Aim- to describe and explain the formation of a variety of river features with the aid of annotated diagrams

1
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.

2
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

3
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.

4
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.
5
Aim- to describe and explain the formation of a
variety of river features with the aid of
annotated diagrams
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.

• Waterfall

6
Aim- to describe and explain the formation of a
variety of river features with the aid of
annotated diagrams
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.
Meander
7
Aim- to describe and explain the formation of a
variety of river features with the aid of
annotated diagrams
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.

• Ox-Bow lake

8
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.
9
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
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.
Isostactic uplift
Isostatic uplift happens when the ice melts and
the land pushes up again.
land
ICE
Weight of ice depresses The land like a sponge
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.
10
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 rivers
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.
11
2004 past paper question
12 marks
12 marks
12
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.