Downstream portion of river: landforms

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Downstream portion of river landforms
River Channel channel is at its widest and
deepest. River also has largest volume of water
and load. Load consists of gravel, sand grains,
silt, clay and dissolved substances. Gradient of
land is very gentle. Lateral erosion is rampant
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MATURE RIVERS

• The downstream portion of a river before it
  enters a lake or the ocean. Flat gradients, slow
  steady currents.

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Downstream portion of river landforms
River Valley wide and flat bottomed. Lateral
erosion by river and weathering and erosion helps
to widen valley. Work of river lateral erosion,
especially along the concave banks of
meanders. Main work is deposition.
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Meanders of a river
A Meander is a loop in a river.(see left)
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Meanders of a river
A meandering river
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Meanders of a river
This is a river with several meanders.
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Downstream portion of river landforms
A) Flood plains and levees
Cross-section of a floodplain
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Downstream portion of river landforms

• A) Flood plains and levees
• a floodplain is a gently sloping/level plain
  built up of successive layers of alluvium
  deposited by a river in times of flood.
• During heavy rainfall, amount of water in the
  river channel increases.
• As the river continues to rise, the water will
  cover any adjacent flat land.

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Downstream portion of river landforms

• A) Flood plains and levees
• land susceptible to flooding in this manner is
  known as the floodplain.
• Once out of the channel, water encounters more
  friction because of increased wetted perimeter.
• This reduces speed of water, resulting in
  deposition.

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Downstream portion of river landforms

• A) Flood plains and levees
• When a river overflows its bank, the larger and
  coarser materials will be deposited first.
• This forms a small, natural embankment alongside
  the channel.
• This embankment is called a levee.
• Smaller sized and finer materials will be
  deposited further away from the river.

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Downstream portion of river landforms

• A) Flood plains and levees
• The thin veneer of silt deposited by each flood
  increases the fertility of the floodplain.
• Successive flooding causes the floodplain to
  build up in height.
• The floodplain may also be made up of materials
  deposited as point bars on the inside of meanders.

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Downstream portion of river landforms
A) Flood plains and levees
Stages in the formation of flood plains and levees
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FLOODPLAIN
The floodplain is a piece of flat land in a river
valley close to the river bank. It is usually
found in lower course rivers.
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Floodplain
A flood Plain is flat land each side of the river
. If there is heavy rain or melting snow the
river will flood.
This is the river Arun, the land is flat here so
it flood when it rains hard.
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Downstream portion of river landforms
A) Flood plains and levees Note although flood
plains and levees are better developed in the
lower course of the river, they can also occur
between the upper course and lower course.
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Downstream portion of river landforms
B) Ox-bow lakes.

• horse-shoed/crescent-shaped lake.

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Downstream portion of river landforms
B) Ox-bow lakes.

• river in the lower course meanders widely across
  the low-lying floodplain.
• Lateral erosion and undercutting occur on the
  concave banks where stream flow is fastest.

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Downstream portion of river landforms
B) Ox-bow lakes.

• deposition occurs on the convex banks because of
  slack water flow.
• Over time, the meander becomes very pronounced
  (a)
• Two neighbouring concave banks will get closer.

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Downstream portion of river landforms
B) Ox-bow lakes.

• narrow neck of land between the two neighbouring
  concave banks will eventually be cut through
  either by lateral erosion on the concave banks or
  strong currents during a flood.

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Downstream portion of river landforms
B) Ox-bow lakes.

• a new straighter river channel is created
  through which the river now flows.
• An abandoned meander loop called a cut-off is
  formed (b).

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Downstream portion of river landforms
B) Ox-bow lakes.

• deposition can now take place next to the river
  bank.
• When enough deposition has taken place, the
  cut-off is sealed off from the main river
  channel.
• An ox-bow lake is now formed. (c)

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Downstream portion of river landforms
B) Ox-bow lakes.
Note An ox-bow lake may not be a permanent
feature as the water may dry up and it will be
then filled with sediments and vegetation.
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Oxbow lake-the start
An Oxbow lake is formed when a meander curves
into a loop..
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1.
Theres a thin bit of land in the loop.
When the land in the loop gets thinner the river
breaks through and makes a new path
2.
3.
The ox bow lake is left
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Ox-bow lake
This is an Ox-bow lake.
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Downstream portion of river landforms

• C) Braided streams
• when rivers level falls rapidly, competence and
  capacity of the river is compromised.
• Load will be deposited.
• These deposits will obstruct he flow of the
  river and cause it to split or divide into
  smaller channels.
• The river will divide into a series of diverging
  and converging segments.

