Coas Geol wk 5 - ch 5
Deltas – where rivers unload deposits
I.Introduction
Named by Herodotus in 450 B.C., for the greek letter , which is the shape of the
Nile delta’s main channel and its distributary channels (fig p.132)
Transitional environments, neither terrestrial nor marine
Formed when the amt sediment delivered at rivermouth exceeds the amt
removed by waves/tides – best developed on low-energy cont. margins
Main terminology diff between estuary and a delta:
 “estuary is an arm of ocean extending into a river (a drowned river mouth)”
 “delta is an arm of land projecting into ocean (or lake)”
A. where deltas develop…
Similar setting is conducive to delta development;
 trailing edge margin
 lots of sed from low and moderate relief terrain
 meandering channels (like Miss R)
trailing edge margin gives stable platform upon which delta can be built, and the
platform reduces incoming wave energy too
there are some exceptions, like US East Coast, which has many estuaries but no
deltas…in these settings, tidal currents disperse the sediment, don’t allow it to
accumulate
many deltas in marginal sea settings that are protected:
 Mississippi
 Rhone
 Nile
 Po
 Many in China
Wk 5 p.2
As expected, no major deltas on western edge of either N or S America, with
convergent, steep, narrow margins that allow big waves to come onshore
Note too that the high-relief drainage area of convergent margin does NOT
translate to lots of sediment delivery, because the AREA in this setting is not
particularly large
B. Age of deltas
Most are very young; existence in large part controlled by sea level
Slowdown of Holocene sea level rise ca. 7000 yrs ago allowed for deltas to start
to develop – relatively stable sea level position (fig p.137)
Some deltas also very old – Miss R has been emptying into GOM for millions of
years, at least since the Miocene…..
Davis points out that the current Miss R delta lobe has been in position only 600
yrs, and it has activley grown since New Orleans was founded in 1717.
II.Environments of Deltas
Like BI s, the type of deltaic environment that develops is function of interaction
between sediment load and wave/tide regime…
First step – river slows as it hits quiet water, sediment load drops out:
river
lake
once sediment begins to drop out,
it forms new ‘bottom” for river, and a number of new channels form in order to
keep distributing sediment further out, so delta progrades outward with classic
bottomset, foreset, and topset beds
Wk 5 p.3
Landward is the delta plain, with a few distributary channels –
Channels are flanked by levees, which are built up by flooding events when river
overflows its banks during flood stage
Levee
Channel
Levee
When levee is breached, again usu during flooding, a crevasse splay forms,
which essentially is a mini-overwash fan
Main channel
flow through breach
Crevasse splay
Moving seaward of delta plain, you get into very marshy conditions, lots of bays,
lots of plant and animal life (fig p.139)
As flooding occurs, entire interdistributary bay area is filled with muddy water,
fine-grained sediment settles out. As sediment is deposited, this area gradually
fills up and transitions from “silt-filled bays to vegetation-covered marshes”
(fig p.140)
over time, flooding brings more sediment to all parts of the delta, and delta slowly
rises in elevation and progrades seaward.
Seaward edge of delta plain is the contact with the ocean – the “subtidal” protion
of the delta known as the delta front
Wk 5, p.4
At the delta front, sandy material falls out of suspension near river mouth, but
clays are carried away.
Sand deposits take a variety of forms – for example, a distributary “mouth bar”
that forms seaward of mouth, and longshore drift can carry this sand into a series
of sandbars near the mouth:
Mouth bar
Longsh drift
Out at the seaward end of the delta front, fine-grained clay settles out slowly,
forming the bottomset beds upon which the delta will eventaully build out upon…
III.contest between river and sea
Somewhat of a contest between the river / landward component, and maritime
processes
Tidal currents tend to work sediments in bodies  to shore
Waves tend to smooth the shoreline and work sediments into bodies parallel to
shore
As long as enough sed is carried to ocean, delta will be maintained – “keep the
marine processes in check”
Key factors to delta’s survival:
 Adequate rainfall (to keep river flowing)
 Soil type that is loose, movable
Wk 5, p.5
Growing deltas have heavy flooding and much sediment dispersal – this causes
great problems for people who live and farm in deltas.
Human activity has had profound influence on deltas – controls on water and
sediment discharge (like with dams) are causing much delta erosion. Nile River
is current example.
IV.Delta shapes
Rivers, waves, tides provide 3 end member points of a ternary diagram
(Fig p.143)
Galloway has developed this scheme – note the variations between rivers,
waves, tides –
 Rivers – show “elongate” shape, lots sed, several distributary channels
 Wave-dominated – shows a more “cuspate” shape, seds smoothed out
 Tide-dominated – shows linear, isolated features reflecting flood and ebb
tides
A. river-dominated
The classic “bird’s-foot” shape, channels crossing a delta plain (Fig p.145)
Need:
“placid” oceanic regime,
a small tidal range,
ample sediment delivery
Miss River is classic example
B. tide-dominated deltas
A key here is there is a large high-to-low tidal range that leads to very strong tidal
currents…
Not a lot of wave energy, not a lot of longshore current energy, so seds line up in
direction of tidal flow,  to coastline – ”long bars perpendicular to coast”
Tide-dominated deltas resemble estuaries – example is Ganges-Brahmaputra
(fig p. 146)
Wk 5 p.6
C. Wave-dominated deltas
 Smooth shoreline, well-developed beaches and dunes
 Longshore currents spread sand along the shore, carry fines away
 Typically smaller than river or tide – dominated, because waves carry away
quite a bit of sediment
Essentially see 2 diff shapes of wave-dominated deltas:
1. symmetrically cuspate when the longshore drift is not strong in one net
direction – Sao Francisco in Brazil
2. strong longshore net drift sets up big barrier spits in direction of drift –
Senegal R. in W. Africa
D. Intermediate delta types – (figs p. 148)
Niger Delta mixes all 3
Nile mixes river- with wave-dominated – note the outer sand bars
Mahakam mixes river and tide
V. Human Intervention
Deltas are choice places for humans – farming, transportation, fishing, military
uses
Upland population growth, forest clearing, etc, results in accelerated erosion and
sediment delivery to the delta – modern example is Amazon River –
But Davis says more typical result of human activity is shrinkage of delta, not
expansion – this happens because velocity decreases, sediment can’t be carried
to the edge.
Davis cites Aswan High Dam in Egypt has decreased Nile fertility – sediment
trapped behind dam, can’t make it to the delta. At same time, waves continue to
attack the existing delta. Without new sediment, erosion is occurring. And
fishing is declining as well.
Same thing happening with the Colorado River where it dumps into Sea of
California – so much water has been siphoned off that virtually no water nor
sediment makes it to the coast.
Chapter closes with Davis speculating about future of co-habitation between
man and delta…..