DELTAS
FORMATION OF RIVER DELTAS
What is a delta?
 A delta is a large deposit of alluvial sediment forming
a low-lying plain found at the mouth of a river.
 River deltas form when a river carrying sediment
reaches:
 (1) a body of standing water, such as a lake, sea, ocean,
or reservoir,
 (2) another river that cannot remove the sediment
quickly enough to stop delta formation, or
 (3) an inland region where the water spreads out and
deposits sediments.
Why study deltas?
 River deltas have been important to humans for thousands
of years because of their extremely fertile soils. Major
ancient civilizations grew along deltas such as those of the
Nile and the Tigris-Euphrates rivers.
 Apart from this, they also provide flat land for settlement.
 Deltas are important ecologically as well. Fish and other
wildlife thrive in deltas, many of which contain mangrove
swamps, sea grass beds and mudflats.
How are deltas formed?
 The formation of a delta is a long and gradual process.
 They are created when the sediment load carried by a
stream is deposited because of a sudden reduction in
stream velocity.
 Deposition occurs as stream velocity drops on entering a
standing body of water, thus reducing the capacity and
competence of the river.
 Vegetation also increases the rate of deposition by slowing
down the water.
 This results in deposition of the bedload and suspended
load carried by the river.
How are deltas formed?
 An important process aiding in the deposition of the river’s
load is flocculation.
 This is the process by which tiny particles in suspension like
clay and silt (which under normal fresh water conditions
would likely never fall out of suspension) coagulate with
the salt in the water by chemical reaction and become
heavy enough to ‘sink’ and be deposited.
 It occurs as fresh water mixes with salt water - e.g where a
river flows into the sea (then called brackish water).
How are deltas formed?
 When a river meets the standing water of an ocean at the
coast, it quickly loses velocity and the heaviest particles
drop out.
 The fine suspended load may be carried further out into
the water before it settles out and sinks to the bottom.
 The sediment deposited in a delta is laid down in layers or
beds each containing various types of sediment. The
position and composition of these beds depend on how far
they were carried by the river’s flow before being
deposited.
How are deltas formed?
 As deposition continues the water becomes more and
more shallow and eventually topset beds begin to rise
above the surface of the water.
 As more sediments are brought by the river to the
delta, especially in times of flooding, the main river
may become choked with sediment.
 When this occurs, the river branches into
distributaries, finding the least resistant path to the
shoreline.
How are deltas formed?
 When sandy deposits block the distributaries, they
then become inactive, and smaller, active
distributaries branch off.
 As the process continues, distributaries constantly
shift position across the surface of the delta.
 Thus, the surface of most deltas is marked by small
shifting channels that carry water and sediments
away from the main river channel.
 These small channels also act to distribute the
stream's sediment load over the surface of the delta.
How are deltas formed?
 As the river channels meander laterally across the top
of the delta, the river is lengthened and its gradient is
reduced, causing the suspended load to settle out in
nearly horizontal beds over the delta's top (i.e. the
topset beds)
 Once above the surface, topset beds become an
extension of the landward alluvial plain.
Delta formation
Composition of a delta
 Deltaic deposits are composed of three types of
depositional layers, each with different types of
sediment:
 Bottomset beds
 Foreset beds
 Topset beds
Composition of a delta
 Bottomset beds - Bottomset beds are nearly horizontal or
flat layers of fine clay and silt that form underwater
farthest from the mouth of the river and the active delta
front. These are the very finest particles that travel the
furthest before very low velocity/flocculation leads to their
deposition.
Composition of a delta
 Foreset beds – Closer to the mouth, yet still underwater,
are foreset beds of sand and gravel that slope steeply
down toward the bottomset beds at angles of 5 to 25
degrees. They make up the main body of deltas. The
sediment particles within foreset beds consist of larger and
more variable sizes, and constitute the bed load that the
river moves downstream by rolling and bouncing along the
channel bottom. When the bed load reaches the edge of
the delta front, it rolls over the edge, and is deposited in
steeply dipping layers over the top of the existing
bottomset beds.