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Downstream portion of river landforms
C) Braided streams
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Downstream portion of river landforms
C) Braided streams
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Downstream portion of river landforms

• C) Deferred Streams
• streams on the flood plain may not be able to
  join the main river directly because of the
  presence of levees.
• They end up flowing down the valley and joining
  the main river further downstream.
• These are known as deferred streams.

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Downstream portion of river landforms
C) Deferred Streams
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Downstream portion of river landforms

• E) Deltas
• flat alluvial depositional plain produced by a
  river when it enters calm water such as a lake or
  tideless sea.

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Downstream portion of river landforms
E) Deltas Conditions for formation 1. Active
erosion along the course of the river. 2. River
flows across resistant rocks so that the
weathered material and eroded rock particles can
add on to the load. 3. Tributaries are present to
add to the load.
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Downstream portion of river landforms
E) Deltas Conditions for formation 4. There are
few/no large lakes along the river to trap the
load. 5. Gentle offshore gradient with shallow
water. 6. Coastal waters are calm/tides are weak
- no strong currents/waves to remove deposited
sediments.
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Downstream portion of river landforms
E) Deltas Conditions for formation 7. Removal of
sediments is slower than deposition. 8. Climate
of are favours fast growth of vegetation that
binds and stabilises the sediment.
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Downstream portion of river landforms

• E) Deltas
• Formation process
• When a river enters the sea, it mixes with the
  surrounding water and its speed is reduced.
• The salty seawater causes the silty particles to
  aggregate/accumulate into larger particles
  (flocculation).

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Downstream portion of river landforms

• E) Deltas
• Formation process
• As these deposits are heavier, they are deposited
  first at the rivers mouth.
• The main river channel will be silted and blocked
  up.
• River overflows and splits into many small
  channels called distributaries.

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Downstream portion of river landforms

• E) Deltas
• Formation process
• As layers upon layers of alluvial materials are
  deposited, a platform of alluvium is built up and
  rises above the water.
• This flat alluvium is called delta.

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Downstream portion of river landforms
E) Deltas Formation process
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Downstream portion of river landforms

• E) Deltas
• Formation process
• The distributaries formed will build up their own
  levees.
• Vegetation (mangroves) that later grows on the
  alluvium helps to bind the alluvial deposits
  together and stabilizes the delta.
• Deltas typically have triangular shapes.

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Downstream portion of river landforms

• E) Deltas
• Formation process
• As the delta grows larger and seawards, it will
  eventually merge with the rivers flood plain and
  no longer possess the appearance of a delta.

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Downstream portion of river landforms
E) Deltas Types of deltas
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Downstream portion of river landforms
E) Deltas Types of deltas Arcuate delta

• triangular/fan-shaped.
• water enters the sea through many distributaries.
• formed when alluvial deposits are evenly spread
  out.
• Longshore currents help to form spits and lagoons
  at the mouths of the distributaries
• Eg. Nile delta

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Downstream portion of river landforms
E) Deltas Types of deltas Birds foot delta

• has distributaries that extend far into the
  water.
• Formed when river discharge is high and load is
  enormous.
• Sediments deposited far exceeds those removed by
  tides and currents.
• Eg. Mississippi delta

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Estuary
There are different types of estuaries. This is
a salt marsh estuary
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Estuary
This is an estuary by the sea .. An estuary
is where the river widens out . This is the
mouth of the river where it runs into the sea.
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Downstream portion of river landforms
E) Deltas Types of deltas Estuarine delta

• sediments are deposited in a long narrow
  submerged estuary.
• Delta does not usually grow beyond the general
  coastline because sediments deposited outside the
  estuary would be washed away by waves and
  currents.
• Eg. Seine delta