Transcript Slide 1

Rivers
• Long and cross profiles
• Processes of erosion, deposition and transportation
• The characteristics and formation of waterfalls and gorges
• The characteristics and formation of meanders and ox-bow lakes
• The characteristics of levees, deltas and flood plains
• Flooding in MEDC’s and LEDC’s
• The short, medium and long term attempts at managing flooding
• The social, economic, environmental and political issues that have an
impact upon strategies and the values and attitudes of interested groups.
Rivers begin in upland areas and flow downhill towards the sea. The start of a
river is called the SOURCE and the end is called the MOUTH. Along the main
river, there are many other smaller rivers which join it called TRIBUTARIES.
These join with the main river at a point called the CONFLUENCE. A river and its
tributaries obtain their water from the surrounding land. The area drained by the
river and its tributaries is called the DRAINAGE BASIN. The boundary of the
drainage basin is called the WATERSHED.
The long profile of a river is a cross-section from the source to the mouth. It
consists of three sections:
Upper section – narrow, steep-sided valleys (v-shaped) where gorges and
waterfalls may form
Middle section – wider valley with floodplain but still with steep valley sides
Lower section – very wide, flat-floored valley with meanders and ox-bow lakes
Processes of erosion, deposition and transportation
EROSION
Attrition-This is when material bashes against each other and breaks up into smaller
and more rounded pieces
Corrosion- This is when material in the river is broken down by chemicals
Corrasion- This is when stones and pebbles rub away at the bed and banks
Hydraulic action- This is the power of the water on the bed and banks
TRANSPORTATION
Saltation- This is when smaller pebbles are bounced along the river bed
Solution- This is when lighter material is dissolved and is carried along in the water
Traction- This is when larger bed load is dragged or rolled along the river bed
Suspension- This is when light material is carried along within the water (in suspension)
DEPOSITION
A river requires lots of energy to transport its load and when this energy is reduced,
the river is forced to deposit the load. An example would be on the inside of a river
bend where the slow moving water has little energy and deposits the load.
The characteristics and formation of waterfalls and gorges
A waterfall is a steep drop in the course of a river. They form when a band of hard,
resistant rock (cap rock) lies over softer, less resistant rock. The softer rock is
quickly eroded by hydraulic action and abrasion, causing the harder rock to be
undercut. The hard rock overhangs until it can no longer carry its own weight. The
overhang collapses and then breaks up in the water below. The great power of the
water at the base of the waterfall causes a plunge pool to form. The bed of the river
below the waterfall contains boulders eroded by splash back from behind the
waterfall.
Over time this process is repeated and eventually a steep-sided gorge forms as the
waterfall retreats up stream.
The characteristics and formation of meanders and ox-bow lakes
Meanders are bends in a rivers course. Water flowing around a meander moves fastest
on the outside of the bend leading to erosion and the formation of a steep river cliff.
On the inside of the bend, water slows down leading to the deposition of sediment. This
forms a gently-sloping slip off slope in the shallow water.
Lateral erosion by meanders leads the widening of the valley floor (floodplain).
Meanders often migrate downstream and become wider and larger over time.
Where there are well developed meanders, a river may take a short-cut across a
narrow meander neck in times of flood and this leads to the creation of sections of
abandoned river called ox-bow lakes. Over time, the ox-bow lakes dry up and leave just
a meander scar.
The characteristics of levees, deltas and flood plains
A flood plain is the wide, flat area of land on either side of the river in its middle and
lower course. A flood plain forms through both erosion and deposition. When material is
deposited on the slip off slope of a meander it gradually builds up over time. When a
river overflows its banks, water pours on to the floodplain and as it drains away fine
material (alluvium) is deposited.
Levees are natural embankments of silt along the banks of a river, which are often
several metres higher than the flood plain.
A delta is a flat area of sand and silt built into the sea. When a river enters the sea
carrying large volumes of fine material, the velocity slows and causes the load to be
deposited in layers. Over time, the deposited material forms small islands separated by
river channels called distributaries.
Flooding in MEDC’s and LEDC’s
Flooding in MEDC’s has a different impact to
flooding in LEDC’s. A large amount of housing in
urban areas of MEDC’s has been built on flood
plains. This has obviously led to extensive flooding
in many MEDC’s (for example, Mississippi river,
USA or River Severn, UK).
The floods are often caused by the amount of
tarmac and other impermeable surfaces which
have been laid down as a result of urbanisation.
This leads to rapid overland flow and rivers
reaching their peak discharge rapidly in times of
heavy rain and leads to extensive flooding.
Emergency services and warning systems mean
that loss of life in MEDCs is often small. Homes
and businesses, however, are often damaged and
the cost can be great (the Mississippi floods of
1993 caused $10 billion of damage).
Plans to prevent flooding are often only put in after
a flood has occurred. However, as MEDC’s are
more economically developed, they are able to
implement effective strategies to reduce the
damage from flooding in the future.
Flooding in LEDC’s has a massive impact on the
economy, health system and people. The floods
can occur due to monsoon rains, snow melt in
spring or just unusually high rainfall (for
example, River Ganges, Bangladesh).
Loss of life may be great as emergency services
struggle to cope with the vast amount of help
that is needed. Communication links are often
destroyed and as the quality of housing is often
of a poor standard, many homes are completely
destroyed leaving people homeless.
Due to the lack of provision for clean up
operations, water often remains stagnant which
leads to the rapid spread of water-borne
diseases, such as cholera and typhoid.
LEDCs may need to seek aid from MEDCs to
assist with the damage caused by severe
flooding.
The short, medium and long term attempts at managing flooding
SHORT TERM
Sand bags - to prevent further damage to property
MEDIUM TERM
Afforestation - trees are planted to absorb water and reduce surface runoff.
Managed flooding - the river is allowed to flood in some places to avoid
destruction close to large settlements
Planning - governments and local councils implement plans to help prevent
damage to property in future
LONG TERM
Dam building- dams are built in order to control the amount of discharge released
River engineering- the river channel may be widened, deepened or straightened
to allow water to flow more quickly
The values and attitudes of interested groups
Environmental groups and residents often prefer options which have as
little impact on the settlement and environment as possible,
such as planting trees
Governments and investment groups often prefer hard engineering options, such
as dam building, as this can often generate large profits and can also attract
people for leisure purposes such as sailing
SUMMARY
• The start of a river is called the SOURCE and ends at the MOUTH
• The long profile of the river consists of three sections;
Upper- where gorges and waterfalls form
Middle- where meanders tend to form
Lower- where deltas and floodplains form
• There are four processes of erosion and transportation
• Deposition is the putting down of material along the river due to a loss of energy
• A waterfall is a steep drop in the course of a river. They form when a band of resistant
rock lies over softer, less resistant rock
• Meanders are bends in a rivers course
• A flood plain is the wide, flat area of land on either side of the river in its lower course
• Levees are natural embankments of silt along the banks of a river, which are often
several metres higher than the flood plain.
• A delta is a flat area of sand and silt built into the sea. When a river enters the sea
carrying large volumes of sand, the velocity slows and causes material to be deposited.
• Flooding in MEDC’s has a different impact to flooding in LEDC’s
• There are short, medium and long term attempts at managing flooding