Origin and evolution of interdistributary delta plains; insights from Mekong River Delta

Abstract

The geometry of river deltas is considered to reflect the interplay of coastal processes, river discharge, and sediment supply. As similar to 25% of the world's population lives on deltaic lowlands, prediction of delta growth is critical. Knowledge of processes responsible for delta geometries is not well established, although such knowledge is critical for the risk management of land use and settlements on deltas. The Mekong River delta of southern Vietnam is one of the largest deltas in the world and offers a unique opportunity to understand the sedimentary evolution of mixed tide- and wave-dominated large river deltas. We constrained the three-dimensional sedimentary evolution of the delta plains, based on optically stimulated luminescence dating of beach ridges coupled with radiocarbon-dated sediment cores. Results show that the beach shoreline in the lower delta plain initiated ca. 3.5 ka by aggradation on basement shoals. The delta plain propagated laterally during the late Holocene, evolving from bars that resulted in asymmetric bifurcation of the river mouth. Asymmetry was caused by the southwestward longshore sediment drift enhanced by the winter monsoon. Bars were successively formed on the wider side of the bifurcated river mouths, and subsequently accreted seaward, being stabilized by tide effects to result in shore-perpendicular elongate delta plains. Our work, based on the Mekong delta, demonstrates for the first time that bar emergence is a key process in the long-term evolution of mixed tide- and wave-dominated deltas.

Full text

GEOLOGY, April 2012 303
INTRODUCTION
The Mekong River delta, southern Vietnam
(Fig. 1A), offers a unique opportunity for under-
standing the sedimentary evolution of mixed
tide- and wave-dominated large river deltas
(Galloway, 1975) that have formed during sea-
level highstands. The annual sediment discharge
of the Mekong River is the ninth largest among
the world’s rivers (Milliman and Syvitski, 1992)
due to the associated drainage of a well-watered
catchment subject to a monsoonal climate and
high sediment supply. This has resulted in the
formation of an extensive delta over the past
8 ka (Nguyen et al., 2000; Tamura et al., 2009),
from southern Cambodia to the South China
Sea coast of southern Vietnam. The low-lying
delta plain has a population of 20 million, and
supports local agriculture, fi shery, and other
industries. The morphological changes of the
Mekong delta thus have a signifi cant environ-
mental impact on the region.
The geometry of the Mekong River delta is
characterized by several shore-perpendicular
elongate delta plains, where sequences of beach
ridges occur (Fig. 1B). The subsurface sedimen-
tary architecture of the delta and its coastal sea
has been recently clarifi ed by many investiga-
tions (e.g., Nguyen et al., 2000; Ta et al., 2002,
2005; Tanabe et al., 2003; Tamura et al., 2009,
2010; Proske et al., 2010, 2011; Tjallingii et
al., 2010; Xue et al., 2010). Compilation of 10
radiocarbon-dated sediment cores of the delta
plain (Ta et al., 2005) revealed the architecture
and temporal evolution of the subaqueous delta
sediment. Contrasts in the sedimentary succes-
sion and landform between the upper and lower
delta plains were attributed to a gradual increase
in wave infl uence as the deltaic shoreline pro-
graded seaward to become more exposed to
ocean waves (Ta et al., 2002, 2005; Tanabe et
al., 2003). However, there is still much uncer-
tainty concerning the continuous evolution
history and processes that have resulted in the
present elongate delta plains, which are typical
of tide-infl uenced deltas (e.g., Reynolds, 1999;
Kuehl et al., 2005; Bhattacharya, 2006). Apart
from sediment supply, river discharge, and
waves, local factors such as basement heteroge-
neity and Asian monsoons also appear to affect
the delta sedimentation, but have not been well
considered. In contrast to established subsurface
data (Ta et al., 2005), the shoreline changes have
been poorly constrained, restricting the under-
standing of the Mekong delta evolution.
This paper presents new views on the three-
dimensional sedimentary evolution of the
Mekong River delta plains controlled by unique
interplays of coastal processes, basement con-
fi gurations, the monsoon climate, and sea-level
changes, based on the geometry and optically
stimulated luminescence (OSL) dating of beach
ridges coupled with previously reported sedi-
ment cores and their radiocarbon ages.
