Unit 1.3B_4
River Landforms
Now you know a bout the processes that
enable the river to change
• What does the river look like?
River Valley Development
A river has a life cycle from source to mouth. The processes at work cause changes to the river valley
and river features in the landscape that it passes over.
Traditionally this cycle has been described in 3 stages, the Upper (Torrent) section, the Middle
(Valley) section and the Lower (Flood Plain) section. The terminology's Youthful, Mature and Old have
also been used to describe the sections.
The stages are not clearly defined but merge from one to the other and rivers show individual
characteristics. There are river features that can be defined within a broad pattern.
A typical long
profile:
upper
middle
lower
Upper Course
Characteristics:
•Deep narrow valley
•(V shaped)
•Interlocking spurs
•Fast flowing river
(erosion predominant)
•Pot holes in the river
bed
•Rapids and
Waterfalls
The main process at work is the downward
or vertical erosion of the stream bed. This
cuts into the landscape deepening the valley
sides.
In the Upper Course
• You see V-shaped valleys, interlocking spurs,
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waterfalls, gorges and rapids.
This because the erosion takes place downwards
only - Vertical erosion
This mainly by abrasion and hydraulic action,
making valley deeper
Most of the transport is by traction and
saltation.
As the main work is vertical erosion, the water
winds it way around obstacles, weaving from side
to side – the point on the valley side ahead is
worn away, increasing the cut into the valley side
– hense V-shaped valley with interlocking spur
Potholes:
Potholes are formed by
corrasion. Pebbles carried
by the river are swirled
around on the riverbed. This
action erodes the rock on
the riverbed forming
potholes. Over time, they
may widen and join with
other potholes to form
larger potholes, and the
whole riverbed is deepened.
Fast flowing water
has more energy
Slip-off
slope
River cliff
forms
Slow, shallow water and deposition on the inside of the bend
Deep, fast water and erosion on the outside of the bend
• As the outer banks of a meander continue to
be eroded through processes such as
hydraulic action the neck of the meander
becomes narrow and narrower.
Oxbow lakes
• Eventually due to the narrowing of the neck, the two
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outer bends meet and the river cuts through the neck of
the meander. The water now takes its shortest route
rather than flowing around the bend.
Deposition gradually seals off the old meander bend
forming a new straighter river channel.
Due to deposition the old meander bend is left isolated
from the main channel as an ox-bow lake.
Over time this feature may fill up with sediment and may
gradually dry up (except for periods of heavy rain). When
the water dries up, the feature left behind is known as a
meander scar.
Levees
(thanks to Mark Bailey)
What are they?
Often several metres higher
than the river’s floodplain, they
are high natural embankments
of silt situated close to the
river channel.
Where are they found?
They are most commonly found in the lower
course of a river where there is a floodplain,
along rivers that flood periodically, have a
slow flow and carry large amounts of load.
How are they formed?
The river before the flood
at regular height.
The river whilst flooding
There is an increase in friction
between the water leaving the
channel and the floodplain when a
river overflows. The water is
shallower on the river banks and
valley floor, thus this is why
deposition of load occurs. After
repeated flooding causing the
deposition of silt onto the banks,
this eventually settles which over
time builds up to form a levee.
The coarser material is deposited
first, and is positioned closer to
the river.
After repeated flooding, the
river bed and levees are raised
due to the constant process of
the deposition of silt. Over
time, this eventually causes the
river to rise above level of
flood plain. This is called an
‘aggraded bed’.
The end of the river
• Eventually the river
flows into a lake or a
sea or an ocean.
• If the water it flows
into has little
current, any
remaining silt is
dropped as it enters
and there is no
current to remove it
and a delta forms.
Mississippi Delta
But in the UK …
• There are plenty of currents to wash away
any silt that is dropped and so our rivers
have mouths instead.
• I google-mapped this one. Where is it?