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
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.
Downstream portion of river: landforms
A) Flood plains and levees
Cross-section of a floodplain
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.
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.
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.
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.
Downstream portion of river: landforms
A) Flood plains and levees
Stages in the formation of flood plains and
levees
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.
Downstream portion of river: landforms
B) Ox-bow lakes.
• horse-shoed/crescentshaped lake.
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.
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.
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.
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).
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)
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.
Downstream portion of river: landforms
C) Braided streams
• when river’s 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.
Downstream portion of river: landforms
C) Braided streams
Downstream portion of river: landforms
C) Braided streams
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.
Downstream portion of river: landforms
C) Deferred Streams
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.
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.
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.
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.
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).
Downstream portion of river: landforms
E) Deltas
Formation process:
• As these deposits are heavier, they are
deposited first at the river’s mouth.
• The main river channel will be silted and
blocked up.
• River overflows and splits into many small
channels called distributaries.
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.
Downstream portion of river: landforms
E) Deltas
Formation process:
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.
Downstream portion of river: landforms
E) Deltas
Formation process:
• As the delta grows larger and seawards, it will
eventually merge with the river’s flood plain
and no longer possess the appearance of a
delta.
Downstream portion of river: landforms
E) Deltas
Types of deltas
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
Downstream portion of river: landforms
E) Deltas
Types of deltas: Bird’s 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
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
Downstream portion of river: landforms
References:
Chong, M. (2001) Aspects of Physical
Geography, Singapore Pekoe Books.
Waugh, D (2000) Geography – An Integrated
Approach, UK: Nelson