
they constructed and analysed?
What is a hydrograph? A hydrograph is a line
graph drawn to show the discharge in a river
after a period of rain.
Why are they used? They are used by river
engineers to show how a particular river responds
to a period of rainfall and can help with predicting
floods and also preventing them.
Key elements of a hydrograph
 Bars = rainfall
 Line = discharge (amount of water in the river)
 Rising limb = rising river as result of floodwater
 Peak flow/discharge = maximum discharge of river
 Recession/falling limb = falling level of river
 Lag time = time difference between peak rainfall and peak
discharge
 Base flow = normal flow of river
 Area of hydrograph above base flow = overland flow
(surface run off + throughflow)
Factors resulting in different
shaped hydrographs
 Rock type:
a. impermeable – doesn’t let water infiltrate through it e.g. clay. Leads to fast
rising/flashy (responds quickly)hydrograph
b. permeable – it lets water through e.g. sandy soil / porous rocks )
c. Porous – rocks with small holes in e.g. limestone/chalk/sandstone
 Land use:
a. forest/woodland – intercepts rainfall. With stemflow, water will slowly infiltrate
into the ground, leading to a slower rising hydrograph
b. Agriculture – are crops growing or have they been harvested, leaving behind
bare soil. The resulting hydrographs would look very different
c. Urbanisation (town/city area). Concreted areas are impermeable, resulting in
rapid surface run-off to drains and then rivers
 Gradient – is the river surrounded by steep, rocky sides (rapid surface run-off)
or are the sides gently sloping (greater infiltration)?
Flooding
What are the causes of a river flooding:
PHYSICAL (natural)
 Prolonged rainfall
 Heavy rainfall
 Snow melt
 Relief (height and shape of land e.g. steep)
HUMAN (man made)
 Deforestation (cutting down trees)
 Urbanisation (towns/cities expanding through construction)
 Dams breaking
Boscastle August 2004
Case Study of a flood in
a MEDC
Mozambique Floods
2000
Case Study of a
flood in a LEDC
Boscastle Flood 2004, UK (MEDC)– causes and effects
Causes:
 400 mm of rain fell in one afternoon
 Really torrential thunderstorm with 60mm rain in 2 hours
 Steep sided valleys lead to rapid run-off
 Below Bodmin Moor (area high land where there was very heavy rainfall)
 Impermeable rocks and thin soils meant rainwater ran off
 High tides held river water back, flooding coastal area
Effects:
 58 properties flooded and 4 demolished
 100 people rescued by 7 helicopters (no one killed)
 1000 residents and tourists were affected and had to leave to find temporary
shelter)
 84 cars wrecked with 34 washed out to sea
 29 out of 31 Cornwall fire stations occupied and had to stay 7 days
 Total cost of damage about £2 million
Mozambique Floods 2004, Africa (LEDC) – Causes and
effects
Causes:
 Mozambique is a lowland country with many rivers
 75% of Mozambique’s annual rainfall (1100mm) fell in only 3 days
 Mozambique suffered a long drought (no rain) before the rains came i.e. the
ground was like concrete and not able to soak up water
 Tropical cyclones (storms) Eline and Gloria brought driving rain, strong winds
and storm surge waves to the region
Effects:
 Crops were flooded and ruined and roads were damaged and destroyed
 Many children suffered stomach cramps and later cholera as a result of hunger
and drinking water that got polluted. Pools of stagnant water provided ideal
breeding grounds for malaria carrying mosquitos
 Muddy water lay up to 5m deep and 50km across Mozambique
 Villages were washed away when the overspill gates of the Kariba Dam were
opened
 At one point there is so much water in the rivers that 8m waves rush across the
Limpopo and Icomati rivers
Responses to the floods in Boscastle and Mozambique
Boscastle:
• Plant trees on upper steep slopes to intercept rainfall
• Straighten the river channel and line it with concrete to get water out of the river
system to the sea as quickly as possible
• Prevent rebuilding on the flood plain around the harbour
• Install control dams in the upper valley
Mozambique:
 South African provided 6 helicopters who helped save 8000 lives in a week.
Mozambique only has one helicopters. 4 Puma helicopters are flown out from
Europe
 Britain cancelled all debts owed by Mozambique and gave £5.8 million in
donations
 The United Nations appealed for £9.1 million in emergency aid for Mozambique
 Charities like OXFAM sent ‘family emergency packs’
 Tents, plastic sheets, blankets, water tanks etc. are shipped from many countries
 Medical kits e.g. basic antibiotics, water purification tablets and medicines are
‘donated’
Approaches to flood
management?
Since the serious 1993 Mississippi floods (USA),
some scientists began asking whether rivers
should be allowed to flood naturally.
There are 3 main strategies in response to
controlling rivers which flood:
1.HARD ENGINEERING (the use of technology to
try to control rivers)
2.SOFT ENGINEERING (trying to work
alongside/with the river
3.Do nothing (this is often necessary in poor
countries)!
How can floods be managed?