DATING OF BEACH RIDGE
Beach ridges are formed within or near the
beach, and are preserved as relict elongate
mounds parallel or subparallel to the shoreline
following subsequent beach progradation. Their
geometry and depositional age thus gener-
ally indicate past shoreline position and shape
(e.g., Mason, 1993). Beach ridges on the lower
Mekong delta plain are convex seaward, and are
associated with branching and recurved shapes,
concordant with recent coastal changes caused
by southwestward accretion of recurved spits
(Tamura et al., 2010). The asymmetric changes
refl ect a net sediment drift induced by the domi-
nance of the northeasterly winter monsoon
waves over the southwesterly summer monsoon
waves. The beach ridges are composed of fi ne
sand, and are generally <5 m above the present
mean sea level (Gagliano and McIntire, 1968;
Ta et al., 2002). While the width of beach ridges
is less than a few kilometers, the muddy inter-
ridge swales are ~5 km wide. Stutz and Pilkey
(2002) suggested that the Mekong delta shore-
line progrades periodically as sandy bars and/
or islands forming on the delta front platform
become linked to the mainland, followed by silt-
ation of the back-barrier lagoon to form inter-
ridge swales. The modern shoreline exhibits no
remarkable lagoon as large as the inland swales,
thus suggesting that the siltation likely becomes
complete soon after the barrier forms, refl ecting
the abundant river sediment supply. The geom-
etry and depositional age of the Mekong beach
ridges thus robustly record the progradation of
the delta plain.
Quartz OSL dating was performed on beach
ridges on delta plains of Tra Vinh, South Ben
Geology, April 2012; v. 40; no. 4; p. 303–306; doi:10.1130/G32717.1; 3 fi gures; Data Repository item 2012087.
© 2012 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
*E-mail: toru.tamura@aist.go.jp.
Origin and evolution of interdistributary delta plains; insights from
Mekong River delta
Toru Tamura
1,2
*, Yoshiki Saito
1
, V. Lap Nguyen
3
, T.K. Oanh Ta
3
, Mark D. Bateman
2
, Dan Matsumoto
1
, and
Shota Yamashita
4
1
Geological Survey of Japan, AIST (National Institute of Advanced Industrial Science and Technology), Central 7, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8567, Japan
2
Sheffi eld Centre for International Drylands Research, University of Sheffi eld, Winter Street, Sheffi eld S10 2TN, UK
3
Institute of Resources Geography, Vietnamese Academy of Science and Technology, 1 Mac Dinh Chi Street, 1 District,
Ho Chi Minh City, Vietnam
4
Department of Earth Science, Chiba University, Yayoi-Cho, Inage-Ku, Chiba 263-8522, Japan
ABSTRACT
The geometry of river deltas is considered to refl ect the interplay of coastal processes, river
discharge, and sediment supply. As ~25% of the world’s population lives on deltaic lowlands,
prediction of delta growth is critical. Knowledge of processes responsible for delta geometries
is not well established, although such knowledge is critical for the risk management of land use
and settlements on deltas. The Mekong River delta of southern Vietnam is one of the largest
deltas in the world and offers a unique opportunity to understand the sedimentary evolution
of mixed tide- and wave-dominated large river deltas. We constrained the three-dimensional
sedimentary evolution of the delta plains, based on optically stimulated luminescence dating
of beach ridges coupled with radiocarbon-dated sediment cores. Results show that the beach
shoreline in the lower delta plain initiated ca. 3.5 ka by aggradation on basement shoals. The
delta plain propagated laterally during the late Holocene, evolving from bars that resulted
in asymmetric bifurcation of the river mouth. Asymmetry was caused by the southwestward
longshore sediment drift enhanced by the winter monsoon. Bars were successively formed on
the wider side of the bifurcated river mouths, and subsequently accreted seaward, being sta-
bilized by tide effects to result in shore-perpendicular elongate delta plains. Our work, based
on the Mekong delta, demonstrates for the fi rst time that bar emergence is a key process in the
long-term evolution of mixed tide- and wave-dominated deltas.

304 GEOLOGY, April 2012
Tre, Central Ben Tre, North Ben Tre, Cai Lay,
and Tien Giang (Fig. 1B). We collected 31
samples in light-tight stainless steel tubes from
boreholes deeper than 50 cm drilled using a
sand auger (Table DR1 in the GSA Data Reposi-
tory
1
). We avoided areas of human disturbance
and eolian dunes so that the OSL ages refl ect the
depositional timing of the shoreline sediment.
