Rivers: Profiles &
Landforms
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River Profile
Source
of river
Upper
Course
Middle
Course
Lower
Course
River speed
Fastest??
Fastest??
Fastest??
Upper Course
Lower Course
Contrasting river landforms
from source to mouth
Channel features
Valley features
Long profile
Upper Course - Channel features
• River channel is rocky.
• Covered with various shapes and sizes
of boulder.
• Discharge is low.
• Under flood conditions rivers energy is
expended on vertical erosion with
hydraulic action and corrosion processes
at work.
• Potholes may form.
Vertical erosion
Pothole formation
Upper Course - Valley features
• Valley sides are steep and form a ‘V’ shaped
cross section.
• Interlocking spurs.
V shaped valley
Steep sides
Zig-zag bends
(interlocking
spurs)
V – shaped valley
Form due to a combination of
the following processes:
Vertical erosion by the river
itself.
Physical weathering (eg:
frost action) which provides
debris to move down slope.
Mass movement (inc: soil
creep & landslides) to move
debris down slope.
Interlocking
spurs
River flows around
interlocking spurs
Upper Course – Long Profile
• Generally the gradient is steep and the
profile is uneven, particularly where
waterfalls and rapids form.
Waterfall formation
Soft rock is easy to
erode, but the hard rock
is resistant.
So over time a ledge
develops.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
Waterfall formation
The water rushes over the
ledge and erodes a plunge
pool by abrasion and
hydraulic action.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
Waterfall formation
The ledge collapses into
the plunge pool, where
the debris helps to
speed up the erosion.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
Waterfall formation
The process is repeated
and the waterfall
gradually retreats
upstream, carving out a
gorge.
Hard Rock – Lava
Soft Rock – Sandstone or Conglomerates
WATERFALL
RETREATS . UPSTREAM
.
. .
OVERHANG
UNDERCUTTING
OF SOFT ROCK
PLUNGE
POOL
Formation of rapids
Resistant rock
Less Resistant Rock
Middle Course - Channel features
• Channel is now wider and has smoother
banks and bed compared to the upper
course.
Middle Course - Valley features
• River erosional energy is now increasingly
expended horizontally rather than vertically.
• Lateral erosion by the river’s meanders
broadens the valley floor into a narrow flood
plain.
• Meanders gradually shift their course
downstream.
Middle Course 1
Middle Course 2
Meanders
A meander starts as a slight bend:
Water flows faster on the outer curve of
the bend (more energy), and slowest on the
inner curve (less energy).
So the outer bank gets eroded while
material is deposited at the inner bank.
Over time the outer bank gets worn away
(river cliff) and the inner one builds up
(river beach). The bend grows into a
meander.
Meanders
(refer to previous notes and diagrams)
• Alternating series of irregularities develop
• Pools – deeper stretches of slow moving water
• Riffles – shallower section of faster flow,
flowing above coarser material
• River develops a winding or sinuous course
• Faster flow on outer bend results in erosion
and formation of River Cliff
• Slower flow on inside of bend results in
deposition and formation of Slip-off Slope
Meanders
• Meanders develop and
migrate laterally and
downstream
• Helicoidal flow further
assists meander
formation and transports
sediment from river cliff to
the slip-off slope on the
inside of the next bend.
MEANDERS
Meanders
Possible break
through point
Most erosion
on the outside
of the bend .. Fastest flow
Flood plain
Possible
ox-bow
lake
Meanders
Middle Course - Profile
Middle Course - Profile
• Gradient is now less steep.
• Overall the profile is smooth – but an
outcrop of resistant rock could still cause a
waterfall.
Lower Course - Channel features
• The channel is now at its broadest and deepest.
• Bedload is carried entirely in suspension and is
solution.
• Deposition now dominates – particularly during
floods.
• Erosion also occurs – in the formation of
meanders
Lower Course – Valley features
• Thanks to lateral erosion the valley sides
may now be several kilometres away.
• Typically it may also contain the following
features:
Floodplain & natural levées
Braided channels
Meanders
Oxbow lakes
Estuaries and deltas
Lower Course
Natural levées
• As the river floods, sediment is dropped over all
the flooded areas but most falls along the river
channel itself.
• This sediment raises the height of the banks is
flooding occurs regularly
• Levees themselves do not prevent flooding
because as the banks are raised, more
sediment is dropped on the river bed, raising the
water level.
Gentle valley sides
River is actually flowing above
The floodplain !!
Flat floodplain
Layers of silt
Deposited during floods
Levees is often
artificially
strengthened
Coarse material
Forms natural
levees
Braided channels
• Formed by the choking of the main
channel by the deposition of a
considerable amounts of the river load.
• The channel splits into several smaller
channels which flow around fresh ‘islands’
of deposited material before rejoining.
Braided channels
Braided channels
Ox-bow lakes
Downstream migration of meanders produce pronounced meander loops which
may form ox-bow lakes during flood conditions
1 Lateral erosion on outside of bends of pronounced meander
2 Narrow neck of meander gradually becomes narrower
3 Neck is cut through by river during floods and river forms new straighter
channel
4 Cut-off is sealed by deposition
5 Ox-bow lake begins to silt up
(Core Higher, P 65, Fig 3.36)
Oxbow lakes
An Oxbow lake starts as a meander.
During a flood the river cuts across the
meander, forming a new channel.
Sediment is deposited along the sides of
the new channel. The loops gets sealed
off and an oxbow lake forms.
The water in it becomes stagnant. The
lake will remain sealed of until either the
river floods into it or it dries out.
Oxbow lakes
NARROW MEANDER NECK
FUTURE
OX-BOW LAKE
Estuaries and deltas
• An estuary is where a river meets the sea
(it is tidal).
• Deltas are essentially the seaward
extension of the floodplain and form where
tides are too weak to remove deposit
sediment.
…………….
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