HARD ENGINEERING
•
Sluice gates (biggest being Thames Barrier)
•
Building dams
•
Straightening the river (straighten meanders)
•
Build up the river banks – naturally and artificially by concrete (levees) e.g.
Exeter
•
Flood relief channels e.g. Exeter
•
Dredging – digging the river bed deeper
SOFT ENGINEERING
•
Afforestation (planting trees)
•
Sandbags
•
Flood warnings/preparation e.g. evacuate
•
Flood plain zoning
‘DO NOTHING’
How do some of these approaches work?
 DAMS – concrete walls (dams) are built across a river to form a reservoir and
control the amount of water in a river i.e. water can be stored during times of high
precipitation and when the threat of river flooding is greatest
 STRAIGHTENING OF MEANDERS – rivers have a mind of their own and will
often meander across a flood plain. Therefore, water in a meander takes longer to
clear an area than water in a straight section of river (so, increasing the possibility of
a river bursting its banks)
 FLOOD WARNINGS/PREPARATION – telling people in advance of a flood gives
them valuable time to prepare for it. The Environment Agency identifies areas at risk
of flooding and issues warnings. This allows people time to take possessions
upstairs; turn off gas, water and electricity; gather together important papers
(passport etc.) and take some basic precautions against flooding e.g. sandbag front
door
 FLOOD PLAIN ZONING – this occurs where the flood risk across different parts of
the floodplain is assessed and resulting land use takes this into account e.g. lowest
value land use will be found nearest the river e.g. pasture for grazing and playing
fields and highest value land use e.g. industry and housing will be located furthest
from a river
COSTS (bad) AND BENEFITS (good) OF
DIFFERENT APPROACHES
HARD ENGINEERING
 Projects include building dams, levees etc. These methods tend
to give immediate results and control of a river
 Problems can worsen in the future or create unforeseen
problems e.g. silt (mud) building up behind dams,
pollution/waste from shipping, industry and cities (Three Gorges
Dam, China)
 Results in big changes to a river and the surrounding landscape
 Expensive
COSTS (bad) AND BENEFITS (good) OF
DIFFERENT APPROACHES
SOFT ENGINEERING
Projects include:
 Letting wetlands (e.g. marshy areas) store water
 Dividing up the flood plain into zones for different uses e.g.
grazing land, crops, settlements and roads. This is called flood
plain zoning. This controls what is built on a floodplain, so that
areas that are at risk of flooding have low value land uses.
 Planting trees – which can increase lag times
 Flood warnings and preparation can allow people time to take
possessions upstairs; turn off gas/electricity; gather up important
papers
 This is cheaper and more environmentally sustainable (can do
this for years without any harm to environment).
Levels of water stress in England
How is water supply managed in the UK?
Background
In the NW, water supply exceeds water demand but in the SE, demand exceeds
supply. There are many reasons for this e.g. heavy annual rainfall, more
reservoirs, less evaporation (summer), fewer larger cities (no London!) and less
demand from farming.
What is water stress?
Water stress is when the amount of water does not meet that required. This may
be due to an inadequate supply at a particular time or may relate to a water quality
problem.
Why has water demand increased in the UK?
 Increased number of households and population in certain areas
 A more affluent (financially better off) lifestyle increases the demand for water
e.g. dishwashers (use more water than hand washing), Jacuzzis, spas etc.
 Growth of car ownership i.e. washing their car
 More people using hoses to water gardens
 The demand for foodstuffs e.g. salads, fruit and vegetables out of season, which
need to be washed and cooked i.e. increasing the use of water
 Less people sharing a house (marrying at an older age)
Is there a solution?
A traditional solution to the problem of demand problems has been to transfer water
from areas of high supply (high, wet areas e.g. Wales ) to those areas of deficit (less
wet areas with many towns and cities e.g. Liverpool, NW England)
CASE STUDY: LAKE VYRNWY, WALES
What have been the costs (bad features/impacts) and benefits
(good/features/impacts) of the dam development?
Economic (money/wealth) = Local people have jobs and income as a result of:
 Cafes, an RSPB Shop, several gift shops which sell local crafts and produce, and
a Tourist Information Centre which sells booklets and pamphlets not just on the Lake
but for most of North Wales and Snowdonia
 The village is a great position for tourists and thousands visit every year. T
 There is sailing and fishing on the Lake
Social (people) = 2 chapels, 3 inns, 10 farmhouses, and 37 houses were lost under
the reservoir when the dam was constructed.
Environmental (surroundings) = The surrounding area has been designated as a
National Nature Reserve, a Site of Special Scientific Interest, a Special Protection
Area and a Special Area of Conservation.
Useful websites - animations
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21604 (waterfall
animation)
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21394 (flood plain
and levees)
http://www.geography.ndo.co.uk/animations3.htm (processes of erosion)
http://www.cleo.net.uk/resources/displayframe.php?src=309/consultants_resourc
es%2F_files%2Fmeander4.swf (meander formation)
http://www.bbc.co.uk/schools/gcsebitesize/geography/riverswater/floodingrev4.sh
tml (Mozambique floods)
Geography at the movies (video clips) http://www.geographyatthemovies.co.uk/ (Coasts)