Composition of a delta
 Topset beds – These are thin, horizontal layers of
differing sizes - coarser sand and gravel (deposited
first as the river loses energy) as well as finer material
from the suspended load that are deposited on the
surface of the delta on top of the foreset beds.
 As a delta increases in size and advances farther out
into the water, the topset beds cover the foreset
beds, which in turn cover the bottomset beds.
Composition of a delta
Parts of a delta
 Topset beds form the delta plain and are subdivided
into two regions:
 the upper deltaic plain which is unaffected by tides and,
 the lower deltaic plain whose boundary with the upper
deltaic plain is defined by the upper limit of tidal
influence.
Parts of a delta
 Fully formed deltas are typically made up of three
parts. These parts are:
 the upper deltaic plain,
 the lower deltaic plain and
 the subaqueous delta
Delta Plain
Upper Deltaic Plain
 This is that part of the
delta that is farthest
inland i.e. nearest to the
land.
 It lies above the high tide
mark and is not affected
by the action of waves or
tides.
 It is usually the area with the
least water and highest
elevation.
 The river or stream that
forms the delta begins to
divide in this area into
smaller channels called
distributaries, which carry
sediments
toward
the
delta's edges.
Lower Deltaic Plain
 This lies immediately
seaward of the upper
deltaic plain, in the
middle of the delta.
 It occupies the area
between high and low
tides
and,
thus,
periodically
lies
underwater.
 The landscape is affected
by
the
action
of
distributaries, tides, and
waves.
 It is a transition zone
between the dry upper
delta and the wet
subaqueous delta.
Subaqueous Delta
 That part of the delta that
lies below the low tide
mark.
 It is closest to the sea or
body of water into which
the river flows.
 This area usually lies past
the shoreline. And as a
result, lies completely
underwater.
Delta Plain
Pro Delta
The Delta Plain is divided into:
 Upper deltaic plain – entirely fluvial
 Lower deltaic plain – modified by tides
• Tidal flats, mangroves, marshes
 Delta slope – deposition of fluvial sediment
 Pro delta – deposition of marine or lacustrine sediment
Subaqueous
Delta
Stages in Delta Formation
 Stage 1: deposition divides the river mouth into several
distributaries. Spits and bars arise and lagoons are formed. The
levées of the river extend into the sea via the distributaries.
 Stage 2: the lagoons begin to get filled in with sediments and
become swampy. The delta begins to assume a more solid
appearance.
 Stage 3: the old part of the delta becomes colonised with plants
and its height is slowly raised as a result. Swamps gradually
disappear and this part of the delta becomes dry land.
 All three stages can be seen in most deltas. As a delta grows
larger, the old parts merge imperceptibly with the flood plain
and they no longer have the appearance of a delta.
Deltas only form where the rate of deposition exceeds the rate of
sediment removal – i.e. where the critical threshold is no longer met
and the system has fallen out of dynamic equilibrium.
Typical Conditions for Delta
Formation
Typical Conditions for Delta
Formation
 Not all rivers form deltas. For a delta to form, the flow of a river must be
slow and steady enough for silt to be deposited and build up.
 The Ok Tedi, in Papua New Guinea is one of the fastest-flowing rivers in
the world. It does not form a delta as it becomes a tributary of the Fly
River. (The Fly, on the other hand, does form a rich delta as it empties
into the Gulf of Papua, part of the Pacific Ocean.)
 A river will also not form a delta if exposed to powerful waves. The
Columbia River in Canada and the United States, for instance, deposits
enormous amounts of sediment into the Pacific Ocean, but strong
waves and currents sweep the material away as soon as it is deposited.
 Tides also limit where deltas can form. The Amazon, the largest river in
the world, is without a delta. The tides of the Atlantic Ocean are too
strong to allow silt to create a delta on the Amazon.
Typical Conditions for Delta
Formation
 Active vertical and lateral
erosion in the upper
course to provide the
extensive
sediments
needed for deltas to
form.
