Hedley et al

Evolution of the Irrawaddy delta region
since 1850geoj_346 138..149
PETER J HEDLEY*, MICHAEL I BIRD*† AND RUTH A J ROBINSON*
*School of Geography and Geosciences, University of St Andrews, St Andrews, Fife KY16 9AL
E-mail: rajr@st-andrews.ac.uk
†Present address: School of Earth and Environmental Science, James Cook University, PO Box 6811,
Cairns, Queensland 4870, Australia
This paper was accepted for publication in September 2009
We present a time series of coastline change for the Irrawaddy delta region of Myanmar using
the earliest available navigation chart from 1850, and a set of topographic maps and satellite
imagery dating from 1913 to 2006. Despite the large sediment load delivered annually to the
gulf by the Irrawaddy and Salween Rivers, the coastline has been largely stable for 156 years,
advancing at an average rate of no more than 0.34 km per century since 1925. The long-term
average rate of increase in land area across the study area between 1925 and 2006 is
4.2 km2
/year, but this masks a period of more rapid accumulation between 1925 and 1989
(8.7 km2
/year), followed by a period of net erosion at a rate of 13 km2
/year until 2006. Less
than 9% of the sediment load delivered to the study region by the Irrawaddy, Salween and
Sittoung Rivers has contributed to the observed progradation, with the remainder being
exported into the Gulf of Martaban to depths below low tide level, or filling any accommo-
dation space created due to subsidence or sea level rise. In contrast to many deltas
worldwide, we suggest that the coastline encompassing the Irrawaddy delta and the Salween
River is more or less in equilibrium, and that sediment deposition currently balances
subsidence and sea level rise. Myanmar has fewer large dams relative to its Asian neigh-
bours, and the Salween is currently undammed. This is forecast to change in the next 5–10
years with extensive damming projects on the mainstem of the Salween under consideration
or construction, and the sediment retention will cause losses in sediment supply to the Gulf
of Martaban, and retreat of the delta. This could impact the densely populated delta region
and Yangon, and further exacerbate the impacts of extreme events such as Cyclone Nargis in
2008.
KEY WORD: Burma, Irrawaddy delta, Salween, sediment flux, coastal sedimentation
Introduction
R
ivers are the major conduits for the transfer of
water, particulates and dissolved material from
the land to the ocean and more than 50% of
this flux enters the global ocean in the tropics (Milli-
man and Meade 1983; Milliman and Syvitski 1992;
Nittrouer et al. 1995). A significant proportion of this
flux is focused on, and delivered via, major delta
complexes at the mouths of rivers such as the Amazon
(Nittrouer et al. 1991), Ganges-Brahmaputra (Good-
bred and Kuehl 1999), Orinoco (Warne et al. 2002),
Niger (Sexton and Murday 1994), Mekong (Tanabe
et al. 2003b), Yangtze (Yang et al. 2006), Chao Phraya
(Tanabe et al. 2003a), Fly (Palinkas et al. 2006) and
Irrawaddy (Stamp 1940; Robinson et al. 2007). Such
delta environments are geologically young, having
developed since about 8000 years ago in response to
a slowdown in the rate of post-glacial eustatic sea-
level rise (Meade 1996; Stanley and Warne 1994
1997).
Deltas are dynamic environments shaped by the
interaction of sediment and water influx from rivers,
tidal regime, sea level, wind patterns and ocean cur-
rents, all of which vary on seasonal to millennial
timescales.The evolution of deltas also reflects anthro-
pogenic changes in basin land use (Woodroffe
et al. 2006). Tropical deltas are highly productive
The Geographical Journal, Vol. 176, No. 2, June 2010, pp. 138–149, doi: 10.1111/j.1475-4959.2009.00346.x
The Geographical Journal Vol. 176 No. 2, pp. 138–149, 2010
© 2009 The Author(s). Journal compilation © 2009 The Royal Geographical Society

ecosystems both on- and offshore, and, as a result,
commonly sustain high human population densities
and agricultural productivity.They contain many of the
remaining large mangrove areas and these are increas-
ingly coming under pressure from high population
densities, resulting in accelerated mangrove clearance
for wood, agriculture and aquaculture (Bird and Teh
2006). This includes direct disturbance arising from
mangrove clearance and embankment construction, as
well as activities such as deforestation and damming in
the river catchments that feed deltaic systems (e.g.