All samples were measured at the Sheffi eld
Centre for International Drylands Research
luminescence laboratory using a TL-DA-20
automated Risø TL (thermal luminescence)/
OSL reader. The standard single aliquot regen-
erative dose protocol was used to determine
the paleodose (De) of cleaned, coarse-grained
quartz extracts using an experimentally derived
optimal preheat temperature for these samples
(Murray and Wintle, 2000). All samples showed
high OSL sensitivity and a well-behaved OSL
signal dominated by the rapidly bleachable fast
component with low thermal transfer and good
recycling. Replicate De values were unscattered,
unimodal, and normally distributed, allowing
fi nal De values to be derived using the central
age model (Galbraith et al., 1999). A single
sample from the innermost beach ridge of North
Ben Tre showed a bimodal De distribution, to
which the minimum age model (Galbraith et al.,
1999) was applied, assuming an overdispersion
of 0.05 as most of the other unimodal samples
have overdispersion of ~0.05. Concentrations
of potassium, uranium, thorium, and rubidium
were quantifi ed by inductively coupled plasma
mass spectrometry, and were converted into
TC1
TV1
VL1
DT1
BT3BT2
BT1 GGN
Silt
Sand
Clay
vffm
c
Silt
Sand
Clay
vffm
c
4.4 ka
3.5 ka
2.3 ka
1.0 ka
7.0 ka
2.5 ka
1.1 ka
3.5 ka
7.3–7.0
7.0–6.8
8.7–8.5
9.1–8.7
10.7–10.5
10.6–10.4
11.6–11.3
3.5–3.4
3.7–3.6
3.7–3.5
3.9–3.7
4.9–4.7
6.2–6.1
3.0–2.8
3.6–3.5
4.1–4.0
4.5–4.3
4.9–4.8
4.9–4.7
5.3–5.1
0.82–0.74
1.1–1.0
1.5–1.4
1.7–1.6
1.9
2.1–2.0
2.8
Undifferentiated
Pleistocene deposits
+5
0
-10
Elevation (m)
-20
-30
-40
-50
Sequence boundary
3.6 ± 0.2, 3.3 ± 0.2
2.8 ± 0.1
2.3 ± 0.1
1.6 ± 0.1 0.97 ± 0.05
0.52 ± 0.03
2.1 ± 0.1
0.29 ± 0.02
0.18 ± 0.01
X
X
′
Upper delta plain
A
Lower delta plain
+5
0
-10
Elevation (m)
-20
-30
-40
-50
3.7–3.5
5.4–5.2
8.2–8.0
4.2–4.0
1.7–1.5
1.4–1.3
0.99–0.80
2.5 ± 0.1
1.3 ± 0.1
1.1 ± 0.1
0.65 ± 0.03
Lower delta plain
Y
Y
′
Sequence
boundary
Undifferentiated
Pleistocene deposits
B C
+5
0
-10
Elevation (m)
-20
-30
02
46
810
Age (ka)
2.2–2.0
25 km
25 km
Holocene marine and
brackish-water sediment
OSL age (ka)
Radiocarbon age (ka)
Sequence
boundary
0.29 ± 0.02
4.9–4.7
1
GSA Data Repository item 2012087, OSL sam-
ples and age results, is available online at www.geo-
society.org/pubs/ft2012.htm, or on request from edit-
ing@geosociety.org or Documents Secretary, GSA,
P.O. Box 9140, Boulder, CO 80301, USA.