 The coastal area into which
the river flows should be
sheltered
with
weak
currents and a small tidal
range (preferably tideless).
 This will result in limited
wave action and little
subsequent transport of
sediment after deposition.
Typical Conditions for Delta
Formation
 The sea adjoining the
delta should be shallow
(e.g. on the continental
shelf) or else the load will
be washed away into the
deep waters.
 The calibre of the load
should be high.
 This will require a large
amount of energy to keep
in transport
Types of Delta
 A tug of war between land and water determines a delta's
shape. It is a battle that pits the strength of a river's flow
and the amount of sediment it carries against wave and
tidal currents.
 Deltas build outward from a coast only if the slope from
the shore is gentle and ocean currents are not strong
enough to carry away the sediment deposited by the river.
 Deltas are classified either based on their shape or on what
controls a river's deposition of sediment.
 The deposition of sediment may be controlled by the river
itself, waves or tides.
Types of Deltas
Types based on shape
Types based on controls of
deposition
 Arcuate
 River-dominated deltas
 Digitate
 Wave-dominated deltas
 Cuspate
 Tide-dominated deltas
 Gilbert deltas
 Estuarine deltas
Arcuate, Digitate and Cuspate
Types of Deltas based on shape
Arcuate Deltas
 Arcuate deltas are the commonest form of delta.
 They are arc or fan-shaped, with the wide portion of the
fan or convex end farthest from the mainland.
 Crossed by many short, well-defined distributaries, these
types of deltas are composed of relatively coarse
sediments like gravel.
 Wave and river activity are fairly well balanced. The
seaward edge of the delta is rather smooth because strong
waves push the sediment back against that edge.
 The Nile Delta in Eygpt is an example of an arcuate delta.
The Nile Delta
•The most common shape of
delta
•Characteristics:
curving
shoreline
(smoothed by long shore
drift),
distinct
pattern
drainage (branching
distributaries),
of
of
typically more ‘gravely’
deposits .
Digitate Deltas
 Where the action of waves is weak and that of a river is
strong, an irregular-shaped delta forms that extends out
into the water well beyond the local shoreline.
 Resembling the spread claws of a bird's foot, this type of
delta is also called a bird's foot delta.
 Fine sediments and shifting distributaries mark this riverdominated delta.
 Bird's foot deltas are not common along ocean coasts
because the action of ocean currents and waves is often as
strong if not stronger than that of rivers.
 The Mississippi Delta, on the Gulf of Mexico, is a bird's foot
delta.
The Mississippi
Delta
•‘Fingers’ of deposition build out
into the sea along the
distributaries channels giving an
appearance like a birds claw.
•Typically with a finer sediment
Cuspate Deltas
 Cuspate deltas form where a river drops sediment
onto a straight shoreline with strong waves that hit
head-on.
 The waves force the sediment to spread outwards in
both directions from the river's mouth, making a
pointed tooth shape with sides that curve inward.
 Few distributaries are found in cuspate deltas.
 The Tiber Delta in Italy and Ebro Delta in Spain are
classic examples.
Ebro Delta
A cuspate delta is shaped
like a tooth by:
gentle but regular
opposing currents in
the water body the
river flows into,
Longshore drift
Strong waves that
approach the coast
head on
Wave –dominated, tide-dominated, Gilbert and estuarine
Types of Deltas based on
control of sediment deposition
River-Dominated Deltas
 These types of delta form where the growth of the delta from
sediment input exceeds the capability of waves, tidal currents
and longshore currents to redistribute the sediment.
 A major example is the Mississippi River Delta – formed where
the Mississippi River empties into the Gulf of Mexico. Wave and
current action in the Gulf is weak. The shape of the delta is
therefore controlled by the large amount of sediment and high
discharge rate of the Mississippi River.
 The delta shape resembles a “bird’s foot” due to the lobes of
sediment that extend into the Gulf of Mexico.
 It is possible that human activity is changing this delta into a
wave-dominated one.