Syvitski et al. 2005). In addition, deltas are likely to
respond rapidly to both natural and anthropogenic
climate and sea-level change, with the potential for
signiﬁcant impacts on populations that live in delta
regions (IPCC 2007; Woodroffe et al. 2006).
The Irrawaddy delta is one of the major tropical
deltas in the world in one of the poorest nations in
Southeast Asia; Burma currently has a low per capita
GDP ($379 in 2007) and low population density (esti-
mated at 72 people/km2
for 2007) (United Nations
Department of Economic and Social Affairs 2008).
This study seeks to quantify historic changes in the
position of the shoreline of the Irrawaddy delta, and
the coast eastwards around the Gulf of Martaban to
the Salween River (Figure 1) in order to elucidate the
processes controlling the evolution of the delta, and to
compare it to other mega-deltas in Asia. Anthropo-
genic activities, including mining, water and hydro-
carbon extraction, sediment retention in dams, and
engineering structures on waterways have led to the
current retreat of many mega-deltas in Asia and the
placement of large coastal urban populations at
greater risk (Syvitski 2008). Although Burma has abun-
dant water resources, only ~18 km3
of water is cur-
rently stored in reservoirs and tanks which represents
~2.5% of the total annual discharge (all rivers) or
~4.4% of the Irrawaddy discharge (Aung 2003;
Robinson et al. 2007; Myanmar Irrigation Works
Department 2004). Although there are a considerable
number of small–medium reservoirs and canal
systems on tributaries, Burma’s mainstem rivers are
still less regulated than other Asian rivers and the
current sediment supply to the Gulf of Martaban is
Figure 1 (A) Map of Myanmar showing its rivers and adjacent coastal area, including the Gulf of Martaban. Bathmetric
contours were provided by V. Ramaswamy and the contour intervals are in metres. The dashed thick line near the 40 m
contour off the Irrawaddy delta is the 36.6 m (20 fathom) contour derived from the 1850 map of Lt. W. Fell (see Figure 2).
(B) Frequency of tropical storms and cyclones from the Indian Ocean for the past half century. Data are from the Climate
Explorer database (e.g. Oldenborgh and Burghers 2005). 1050 ¥ 667 mm (72 ¥ 72 dpi)
Evolution of the Irrawaddy delta region since 1850 139

less attenuated by damming. However, over the next
5–10 years, major mainstem damming projects will
progress on the Salween River and further dams are
under consideration on tributaries of the Irrawaddy
and Chindwin (Figure 1). This paper documents the
historical and current status of the Irrawaddy delta
region and serves as a reference for tracking how
future changes in the delivery of riverine sediment
impact on the position of the coastline.
Mangroves are a key element in the evolution of
tropical deltas such as the Irrawaddy, acting as
sediment traps and primary colonisers of shallow
submerged sand bars, and as bioshields against the
impacts of large storm events and tsunamis (Osti et al.
2009). Almost 50% of the world’s mangrove forests
have been lost in the past 20 years and the ecological
status of Irrawaddy delta mangroves is now in con-
tinuous decline (Blasco and Aizpuru 2002) due to
increased rice production and harvesting of man-
groves for fuel over the last 30 years (Oo 2002). Stamp
(1925, 267), based on the late nineteenth to early
twentieth century period of British involvement in the
region, commented that ‘very little damage . . . [to
mangrove forests] . . . is caused by wind, and
cyclones are unknown’. However, based on data from
the last decades (Figure 1), while the number of tropi-
cal storms per year in the Indian Ocean dropped in
the mid 1970s, the frequency and number of tropical
cyclones has increased, particularly since the mid-
1990s (Oldenborgh and Burghers 2005). There is only
one known Indian Ocean cyclone that had landfall in
Burma prior to 2008; Mala hit the coast to the north of
the delta (latitude 17.5°N) in 2006 (Fritz et al. 2009).
