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A delta is a landform that is formated at the mouth of a
riverwhere that river flows into
an ocean,sea, estuary,lake,reservoir, flat arid area, or another
river. Deltas are formed
from the deposition of the sediment carried by the river as the
flow leaves the mouth ofthe river. Over long periods of time, this
deposition builds the characteristic geographic
pattern of a river delta.
Herodotus the great historian coined the term delta for theNile
River delta because the
sediment deposited at its mouth had the shape of the
upper-caseGreek letterDelta: .
Types of deltas
Delta lobe switching in the Mississippi Delta, 4600 yrs BP, 3500
yrs BP, 2800 yrs
BP, 1000 yrs BP, 300 yrs BP, 500 yrs BP, current
Deltas are typically classified according to the main control on
deposition, which isusually either a river, waves, ortides.[2]
These controls have a large effect on the shape of
the resulting delta.
[edit] River-dominated deltas
River dominated deltas, such as the Mississippi River Delta,
usually take on a multi-
lobed shape that results from repeated sequences of channel
occupation, offshoredeposition, and channel avulsion. (See delta
switching.) When a single channel is
occupied for a long period of time, its deposits extend the
channel far offshore, and
causes the delta to resemble abird's foot; the term "digitate
delta" is sometimes used aswell. These deltas are often
characterized by a main channelthat divides itself into
several distributary channels.
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The "bird's-foot" delta of the Ural River
Digitate deltas can be often seen on sediment-rich rivers
flowing into lakes. Among the
examples are the delta of theUral Riverin Kazakhstan ( 4653N
5137E / 46.883N
51.617E),[3]
the delta ofSaskatchewan Riverat its fall intoCedar
LakeinManitoba,[4]
orthe Mitchell River silt jettiesat the fall of the Mitchell
Riverinto Lake King (part ofAustralia's Gippsland Lakes).
Smaller formation of this type can be seen on rivers and
irrigation channels depositing
sediment into human-built reservoirs. One example is the
sediment-formed peninsula at
the point ( 452750N 443733E / 45.46389N 44.62583E) where the
KumaManych Canalflows into the Chogray Reservoirin southern Russia.
As these structures
were completed in the late 1960s, the peninsula must be the
product of just 40 years'
worth of sedimentation.
[edit] Wave-dominated deltas
In wave dominated deltas, wave erosion controls the shape of the
delta, althoughdeposition still outweighs the amount of erosion and
the delta is able to advance into the
sea. Deltas of this form, such as theNile Delta, tend to have a
characteristic Greek-
capital-delta shape .
The Ganges Delta in India and Bangladesh is the largest delta in
the world and it is alsoone of the most fertile regions in the
world.
[edit] Tide-dominated deltas
Erosion is also an important control in tide dominated deltas,
such as the Ganges Delta,which may be mainly submarine, with
prominent sand bars and ridges. This tends to
produce a "dendritic" structure.[2] Tidal deltas behave
differently from river- and wave-
dominated deltas, which tend to have a few main distributaries.
Once a wave- or river-distributary silts up, it is abandoned, and a
new channel forms elsewhere. In a tidal delta,
new distributaries are formed during times when there's a lot of
water around - such asfloods orstorm surges. These distributaries
slowly silt up at a pretty constant rate until
they fizzle out.[2]
[edit] Gilbert deltas
A Gilbert delta (named afterGrove Karl Gilbert) is a specific
type of delta that is formedby coarse sediments, as opposed to
gently-sloping muddy deltas such as that of the
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Mississippi. For example, a mountain river depositing sediment
into a freshwater lake
would form this kind of delta.[5][6] While some authors describe
both lacustrine and
marine locations of Gilbert deltas[5], others note that their
formation is more characteristicof the freshwater lakes, where it
is easier for the river water to mix with the lakewater
faster (as opposed to the case of a river falling into the sea
or a salt lake, where less dense
fresh water brought by the river stays on top longer).[7]
G.K. Gilbert himself first described this type of delta on Lake
Bonneville in 1885.[7]Elsewhere, similar structures can be found
e.g. at the mouths of several creeks flowing
into Okanagan Lakein British Columbia and forming prominent
peninsulas atNaramata (
493530N 1193530W / 49.59167N 119.59167W), Summerland (
493423N
1193745W / 49.57306N 119.62917W), orPeachland ( 494700N 1194245W
/
49.7833333N 119.7125W)
[edit] Estuaries
Other rivers, particularly those located on coasts with
significant tidal range, do not forma delta but enter into the sea
in the form of an estuary. Notable examples include the
SaintLawrence Riverand the Tagusestuary.
[edit] Inland deltas
Okavango Delta
In rare cases the river delta is located inside a large valley
and is called an inverted river
delta. Sometimes a river will divide into multiple branches in
an inland area, only to
rejoin and continue to the sea; such an area is known as an
inland delta, and often occuron former lake beds. The Inner Niger
Delta is the most notable example. TheAmazon
has also an inland delta before the island ofMaraj.
In some cases a river flowing into a flat arid area splits into
channels which then
disappear in the desert.Okavango Delta in Botswana is one
well-known example.
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