Mississippi Delta
Wave-Dominated Deltas
 A wave-dominated delta is one where wave erosion
controls where and how much sediment remains in the
delta after a river drops it.
 Strong wave action at the mouth helps to move sediment
and controls the size and shape of the delta.
 These deltas have relatively smooth shorelines.
 The Nile delta (shaped by waves from the Mediterranean
Sea) and Senegal delta (shaped by waves from the Atlantic
Ocean) are both wave-dominated deltas.
Nile Delta
Tide-Dominated Deltas
 A tide-dominated delta is one that forms where tidal
currents are stronger than river outflows.
 The currents re-distribute the sediment forming sand-filled,
funnel-shaped distributaries.
 It has a dendritic structure (branched, like a tree) due to
newly-formed distributaries during times of high water
(flood) and lobes perpendicular to the shoreline.
 Tide-dominated deltas usually form in areas with a large
tidal range, or area between high tide and low tide.
 The massive Ganges-Bramhaputra delta, in India and
Bangladesh, is a tide-dominated delta, shaped by the rise
and fall of tides in the Bay of Bengal.
Ganges-Brahmaputra Delta
Summary
The Mississippi Delta – A
River Dominated Delta
The Mississippi delta is dominated by
fluvial processes, which produce bird
foot extensions. The Mississippi River
pours more than a million metric tons
of sediment into the ocean each day.
Eventually the river finds a shorter
route to the ocean and abandons its
active distributary channel for a
shorter course. The abandoned
distributary ceases to grow and is
eroded back by wave action.
Abandoned river channels and
inactive sub-deltas have developed
on each side of the present river.
The Nile Delta – A Wave
Dominated Delta
The Nile delta is dominated by wave
action which produces an arcuate
delta front.
The Mekong Delta – A
Tide Dominated Delta
The Mekong delta is dominated by
tidal forces which produce wide
distributary channels.
The Niger Delta – A
Hybrid Delta
The Niger delta has formed where
stream deposition, wave action, and
tidal forces are about equal. An
arcuate delta front and wide
distributary channels are thus
produced.
Gilbert Deltas
 Gilbert deltas are formed as rivers deposit large,
coarse sediments. Although they can form in oceans,
such deltas are usually confined to rivers emptying
into freshwater lakes in mountainous rivers. They are
usually steeper than the normal flat plain of a wavedominated or tide-dominated delta.
 Lacustrine deltas are good examples such as the
Kander River Delta in Lake Thun, Switzerland and St.
Clair River Delta in Canada.
St. Clair River Delta – the world’s
largest lacustrine delta.
Estuarine Deltas
 When a river is located near coasts that have large tidal
variation they do not always form a traditional delta. They
instead form estuaries, or a river that meets the sea.
 Estuarine deltas form as a river does not empty directly
into the ocean, but instead forms an estuary.
 The load is deposited along the sides of the estuary.
 An estuary is a partly enclosed wetland that features
a brackish water (part-saltwater, part-freshwater) habitat.
 The Yellow River forms an estuary, for instance, as it
reaches the Bohai Sea off the coast of northern China. The
River Shannon Estuary is another example.
The Yenisei and Ob Rivers
Other Types of Deltas
 Inland deltas, which empty into a plain, are extremely
rare.
 The Okavango delta in Botswana is probably the most
well-known—and so unusual it is recognized as one
of the "Seven Natural Wonders of Africa.“
 Water from the Okavango River never reaches
another body of water. The delta spreads water and
silt across a flat plain in the Kalahari Desert before
being evaporated.
Other Types of Deltas
 Inverted deltas look like the opposite of a classic arcuate
delta.
 The distributary network of an inverted delta is inland,
while a single stream reaches the ocean or other body of
water.
 The delta of the Sacramento-San Joaquin River in northern
California is an inverted delta. The rivers and creeks of the
Sacramento and San Joaquin distributary networks meet in
Suisun Bay, before flowing to the Pacific Ocean through a
single gap in the Coast Range, the Carquinez Strait.