Cyclone Nargis tracked eastward across the delta in
early May 2008, with winds reaching 165–212 km/
hour (Fritz et al. 2009; NASA 2008). The cyclone, and
accompanying heavy rainfall (up to 600 mm), inun-
dated more than 14 402 km2
of the delta causing cata-
strophic damage and loss of life in the region (NASA
2008), and has increased the vulnerability of the delta
during storms due to the reduced density of man-
groves. Our study additionally provides a baseline on
which shoreline changes since Cyclone Nargis, and
the impacts of future reduction in mangrove forest
density, can be evaluated.
Study area
The Irrawaddy River feeds one of the two largest delta
systems in Southeast Asia and the total catchment area
of rivers draining into the Gulf of Martaban is about
0.7 ¥ 106
km2
(Robinson et al. 2007). The Irrawaddy
and Chindwin Rivers (Figure 1) have been in existence
since at least the Eocene, and the sediments carried by
the palaeorivers from the Himalayas to the ocean
have gradually infilled the Burma Trough with about
900 km of progradation over the last ~50 million years
(Nyi Nyi 1967; Bender 1983). The extensive wedge-
shaped modern delta probably originated around
7000–8000 years ago when other major deltas in
Southeast Asia developed (e.g. Tanabe et al. 2003a),
and now comprises around 20 571 km2
of flat, low-
lying fertile delta plain, with five major and many
smaller distributaries (Stamp 1940; Orton and
Reading 1993; Stanley and Warne 1994; Woodroffe
2000). It is classified as a mud-silt, tide-dominated
system with a mean tidal range of 4.2 m (Hayes 1979;
Orton and Reading 1993), and tidal influence extends
almost 300 km inland as far as Henzada at the apex of
the delta (Figure 1), with saline water penetrating up
to 100 km upstream (Aung 2003). The maximum
spring tidal range increases eastwards from Diamond
Island (2.8 m) to Elephant Point (6.4 m) (Figure 2).
Tidal currents as high as 3 m/s have been reported
from the Bassein estuary (Figure 2) when ebb tides
and river flooding coincide (Volker 1966).
Much of the region was initially forested and man-
groves grow in tidally influenced areas, but the total
mangrove forest area in the delta decreased from
2345 to 1786 km2
between 1924 and 1995 as a result
of logging and clearance for agriculture and aquacul-
ture (Oo 2002; Adas 1974). The population of the
coastal and delta areas grew from 9.6 million in 1983
to 12.7 million in 1997 (27% of the country’s popu-
lation for 1997 and a population density of 583
people/km2
), and the continued population increase
has affected land use and coastal resources (Oo
2002); the majority of the population are involved in
agriculture, and rice production accounts for the
largest proportion of the cultivated area (2 million ha
in 1996; Aung 2003).
A reassessment of the original data of Gordon
(1879–80 1885) from surveys conducted throughout
the 1870s suggests that the Irrawaddy delivers
442 Ϯ 41 km3
of water containing 226–364 mega-
tonnes (Mt) of sediment to the ocean via the delta
every year (Robinson et al. 2007), carrying a prelimi-
nary estimate of 3.1–5.2 Mt/year of organic carbon
(Bird et al. 2008). Because of the monsoonal nature of
the climate, approximately 80% of the water and 92%
of the sediment discharge occurs between June and
November. Most of the water debouches to the ocean
via the central distributaries of the delta, with com-
paratively little entering via the western (Bassein) and
eastern (Rangoon) distributaries (Figure 2). There are
no major dams on the trunk stream of the Irrawaddy,
but about 1300 km of embankments were built in the
late nineteenth and early twentieth century to protect
agricultural land. These embankments, and the artifi-
cial cutting of meander loops, have had the effect of
limiting overbank flooding and deposition of sus-
pended load in the delta (Volker 1966; Stamp 1940).
The modern coast is marked by shore-parallel sand
ridges, with the offshore bathymetry characterised by
140 Evolution of the Irrawaddy delta region since 1850

a low gradient continental shelf about 120 km wide.