4.84 ± 0.23
4.55 ± 0.22
5.57 ± 0.26
1.76 ± 0.09
1.52 ± 0.08
0.40 ± 0.02
0.17 ± 0.01
1.20 ± 0.08
0.81 ± 0.04
0.65 ± 0.03
0.28 ± 0.01
1.09 ± 0.05
1.27 ± 0.06
2.45 ± 0.12
3.33 ± 0.17
1.37 ± 0.07
3.57 ± 0.19
2.78 ± 0.14
2.32 ± 0.12
2.08 ± 0.10
1.62 ± 0.09
1.59 ± 0.09
1.11 ± 0.06
0.97 ± 0.05
0.77 ± 0.04
0.52 ± 0.03
0.29 ± 0.02
0.07 ± 0.01
0.18 ± 0.01
0.04 ± 0.01
3.33 ± 0.17
Tidal flat:
4.50–4.88 ka
TA
GGN
BT1
BT2
BT3
GC1
TV1
TC1
VL1
DT1
CD1
X
X
′
Y
Y
′
106°E
106°30
′
10
o
N
N9°30
′
10°30
′
N
25 km
Tra Vinh
South
Ben Tre
Central
Ben Tre
North
Ben Tre
Cai Lay
Tien
Giang
10 m
20 m
B
Upper delta plain
- Flood basin
- Flood plain
- Back swamp
Lower delta plain
- Coastal plain
- Salt marsh
- Mangrove marsh
Abandoned channel
Channel bar or islet
Beach ridge
Drill core
OSL age (ka)
E
South China Sea
CAMBODIA
VIETNAM
PHNOM PENH
HO CHI MINH CITY
N11
100 km
KS
PK
PSG
E106
N9
E108
BT2
Mekong River delta
72-2
73-3
69-2
70-2
Mekong River
Bassac River
Fig. 1B
GGM
Gulf of
Thailand
A
DT1
Figure 1. A: Location of Mekong River delta.
Locations of sediment drill cores reported by
previous studies are shown. Cores KS, PK,
and PSG are from Tamura et al. (2009); core
GGM is from Proske et al. (2010); cores DT1
and BT2 are from Ta et al. (2002, 2005); and
cores 69–2, 70–2, 72–2, and 72–3 are from
Tjallingii et al. (2010). B: Geomorphology of
Mekong River delta (simplifi ed from Nguyen
et al., 2000) and bathymetry of coastal sea
relative to mean sea level (Ta et al., 2005).
Beach ridges were redefi ned using Landsat
image taken in 1989 and after identifi cation
(Gagliano and McIntire, 1968; Nguyen et
al., 2000). Delta-front platform extends from
shoreline to 4-m-deep isobath, offshore of
which is delta-front slope. Delta-front slope
grades offshore into prodelta and shelf at wa-
ter depth of 18–20 m. Sediment cores TA and
GGN are from Proske et al. (2011), and other
cores are from Ta et al. (2002, 2005). Optically
stimulated luminescence (OSL) and radiocar-
bon ages are expressed relative to A.D. 2010.
Figure 2. A, B: Cross sections along shore-perpendicular transects of Tra Vinh and Central
Ben Tre delta plains (modifi ed from Ta et al., 2005). Transect locations are shown in Figure 1B.
Isochronous lines were redefi ned according to optically stimulated luminescence (OSL) ages
of beach ridges. OSL and radiocarbon ages are expressed relative to A.D. 2010. C: Relative
sea-level curve of Mekong River delta since 10 ka (Ta et al., 2002; Hanebuth et al., 2011).

GEOLOGY, April 2012 305
dose rate based on data from Adamiec and Ait-
ken (1998) and Marsh et al. (2002). Past changes
of moisture content are not known, so an error
of 5% was applied to the present-day annual
average to account for them. Cosmic dose rate
was estimated based on Prescott and Hutton
(1994). OSL ages are expressed relative to A.D.
2010 with associated 1σ uncertainties. Results
of OSL dating are summarized in Table DR1.
All radiocarbon ages reported from previous
studies are also expressed relative to A.D. 2010
for comparison with OSL ages.
RESULTS AND DISCUSSION
OSL ages of the beach ridges illustrate the
seaward progradation of the delta, being con-
cordant with radiocarbon ages of sediment cores
(Fig. 1B). Beach ridges in Cai Lay have ages
ranging from 4.6 to 5.6 ka, agreeing with radio-
carbon ages of tidal fl at facies in core TA nearby
(4.50–4.88 ka; Fig. 1B; Proske et al., 2011). The
tidal fl at facies in core GGM, located ~50 km
northwest of Cai Lay (Figs. 1A and 1B), has a
radiocarbon age of 6.37–6.72 ka (Proske et al.,
2010). These ages show that in the northern part
of the delta the shoreline prograded during the
mid-Holocene sea-level highstand (Fig. 2C) to
form a sandy beach in Cai Lay ca. 5.6–4.6 ka.
No age reversals are identifi ed when OSL ages
from the Tra Vinh and Central Ben Tre plains
are compared with radiocarbon ages from sedi-
ment cores (Figs. 2A and 2B).
