Landforms in the Lower Course
Meanders and Ox-bow Lakes
Lesson Objectives
You should be able to:
• describe and explain the asymmetrical shape of
the channel cross-section across a meander.
• explain how an ox-bow lake is formed.
Asymmetrical Shape
3 (c) (ii) Suggest reasons for the
shape of the river cross section
between P and Q. (2)
• (There should be recognition of the asymmetrical shape
of the cross section, with a steep sided river cliff on the
outer bank and gentler slip-off slope on the inner bank).
• The faster flow of water (helicoidal flow) on the outer
bend of the meander (1), causes more erosion and
undercutting of the bank (1). Slower flow on the inner
bend (1) may lead to deposition of material (1). Credit a
labelled diagram if it helps to explain the cross section.
(2 marks)
• Need to refer both sides of channel for 2 marks.
Meander Bend on the River Conwy
Explain why there is more deposition at A than at B. (3)
CONVEX - A
Slow
Shallow
Deposition
Slip-off Slope
VARIABLES
CONCAVE - B
SPEED OF FLOW
Fast
DEPTH
Deep
PROCESSES
FEATURE
Erosion
River Cliff
Explain why there is more deposition
at A than at B. (3)
Explain why there is more deposition
at A than at B. (3)
Explain why there is more deposition
at A than at B. (3)
• A* Knowledge:
• The force that pushes the river’s water towards
the outer bank on a bend is called centrifugal
force. (It’s the same force that throws you
outwards on a roller coaster or roundabout)
• Eroded material on one meander is transported
across the channel to be deposited on the
beach/slip-off slope of the next meander by
helicoidal/helical flow. (This is a ‘corkscrew’
motion that sometimes creates little whirlpools
on the river’s surface)
Questions on Meanders
• What is a meander (1)
• Draw and label a cross-section from A – B
through the meander. (3)
• Explain how the features of a meander are
formed. (4)
What is a meander (1)
• A meander is a winding curve or bend in a river.
Different questions may require the
same information.
• Give reasons for the formation of meanders.
• Briefly state why meanders form.
• Account for one feature found in the
middle/lower course of a river. (2-4)
• A meander is a winding curve or bend in a river.
They are typical of the middle and lower course
of a river. This is because vertical erosion is
replaced by a sideways form of erosion called
LATERAL erosion, plus deposition within the
floodplain.
Questions on Meanders
• What is a meander (1)
• Draw and label a cross-section from A – B
through the meander. (3)
• Explain how the features of a meander are
formed. (4)
Draw and label a cross-section from
A – B through the meander. (3)
Questions on Meanders
• What is a meander (1)
• Draw and label a cross-section from A – B
through the meander. (3)
• Explain how the features of a meander are
formed. (4)
Explain how the features of a
meander are formed. (4)
• The major features found on a meander are river
cliffs and slip-off slopes. A river cliff forms on the
outside of a river bend as a result of faster
flowing water and its load causing erosion and
undercutting the river bank. A slip off slope forms
on the inside of a river bend where the river is
flowing more slowly and deposition takes place.
1 (a) Describe how a river erodes
material from its bed and banks (3)
•
•
•
•
•
•
Hydraulic action, abrasion and corrosion/solution are the relevant
processes.
There should be reference to how the processes work:
Hydraulic action is the removal of material by the sheer force of water. The
turbulent flow of the water picks up loose material due to frictional drag. The
higher the velocity – the more effective the process.
Abrasion occurs when material being carried by the river hits the bed and
banks and so wears them away via a scouring/sandpapering effect. The
load used for this purpose will vary in size.
Corrosion is the removal of certain rocks in solution – such as chalk and
limestone where the mineral mixes with the rainwater and is carried away
within it.
Allow 1 mark for a list of two or more terms. 3 x 1
1 (a) Describe different types of load
a river carries. (4)
• There are three types of load – bedload,
suspended load and dissolved load.
• Bedload consists of that which remains on the
bottom of the river channel and represents the
largest material, such as cobbles and boulders.
At times of lower flow, it also includes smaller
particles, including sand¹.
• Suspended load is that held within the body of
the water and includes smallest and smaller
particles of clay, silt and sand, depending on the
speed of the water.
1 (a) Describe different types of load
a river carries. (4)
• Dissolved load is that which is held in solution
where certain types of rock such as chalk and
limestone are present within the water, thus are not
visible.
• Allow 1 mark for a list of either types (two or more)
and size (two or more) if different sizes identified
e.g. clay and boulders, a further mark is available
for an accurate figure indicating size of each e.g.
0.001 and 600mm. 4×1 (4 marks)
River Competence vs
Capacity
At times of lower
•
This
point
in
the
answer
flow,
it
also
refers to the competence
includes smaller
of the river i.e. the largest
particles,
sized particle the river is
including sand¹.
able
to
carry.
Competence varies with
energy. Thus, when there
is low flow in the river,
energy is lower and
competence falls.
Study the figure below, a photograph
which shows part of a river meander.
(7)
Draw a simple cross-section across
the meander, and mark and label a
river-cliff and the slip-off slope. (3)
River Cliff
Slip-off Slope
Explain why the meander crosssection has this shape. (4)
• The meander cross-section is asymmetrical.
The river cliff is formed on the outer bend of
the meander. This is where the water is deeper
and moves faster. Hydraulic action and
abrasion wear away at the side of the channel,
undercutting the river bank to form a river cliff.
• On the inside bend of the meander, the water is
shallower and moves more slowly. As a result
the river’s load is deposited to form a gentle
slope called a slip-off slope composed of
alluvium.
Explain the formation of ox-bow lakes.
OX-BOW LAKES
Stages in ox-bow lake formation
Stages in ox-bow lake formation
Wavelength and amplitude of
meanders increase downstream
because of erosion and deposition.
With the aid of diagrams, explain the
formation of an oxbow lake. (6 marks)
An ox-bow lake is a crescent/horseshoe-shaped lake found
on a river’s floodplain in its lower course.
As the river meanders across the valley, it erodes laterally and
deposits as well. In a meander the thalweg is close to the
outside bend. A faster, helicoidal flow causes erosion of this
bank through hydraulic action and abrasion – undercutting the
concave bank to form a river cliff.
Shallower water, greater friction and lower velocity results in
deposition on the convex inner bend and the formation of a
slip-off slope. This combination of lateral erosion on the
outside and deposition on the inner bend causes meander
wavelength and amplitude to increase downstream.
With the aid of diagrams, explain the
formation of an oxbow lake. (6 marks)
Over time the size of the meander increases such that a
narrow piece of land is all that separates two adjacent
concave meander loops. This is called the meander neck.
Erosion gradually narrows the meander neck until, during a
flood, the river breaks through and flows along a straighter
and shorter course, forming a meander chute and cut-off.
The old meander will contain water but will eventually be
abandoned as new deposition seals of the ends to form an
ox-bow lake. Over time further deposition will fill in the ox-bow
lake to form a meander swamp and then a meander scar.
With the aid of diagrams, explain the
formation of an oxbow lake. (6 marks)
Photo of an old meander in England. Deposition
has almost filled in the ox-bow lake and will
eventually form a meander scar.
At this meander, the neck is very
narrow. An ox-bow lake should form
at the next flood.
Cocha Brasko and Rio Manu at confluence with Rio
Pinquen in lowland tropical rainforest, Manu National
Park, Madre de Dios, Peru.
Ox-Bow Lake in Butterby, UK
Ox-Bow Lake
Undercut Bank
Oxbow Lake
Old Channel
Slip-off Slope &
Point Bar
Channel Meander along the Sacrament River
Formation of an Oxbow Lake