To the east, the continental shelf increases in width to
around 250 km in the centre of the Gulf of Martaban,
with the shelf break at 110 m depth. Most of the Gulf
of Martaban is covered by modern muds and silts,
with relict shelf sands dominating the outer shelf at
depths below 20–30 m (Rodolfo 1969a 1969b;
Ramaswamy et al. 2004 2008; Rao et al. 2005).
The shelf in the centre of the Gulf of Martaban is
incised by the Martaban Canyon (Figure 1), striking
roughly south to abyssal depths of over 2000 m in the
Andaman Sea, and modern muddy–silty sediments
extend further out onto the shelf to depths of at least
100 m along the ﬂanks of this feature (Rodolfo 1969a
1969b; Ramaswamy et al. 2004; Rao et al. 2005;
Curry 2005). The present width of the canyon is
150–200 km, and its position offshore of the Sittoung
River, which enters at the head of the gulf (Figure 1),
indicates that the Martaban Canyon is a relict feature
of a much larger palaeo-Irrawaddy drainage system
that occupied the modern Sittoung River route until
the early Miocene (Nyi Nyi 1967). The Sittoung River
has a low annual sediment load, and its outlet to the
Gulf of Martaban deﬁnes a funnel-shaped estuary,
whereas the Salween delivers an estimated 110–180
Mt/year of sediment to the Gulf of Martaban (Robin-
son et al. 2007). In total, the Irrawaddy, Salween, Sit-
toung and smaller tributaries deliver an estimated
Figure 2 Digitised maps of the Irrawaddy delta and the Gulf of Martaban from copies of maps produced by Lt. W. Fell
(1850) and Mr G. H Barnett (1913). Numbers refer to locations mentioned in the text. 1109 ¥ 987 mm (72 ¥ 72 dpi)

752 km3
of water and 370–600 Mt of sediment per
year to the gulf (Robinson et al. 2007). It should be
noted that these values are based on sediment con-
centrations measured just above Henzada on the
Irrawaddy and assume 100% transport efficiency
through the delta. There are currently no available
quantitative data on sedimentation rates within the
delta to test these assumptions.
As a result of the seasonally reversing Asian
monsoon, circulation in the Andaman Sea is broadly
cyclonic from May to September (southwest
monsoon) and anti-cyclonic from December to Feb-
ruary (northeast monsoon). High suspended load
inputs from the rivers surrounding the Gulf and strong
tidal currents lead to substantial sediment suspension
and re-suspension, resulting in the Gulf remaining
turbid year-round, with surface suspended sediment
concentrations up to 100 mg/litre persisting up to
100 km from the coast (Ramaswamy et al. 2004). A
well-defined turbidity front oscillates north and south
by 150 km in phase with spring–neap tidal cycles,
with the area of the turbid zone varying from over
45 000 km2
during spring tides to less than
15 000 km2
during neap tides (Ramaswamy et al.
2004).
Methodology
A number of cartographic resources of varying accu-
racy are available for the region (Table 1). The earliest
available map of the delta and Gulf of Martaban,
including bathymetry, was produced in 1850 by Lt. W.
Fell. Several more detailed maps of the delta were
produced and periodically updated into the early
twentieth century, with a version dating from 1914
chosen for this study. The maps were digitised but are
not considered of a suitable quality for a quantitative
comparison with later maps and imagery; however,
they still contain valuable information on the general
morphology of the delta and the shoreline. Both maps
were provided as 600 dpi scanned TIFF files of origi-
nal paper copies by the British Library.
The HIND series of 1:253 440 maps were produced
in 1944–6 by the Survey of India from ground surveys
conducted between 1919 and 1930. This is the earli-
est series of maps that is sufficiently accurate and of
high enough quality (reproduction and preservation)
to georeference. It is possible that data from aerial
surveys conducted in 1924 (Stamp 1925) until 1944
are included in the maps, but for the purposes of this
study, these maps are assumed to relate to the situa-
tion in 1925. The eight relevant map sheets were
provided as 600 dpi scanned TIFF files of original
paper copies by the British Library. From the 1970s
onwards, cloud-free satellite imagery of the region
was selected from the georeferenced Terralook JPEG
image archive, derived from the Landsat-MSS (1973),
Landsat-TM (1989), Landsat-ETM+ (2000) and Terra-
Aster (2006) sensors (Table 1).