Isochronous lines of deltaic sediment in cross
sections along transects of the Tra Vinh and Cen-
tral Ben Tre plains are revised from those pre-
sented by Ta et al. (2005) according to the new
OSL ages (Figs. 2A and 2B). The initiation of
the Tra Vinh beach-ridge system is assumed as
ca. 3.5 ka, based on two OSL ages of the inner-
most ridge, 3.57 ± 0.19 ka and 3.33 ± 0.17 ka.
Along the Tra Vinh transect sediment aggrada-
tion over a basement shoal in the lower delta
plain occurred between 4.4 and 3.5 ka, initiating
the beach shoreline (Fig. 2A). The Central Ben
Tre beach initiated ca. 2.5 ka, before which time
the progradation was not pronounced and aggra-
dation is inferred to have occurred. Clinoforms
younger than 3.5 ka and 2.5 ka are defi ned in the
Tra Vinh and Central Ben Tre transects, respec-
tively, clearly illustrating the progradation of the
shoreline and delta front. The clinoforms gradu-
ally steepen toward the present coast except for
the very gentle and wide delta-front platform of
Central Ben Tre.
Beach shorelines in the lower delta plain are
laterally younger northeastward (Fig. 1B). The
South Ben Tre beach initiated almost simultane-
ously with Tra Vinh. The initiation of the Cen-
tral Ben Tre beach is ~800 yr later than that of
South Ben Tre. The North Ben Tre delta plain
has remarkably younger OSL ages than Cen-
tral Ben Tre. The majority of Tien Giang beach
ridges have prograded over the past 400 yr.
The age structure of the beach ridges suggests
that the delta-plain formation propagated later-
ally during the late Holocene (Fig. 3). A large
river mouth opening southeastward occurred
between the shorelines of Cai Lay and Tra Vinh
ca. 3.5 ka. The river mouth roughly refl ects the
main incised valley revealed by cores KS and
PSG (Tamura et al., 2009), cores DT1 and BT2
(Ta et al., 2002, 2005), and seismic profi les and
piston cores offshore (Tjallingii et al., 2010)
(Fig. 1A). The sediment body in South Ben Tre
is inferred to have emerged as a longshore bar
in the river mouth. The initial growth process
of the longshore river-mouth bar is uncertain,
but from cross sections of the Tra Vinh and
Central Ben Tre plains, the initial beach shore-
line was exposed subaerially, followed shortly
by, or simultaneously with, rapid aggradation
of the inner part of the bar. The mouth bar
then accreted seaward and probably landward
(Olariu and Bhattacharya. 2006) to become a
shore-perpendicular elongate delta plain. Sub-
sequently, the Central Ben Tre and North Ben
Tre delta plains formed in a similar way. The
emergence of the longshore mouth bar splits
the river mouth into two distributaries. Note that
the northeastern side of the bifurcated mouth is
always wider, where the subsequent mouth bar
emerges to cause further bifurcation. The sedi-
ment drift due to the dominant winter monsoon
distorts the mouth bar southwestward, resulting
in the asymmetric bifurcation.
The inferred successive formation of the long-
shore river-mouth bars and the resultant bifurca-
tions of the river mouth account for the distribu-
tary network. Apexes of delta plains in South
Ben Tre, Central Ben Tre, and North Ben Tre
in turn defi ne bifurcations of the distributaries.
The bifurcation initially caused by the mouth bar
emergence was preserved as the tidal infl uence
stabilized the distributaries (Bhattacharya, 2006).
Avulsion was thus not critical to the branching of
distributaries. However, the distributary between
North and Central Ben Tre was choked by sedi-
ment so that the Central Ben Tre shoreline was
abandoned, and it has showed negligible progra-
dation over the past several hundred years. The
offshore delta-front slope in contrast has con-
tinued to prograde. Consequently, the shoreline
and delta-front slope were detached, resulting in
a wide delta-front platform (Figs. 1B and 2B).
Whereas the shoreline is composed of sand sup-
plied from the adjacent distributary by longshore
drift, the progradation of which has been retarded
by the choking of the distributary, mud and very
fi ne sand are transported in a river plume more
widely offshore, and partly deposited on the
delta-front slope. The delta-front slope is thus
less sensitive to the adjacent distributary. The Tra
Vinh shoreline has prograded more constantly
until the present, accounting for its relatively nar-
row delta-front platform.
