Fluvial landforms (1)
Landforms of erosion:
-V-shaped valley and interlocking spurs
-Rapids and waterfalls
-Potholes
Landform of deposition:
-Braided channels
Upper course
 Vertical erosion is the dominant process
 River channel narrow and shallow, often lined
with large angular boulders
 Quite steep gradient
 Velocity quite low because so much energy
used in overcoming friction with the rocky bed
and banks of the river
Large boulders from physical weathering and mass movement from the
valley side increase friction and reduce velocity.
Upper course channel
V-shaped valleys and interlocking spurs
The transport of largecalibre sediment may
cause intensive vertical
erosion which produces
a steep-sided V-shaped
valley profile.
V- shaped valley and interlocking spurs
• Vertical erosion leads to the development of
steep-sided, narrow valleys shaped like the letter
‘V’
• The valley sides are steep due to soil and loose
rock being washed downhill following periods of
heavy rainfall.
• The material is then added to the load of the river
and transported when the discharge is high.
• The river itself is forced to wind its way around
hillsides made of hard rock, called interlocking
spurs.
Upper course: Rapids and waterfalls
Rapids form as a result of the river cutting down rapidly in a localised section of the
river.
The main characteristics of rapids are distinctly steeper gradients marked by steps in
the channel and high turbulence, which is the result of large bed load in the channel or
steps.
Rapids form:
- Over different bands of geology that vary in resistance.
Where the river flows from hard to soft rock, the softer
rock is eroded relatively rapidly.
- Where there is a sudden change in the gradient of the
river.
- May form downstream of a waterfall.
Rapids
Waterfalls are
steep steps in
the long
profile of a
river’s course.
Waterfalls
Thornton Force (Ingleton,UK)
Formation of waterfall and a gorge
Watch a short animation which shows the formation of a waterfall and a gorge
Explain the formation of a waterfall. (7)
They may be formed by a band of resistant rock
occurring on the river bed. The hard rock forms a cap
rock, underlain by the soft rock. Differential erosion
results and the softer rock is eroded faster than the
hard rock, creating a steep gradient in the bed. The
increased velocity gained by the water as it falls over
the step further increases the rate of erosion of the
soft rock. Abrasion and hydraulic action cause
undercutting and the formation of a plunge pool at the
base of the fall. Eventually the overhanging, more
resistant rock collapses due to gravity and the position
of the waterfall retreats in an upstream direction. If
this process is repeated over time, a gorge-a narrow,
steep-sided valley- will be formed downstream of the
waterfall.
A waterfall and its gorge-Bonheur River-France
Case Study: High Force Waterfall
Outcrop of resistant,
igneous rock, called the
Whin Sill overlies weaker
sandstone, shale and
limestone. Undercutting,
collapse and retreat led to
the formation of a 22m-high
waterfall with a gorge
stretching 500m
downstream.
Questions may include photos and usually ask either for an annotated sketch
diagram or a description/explanation of the features of the landform.
Potholes
Potholes are cylindrical holes ‘drilled’ into
the rock by turbulent high-velocity flow.
Vertical eddies may be strong enough to
rotate small pebbles, which grind a
depression in the rock. The water contains
some of the load being carried by the river
and this hits the base and sides of the hole
to both widen and especially deepen it,
smoothing the edges. This is a particular
application of the abrasion process, known
as pothole drilling. It is most effective at
times of high velocity and discharge. As
vertical erosion is the dominant process,
potholes tend to form well above base
level where potential energy is relatively
high.
The photo shows potholes at low water
level. The ‘grinder pebbles can be seen at
the bottom.
Explain the formation of channel landforms typically
found in the upper course of a river. (15)
This question requires both description and
explanation of relevant features: waterfalls, rapids
and potholes.
The formation of at least two channel features
must be covered. There may be use of supporting
case studies, such as High Force Waterfall on the
River Tees.
Braiding occurs when the river is forced to split into several
channels separated by islands.
Temporary bars
- River supplied with large
loads of sand and gravel
transported as bed load.
- Small HR as channel very
wide in relation to its
depth  deposition of
large material which form
temporary bars.
Most likely to occur when
- River has variable
discharge.
- Banks formed from sand
and gravel, easy to erode.
-Common in Alpine rivers
with significant snow
melts as well as in climates
that experience extreme
rainfall events: Many arid
and semi-arid regions have
braided river channels.