In the case of the HIND topographic maps, the
‘coast’ as identified on the maps was taken to be the
seaward limit of the land and digitised using Terralook
1.0 and ArcGIS. The ‘coast’ on the satellite imagery
was taken to be the seaward margin of vegetation. In
addition, unvegetated exposed sediment further off-
shore was digitised, although the area of exposed
unvegetated sediment is critically dependent on tide
level, which varies between scenes and years.
The source of uncertainties and errors that arise
when comparing paper maps, aerial photographs or
satellite images include surveying errors, variations in
the degree of generalisation on maps, map registra-
tion, distortion occurring to photocopied maps,
and errors incurred during the digitising process
Table 1 Description of cartographic resources and satellite imagery used in this study
Publication date Acquisition mode Notes
1852 Hydrographic survey Chart of the coast of Pegu and Gulf of Martaban, published by J. Walker;
British Library shelfmark: Maps 147.e.19.(65.); based on survey in 1850 by
Lt. Fell
1914 Ground survey Map accompanying report entitled ‘Note on the Protective Embankments in
the Irrawaddy Delta, 1862–1912’ by Mr C G Barnett; British library
shelfmark: I.S.BU.53/14; based on ground surveys to 1913
1944–45 Ground and aerial
survey
Burma 1:253,440. HIND first edition, unlayered; sheets 85L, 85P, 86I, 86M,
94D, 94C, 94G, 94H; British Library shelfmark: Maps 58765.(27.);
published by the Indian Survey based on surveys between 1919 and 1930
1973 Landsat MSS Individual scenes acquired for Jan and Mar 73; Jan 74; Nov 78
1989 Landsat TM Individual scenes acquired for Jan and Feb 1989
2000 Landsat ETM+ Individual scenes acquired for Dec 1999; Feb, Mar and Nov 2000
2006 Terra ASTER Individual scenes acquired for Jan, Feb, Apr and Dec 2006

(Heywood et al. 2006). In order to investigate the
degree to which any apparent changes in the delta can
be attributed to genuine land changes, rather than
errors associated with map quality, registration of the
sources and digitising, a comparison of the digitised
coastlines located along the rocky western Arakan
coast between the HIND series maps and the 1973
satellite imagery was made to quantify the amount of
land change that can be attributed to the above errors.
Between 1925 and 1973, an apparent total of
30.7 km2
of land was gained, 2.1 km2
was lost, with a
net change of 28.6 km2
. Between 1973 and 2006,
10.3 km2
was apparently gained, 7.1 km2
was lost,
with an apparent net gain of 3.2 km2
. We recognise
that this error analysis is spatially biased to the
western edge of the maps and arises because there are
no other fixed points common to all maps that allow
more spatial coverage of error. However, the errors are
considered small in the context of total land changes
as they represent 4.2% of the land gained and 0.5% of
the land lost between 1925 and 1973, and 1.1% of
the land gained and 0.8% of the land lost between
1973 and 2006 (Table 2). The uncertainty in the cal-
culation of coastline change is greatest for the older
maps (4–5%) and less when based on the comparison
of more recent maps and satellite imagery (~1%).
To assess the areal extent of land gained and lost
through time, the digitised maps and satellite imagery
were overlain, the coastline was digitised within
ArcGIS, and difference polygons were created. Areal
estimates of net land gained (Table 2) have been con-
verted to sediment volumes assuming a vertical
dimension equivalent to the maximum tidal range of
6.4 m and this was converted to sediment mass
assuming a minimum density of 0.5 tonnes/m3
for
fine-grained muds and a maximum density of 1.5
tonnes/m3
for coarse-grained sands (Bird et al. 2004).
Estimates of annual sediment flux into the Gulf of
Martaban were taken from Robinson et al. (2007) and
are used to compare the proportion of estimated sedi-
ment mass deposited over the last 81 years to the total
riverine sediment flux over the same period. The
uncertainties described above are increased because
the rate of vertical accretion and values of sediment
density are not quantified for the Irrawaddy delta
region, and we have therefore adopted a simple
approach by estimating the maximum and minimum
sediment mass changes using the range of plausible
values for annual sediment flux and sediment density.