A gap in beach-ridge age of more than
1000 yr occurs between Cai Lay and the lower
delta plain. This hiatus corresponds to a period
of relative sea-level fall (Fig. 2C; Ta et al., 2002;
Hanebuth et al., 2011), although sea-level fall is
generally thought to promote beach-ridge for-
mation (e.g., Taylor and Stone, 1996). Beach
ridges in Cai Lay, in contrast to those in the lower
Abandoned
Choking
3.5–3.3 ka
2.5–2.1 ka
1.5–1.2 ka
Present
Cai Lay
Tidal flat
Tidal flat
progradation
Tra Vinh
South Ben Tre
Central
Ben Tre
Cai Lay
Tra Vinh
Wave
exposure
returned
Delta plain
Aggradation
Delta-front slope
Beach and beach ridge
Net longshore drift
Age control
Cai Lay
Tra Vinh
Cai Lay
Tra Vinh
Tien Giang
North
Ben Tre
Tien Giang
Figure 3. Schematic illustration showing
evolution of Mekong River delta plains. River
mouth at 3.3–3.5 ka refl ects main incised val-
ley of Mekong River delta. Longshore drift of
sand is driven by dominant winter monsoon
and associated waves and currents.

306 GEOLOGY, April 2012
delta plain, exhibit no evidence of southwest-
ward longshore drift (Fig. 1B). The northeast-
erly winter monsoon and associated waves and
currents had less effect on Cai Lay, which was
sheltered by the coast and hills east of the delta
(Fig. 1A). The southwesterly waves, related
to the weaker summer monsoon, were more
important for beach-ridge formation in Cai Lay.
From 4.4 to 3.5 ka, aggradation occurred in Tra
Vinh, thus attenuating the southwesterly waves
that reached the Cai Lay shore. This may have
led to the formation of a non-beach shoreline,
probably a muddy tidal fl at, around Cai Lay
with consequently no beach-ridge formation.
The sheltered coast and muddy progradation
continued until ca. 1.8 ka, at which time pro-
gradation had become suffi cient for the shore
to become exposed once again to larger waves
that formed the innermost beach ridge in Tien
Giang (Fig. 1B). Farther southwestward, the
initial beach shoreline of Tra Vinh determined
the position of the lower delta plain (Figs. 1 and
3). It evolved from aggradation at the landward
edge of the basement shoal (Fig. 2A). The base-
ment confi guration thus appears to have affected
the shape of the delta plain.
CONCLUSIONS
Characterization of the late Holocene shore-
line changes based on OSL ages of beach ridges
has helped to clarify the origin and continuous
evolutionary history of the Mekong River delta
plains. The delta plains did not initiate synchro-
nously; their formation occurred through the suc-
cessive emergence of longshore river-mouth bars
that aggraded into beach ridges. The river-mouth
bar formation was affected by asymmetric sedi-
ment drift due to winter monsoon waves. Delta
plains were stabilized by the infl uence of tide,
and prograded seaward to result in their shore-
perpendicular elongate shapes. The successive
formation of river-mouth bars accounts for the
distributary network of the northern Mekong
delta. The initial formation of the beach shoreline
in the lower delta plain appears to be determined
by the basement confi guration. The geometry
of the Mekong River delta has thus been con-
trolled by contributions of the river, tides, and
monsoon waves, and was also affected by base-
ment confi guration. The detailed chronology
of beach ridges generates critical knowledge of
the long-term deltaic shoreline behavior in the
past. As such, the approach, if applied to other
wave-infl uenced deltas around the globe, would
provide the essential basis upon which modeling
studies could predict future changes of deltas.
ACKNOWLEDGMENTS
We thank Adam Dunajko and Rob Ashurst for their
help in processing luminescence samples. This work
was done while Tamura worked at the University of
Sheffi eld with the aid of Japan Society for the Promo-
tion of Science Postdoctoral Fellowships for Research
Abroad, and was partly supported by NAFOSTED
(The Vietnam National Foundation for Science and
Technology Development; Projects 105.09-2010.05
and 105.09-2010.02), and the Megadelta Project of
the Ministry of Environment of Japan. The manu-
script was improved by Liviu Giosan, an anonymous
reviewer, and Bradley Opdyke.
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Manuscript received 5 August 2011
Revised manuscript received 26 October 2011
Manuscript accepted 30 October 2011
Printed in USA