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RIVERS
The work of the River Severn
As the river Severn passes on its journey from its source in mid Wales to its mouth in the
Severn Estuary it does three types of work — it
ERODES
TRANSPORTS
DEPOSITS.
EROSION is the wearing away of the bottom and sides of the rivers by the force of the
water, and material carried by the water. The river Severn erodes in three main ways.
ABRASION is when the flowing river Severn throws particles of fine sand, gravel and stones
it is carrying against the bank & bed of the river breaking loose and dislodging more
material. In floods damage is also caused by uprooted trees and branches carried in the fast
flow of the river.
ATTRITION is when the material carried downstream knock against each other wearing
each other smaller. This is why the further downstream you go the smaller the material
gets.
HYDRAULIC ACTION is when just the power & force of the water crashing into the banks
can attack, and wear away the sides of a river. This is more noticeable when the Severn is
in full flood, and you get banks collapsing.
TRANSPORTATION is the movement of the material eroded by the river (called the load)
from one place to another.
Rivers transport material in SOLUTION (material dissolved in the water but not seen by the
naked eye), in SUSPENSION ( material floating on & under the water), as SALTATION
(where material is bounced along the stream bed), and by TRACTION (where large material
is rolled and pushed along the river bed when flooding takes place).
DEPOSITION is the dropping of the load when the speed of the river decreases, for
example when it reaches the sea.
Features of a river
The work of the river, in eroding, transporting and depositing material on its route means
that different features are formed by the river:
1. WATERFALLS & RAPIDS
These are often caused when a river reaches a layer of rock which is hard, and therefore
resists erosion. The soft rocks on the downstream side are eroded more quickly than the
hard rock, and therefore a waterfall develops. There are no real waterfalls on the river
Severn around Ironbridge. There are however small rapids (white water) in the gorge
section where the eroding limestone stream bed causes some potholes and small eddy
currents.
2. MEANDER
As a river reaches lower land it begins to wander from side to side across the valley floor.
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Erosion, particularly in flood conditions tends to be horizontal (sideways). The bend in the
river caused by this wandering is called a meander. There are some excellent meanders
west of Ironbridge towards the village of Cressage.
3. OX-BOW LAKE
When the river water turns around the bend of a meander the water on the outside of the
bend moves faster than the water on the inside. Faster moving water erodes more quickly
and therefore gradually the outside bend the river is eroded back. Eventually 2 bends in
the river may meet each other and the river breaks through taking the most direct route.
The old bend in the river is then left as a lake, which is called an ox-bow lake. In time old
oxbows silt up and only traces of them can be seen in the landscape.
FORMATION OF OX-BOW LAKE
4. RIVER CLIFF & SLIP OFF SLOPE
The water flowing around the bends of the River Severn move in a corkscrew movement.
This means that erosion takes place on the outside of the bend and deposition takes place
on the inside. Where the river is eroding most quickly on the outside of the bend a steep
side is cut into the land, and this is called a river-cliff
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A PLAN & SECTION OF A RIVER BEND
 water direction
deposition
F= fast water flow
D= deepest water
S =slower water flow
Sh=shallowest water
5. FLOOD PLAIN
As the River Severn flows along the valley floor between Shrewsbury & Ironbridge it can
overflow and flood after a period of heavy rainfall further upstream. With the flooding a
deposit of fine silt and mud spreads onto the valley floor. The part of the valley floor that is
sometimes covered by the river when it floods is called the flood plain . Ox-bow lakes are
often found in a flood plain.
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6. SEDIMENT
The material that is carried by the Severn and then deposited is called sediment. This may
be the remains of rocks that were eroded by the river in Wales and have then been broken
down by rubbing against each other and the sides and bottom of the river valley to become
almost like sand. The sediment is deposited in layers on the valley floor, on the bends in the
river and at the mouth of the river as it enter the sea. To make it confusing sediment is also
called silt, alluvium sand & gravel depending on the size & type of material deposited.
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LEVEES & FLOOD BANKS
As the river deposits material on its bed and banks, this material builds up, and sometimes
the level of the river may be above that of the flood plain. The banks of the river are called
levees (from the French verb lever meaning to rise or get up). Artificial banks may be made
out of steel mesh cages full of stones, concrete, steel plates and earth to stop a river flooding
onto farm land, roads, railtracks and housing.
8. VALLEY & RIVER SHAPES
If you followed the course of the River Severn from it’s source in Mid Wales at
Plynlimmon, the altitude at the start would be 752 meters in height. As the river flows east
and then southwards the river drops in height until it reaches the sea at 0 meters. A graph
drawn of this drop in altitude is called a River Profile. The Severn is different from most
UK rivers in that halfway along its course the river is forced to flow through the Ironbridge
Gorge and this is like a bottle neck forcing the river to flood over the flood plain west of
Ironbridge. It also makes the water in the gorge fast & dangerous at times. After the
straight section of the gorge bottle neck the Severn starts to meander again on it’s way to
the sea.
Severn source in
Wales
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!
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steep early
section
Ironbridge
Gorge
almost flat
lower section
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Severn
Estuary
stes
RIVER SEVERN PROFILE
If a cross - section slice is drawn across the Severn valley at different points on its profile,
the valley forms different shapes depending if the river is in the hills & mountains of
Wales, in the Ironbridge Gorge or in the flood plain sections north or south of the gorge..
9. RIVER TERRACES
There are some clear sets of river terraces on the Severn near Ironbridge. A river terrace
shows the width & height of the river many years ago. In some cases 17000 years ago when
there was a lot more water in the Severn with the melting snow fields in Wales. Even 10000
years ago the Severn was much wider than it is today. It is hard to imagine that the
roadside lay by and viewpoint at Leighton (619049) west of Ironbridge was once the north
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bank of the Severn.
T = terrace
C = confluence
where two
streams /rivers
meet
PHOTOGRAPHS
Look at the photographs to see some of the river features mentioned above.
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