Transcript Aim- to describe and explain the formation of a variety of river

Aim- to describe and explain the formation of a variety of river features with
the aid of annotated diagrams

River Velocity
The THALWEG of the stream
channel is the maximum velocity
that weaves from side to side
wriggling like a snake down the
channel as it is deflected from one
bank to another.


In straight channels, riffles tend to develop at regular intervals. These
typically slope alternately towards one bank and then another, and the
thalweg winds between them.
Between the riffles occurs deeper pools. The spacing of these features is
associated with the channel width, and usually the space between on riffle
and the next is five to seven times the channel width.
Aim- to describe and explain the formation of a variety of river
features with the aid of annotated diagrams
Sediment Transportation
One of the most important relief forming activities of streamflow is its movement of
broken rock debris and dissolved matter the Stream Load.
This is governed by two factors;

Its energy

Amount of rock available
A stream can wear away the bottom and sides of a rock channel, but to do so it must
carry rock fragments.

Sediment already deposited

Mass movement downslope
Different amounts and sizes of fragments occur in a stream at different points along the
course of a river.

Where steep slopes are close to the channel (upper course) large blocks and
boulders may fall in of rocks are well jointed or weathered.

Flat floodplain (lower course) the stream has available only the materials that carried
out and deposited itself.
Total Stream Load is composed of;

Solute

Suspended

Bed load

Aim- to describe and explain the formation of a variety of river
features with the aid of annotated diagrams



Solute- dissloved load,chemical weathering soil and rock produces ions of
elements/compounds dissolved in water.
Suspended load- particles eroded from the stream bed or banks may be
carried along in a flow of water of suspended load (held up in the water).
Such particles are usually no larger than sand or gravel.
Bed load -rock fragments rolled along the stream bed.
Aim- to describe and explain the formation of a variety
of river features with the aid of annotated diagrams
V-shaped Valley
The river erodes vertically at a greater
rate than it does horizontally deepening
the valley.
The river continues to erode vertically,
the river banks become less stable and after
A period of heavy rain, due to gravity they
collapse.
This creates a v-shaped valley.
The process is repeated, deepening
The valley further.
Aim- to describe and explain the formation of a variety of
river features with the aid of annotated diagrams

Waterfall
Many waterfalls form when
rivers meet a band of softer
less resistant rock after
flowing over a relatively hard
resistant rock. The softer
rock is worn away more
quickly, and the harder rock
undercut.
The overhead hard rock
forms an overhang, which
will eventually collapse, to
form a deep plunge pool.
This process is repeated
causing the waterfall to
retreat upstream creating a
gorge in its wake.
Aim- to describe and explain the formation of a variety
of river features with the aid of annotated diagrams
Meander
Within sections of the river channel, the flow
tends to wind from side to side through a
pattern of deep pools and shallower riffles.
Riffles are formed by bed load deposits.
A meander forms when the river channel
bends, most of the water is directed to the
outside of the bend. This reduces friction
and increases the speed of the river at this
point. The river therefore has more energy
to transport through suspension, which will
lead to erosion of the outside bank by
corrasion. The bank will be undercut,
collapse and retreat to leave a small river
cliff.
Meanwhile, there is less water on the inside
of the bend, there is an increase in friction
and a decrease in velocity. As the river loses
energy, it deposits some of its load to form a
slip off slope.
Aim- to describe and explain the formation of a variety
of river features with the aid of annotated diagrams

Ox-Bow lake
Following on from the
development of a
meander…..
Continued erosion
(corrasion) on the outside
of the bend results in the
neck of the meander
getting narrower until,
usually at a time of flood,
the river cuts across. The
fastest current will now be
flowing in the centre of the
channel and deposition is
now next to the banks.
The original meander is
blocked off to leave a
crescent shaped Ox-Bow
Lake.
Aim- to describe and explain the formation of a variety
of river features with the aid of annotated diagrams
Floodplain and levee
In the lower course the river widens its
valley through lateral erosion. At times of
high discharge the river has considerable
amounts of energy, which it uses to
transport material through suspension.
When the river overflows its banks, it will
spread out over the surrounding area which
is flat. This sudden increase in friction will
reduce the velocity of the river causing it to
deposit its load (silt). Each time the river
floods another layer of silt is added creating
as flat floodplain.
The coarest material will be dropped first,
and this can form a natural embankment,
called a Levee next to the river. Levees can
help to prevent further flooding.
Aim- to describe and explain the formation of a variety of river features with the aid of
annotated diagrams
river
Current
floodplain
Height of old floodplain
Isostactic uplift
ICE
Weight of ice depresses
The land like a sponge
Rejuvenation is when the river is given a
new lease of life and begins to erode to a
new lower level. This can happen when
the land was depressed during the last ice
age rebounds, known as isostatic uplift.
land
Isostatic uplift happens when the
ice melts and the land pushes
up again.
Or when there is a change in sea level, i.e. sea levels fell during the ice age, creating a new base
level for the river to erode down to. As the river erodes to its new base level it creates a new
floodplain lower than the old one. Part of the old floodplain remains, above the new one. These
terraces are often used as flood free sites for settlement and industry.
Aim- to describe and explain the formation of a variety
of river features with the aid of annotated diagrams
Delta
Accumulation of silt deposited
on the seabed at the month of
the river. Because the river’s
velocity is much reduced
when it joins the sea, it must
deposit its load. If the load is
built up above sea level then
mud banks form to create a
delta.
Estuary
The mouth of a river where tidal effects can be seen. Most estuaries are
funnel shaped, becoming wider towards the sea.
2004 past paper question
12 marks
12 marks
2004 answer










Main points which might be made are;
Erosion;
In the upper valley great deal of vertically erosion- due to steep gradient of streams,
volume of water and velocity. Large bedload can cause considerable erosion at times
of very heavy rainfall.
Middle valley-erosion mostly sideways- particularly at meanders-where the fastest
flow on the outside of the bend causes erosion.
Lower valley- very little erosion- main work of river is now transportation. Broader
deeper channels mean energy more evenly distributed.
Deposition
In upper valley- tends to be dominated by large stones and boulders- they can be
transported at times of high discharge, but at other times are dumped in the stream.
In the middle valley- deposition of sand, silt and gravel at the inside of bends of
meanders due to the slower flow and hence lower energy of the river at this point.
In the lower valley- silt deposited (this can form mudflats) as the river approaches the
sea and slows down- gentler gradient, so load is deposited.
Credit can be awarded for details on the formation of a river delta.