Middle course of the river
Quick revision for spot test
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© Boardworks Ltd 2005
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What is erosion?
I am a type of erosion that is like sandpaper
How does Hydraulic action work?
I am the type of erosion that knocks off edges
off rough rocks.
What is solution/corrosion?
What is weathering?
How does physical weathering work?
Plants and animals break the rock apart- what
am I?
The slowest form of mass movement
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River Profiles – a cross-section through a river’s
course from source to mouth.
DONE
The Middle and Lower
Courses of a River
What are the main processes that operate in the
middle and lower course of a river?
• The river is now flowing over flatter land and so the
dominant direction of erosion is lateral (from side to side).
• The river has a greater discharge and so has more energy
to transport material (load).
• Deposition is also an important process and occurs when
the velocity of the river decreases or if the discharge falls due
to a dry spell of weather.
Why do you think that a river in the middle and lower
course has a higher discharge than near the source?
Types of erosion?
• Attrition =
• Solution =
• Abrasion =
• Hydraulic Action =
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What landforms are found in the middle and lower course?
1) Meanders
2) Oxbow Lakes
3) Flood plains and
Leveés
4) Deltas
DEPOSITION and
EROSION create
these landforms
DEPOSITION created
these landforms
Hold hands and run go in a wave- who goes fastest?
Q: Do you think there will be erosion or deposition on the outside
of the bend?
Watch youtube clip
http://www.youtube.com/watch?v=4qKS_Nk7UmY&safe=active
Draw a diagram of how meanders and oxbow lakes form (p69
and 70)
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Meander = a bend in a river
Meanders
As the course of a river approaches its
middle stages it flows over flatter land.
Lateral erosion dominates as the river
swings in large bends known as
meanders. Meanders constantly change
their shape and position.
Water is pushed to the outer bend .
This reduces friction with the bed and
banks. So the river has more energy
for transporting material which can
erode the outside bank via abrasion.
A cross section through a meander
A cross section through a meander
How are Oxbow lakes formed?
Explain the formation of an oxbow lake
http://www.youtube.com/watch?v=PWkGgM
6b_Yk&feature=related
new course
of the river
oxbow lake
When the river floods it
breaks through the thin
Meander
neck
meander
neck
and the
becomes
river
takes thesmaller
easier,
straight course. This
leaves the meander
loop ‘cut off’ as an
oxbow lake. Over time,
the oxbow lake will
become colonised by
vegetation.
How well do you understand oxbow lakes?
Explain why there is more deposition at ‘A’ rather
Meander bend
on the
than
at River
‘B’ (4Conwy
marks)
A
B
Examination question
Study the photograph below:
B
A
C
Name the 3 features labelled A, B and C (3)
Choose one of the features and explain its formation. You
may use diagrams to help you.(6)
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How are floodplains formed?
How are floodplains and levees formed?
Floodplains and leveés are formed by deposition in
times of river flood. The river’s load is composed of
different sized particles. When a river floods it deposits
the heaviest of these particles first. The larger particles,
often pebble-sized, form the leveés. The sands, silts and
clays are similarly sorted with the sands being deposited
next, then the silts and finally the lightest clays. Every
time the river floods deposition builds up the floodplain.
Floodplain formation
This is a cross section of a floodplain.
How much do you know about rivers?
How are Deltas formed?
• Deltas are found at the mouth of a
river, where the river meets the sea.
• At this point the river is carrying too
much load for its velocity and so
deposition occurs.
• The top of the delta is a
fairly flat surface. This is
where the coarsest river
load is dropped.
• The finer particles are
carried into deeper water.
The silt is dropped to form
a steep slope on the edge of
the delta while the clay
stays in suspension until it
reaches the deeper water.
How are Deltas formed?
Key ideas on the middle and lower course of rivers.
Deposition is the main process operating in the middle and
lower upper course of a river.
Erosion is still important and occurs laterally (side to side).
Discharge is greater than at the source.
River Velocity is greater than at the source.
Meanders and Oxbow lakes are formed by erosion and
deposition.
Deltas and leveés are formed by deposition.
Time to test your
understanding
How are the river landforms formed?
Which of the following landforms are the result of a
mixture of erosion and deposition? (2 marks)
v-shaped valleys
deltas
waterfalls
rapids
meanders
oxbow lakes
interlocking spurs
floodplains
leveés