The values used to estimate the percentage of sedi-
ment mass deposited do not include any deposition
related to filling available accommodation space
(subsidence and sea level changes).
Results and discussion
The early maps from 1850 and 1913 are reproduced
in Figure 2. While the maps are not of a quality that
allows a quantitative comparison, it is clear that the
major morphological features of the delta were
present in 1850, while the more detailed map from
1913 is qualitatively and broadly indistinguishable
from subsequent maps and imagery up to the present
day (Figure 1). Purian Point (a 12 km long NNE-
trending bedrock high) and Krishna Shoal are used
as fixed points of reference for the 1850 map, and
there appears to be no significant advances of the
delta between 1850 and 1925 based on a visual com-
parison of the 1850 map and the 1920s surveys (upon
which the HIND series topographic sheets published
in the 1940s were based).
Quantitative comparisons of the georeferenced
coastlines from 1925, 1973 and 2006 are provided in
Figure 3, as well as the area of unvegetated sediment
Table 2 Summary of gain, loss and net change over the time slices used in this study for both vegetated coastline and
unvegetated sediments seaward of the vegetation line and offshore for the entire study area with cumulative values for
1925 to the 2006 and 1973 to 2006. Figures marked with an asterisk refer only to the Irrawaddy Delta itself between
Purian Point and Elephant Point (see Figure 1)
Vegetated coast Unvegetated sediment
Time period Years
Gain
(km2
)
Loss
(km2
)
Net change
(km2
)
Net change
(km2
/year)
Area
(km2
)
Net change
(km2
)
1925 – – – – – 2116 (1925) –
1925–73 48 725 428 297 6 1746 (1973) -370
1973–89 16 539 280 258 16 1794 (1989) 48
1989–2000 11 160 328 -168 -15 1291 (2000) -503
2000–06 6 206 253 -47 -8 2015 (2006) 723
Cumulative from 1925 81 1629 1289 340 (73*) 4.2 (0.90*) – -102
Cumulative from 1973 33 905 861 44 (18*) 1.3 (0.55*) – 268

exposed at the time the satellite imagery was
obtained. A comparison of the maps and images con-
firms little change in the coastline position from
Purian Point to Elephant Point between the 1920s and
2006, with some significant areas of advance at the
easternmost end near Elephant Point. Table 2 provides
a summary of the areas of coastline gained and
lost between successive time slices, and the area
of exposed unvegetated sediment. Overall, the
Irrawaddy delta gained 55 km2
of land between
Purian Point and Elephant Point from 1925 to 1973,
and an additional 18 km2
up to 2006, averaging less
than 1 km2
/year. This is equivalent to an average rate
of advance distributed equally across the 250 km
coastline of no more than 3.6 m/year or 360 m per
century. However, based on Chhibber (1934) (and
later citations of his work in Pascoe 1950; Rodolfo
1969b; Bender 1983; Aung 2003), the Irrawaddy delta
is rapidly advancing seawards, with quoted rates of
advance ranging from 25 to 61 m/year. Chhibber
(1934) calculated an average value of 4.1 km per
century from two bathymetric surveys in 1860–70 and
1909–10. In the time between the surveys, the 10
fathom (18.3 m), 15 fathom (27.4 m) and 20 fathom
(36.6 m) contours advanced seawards by 16.5, 33 and
41 km, respectively, east of longitude 95.5° E in the
Gulf of Martaban. The average value derived by
Chhibber (1934) would imply 6.4 km (4–10 km) of
advance since the time of the earliest 1850 map
obtained for this study. We question the long-term
applicability of the rates quoted by Chhibber (1934)
because the 36.6 m (20 fathom) contour from the
1850 map overlies the present day 40 m contour from
Ramaswamy et al. (2008), and therefore shows no
appreciable seaward shift over 150 years (Figure 1). It
should be noted that the estimates of Chhibber (1934)
are based on a period of time that included extensive
embankment construction, river straightening and
meander loop blocking on the Irrawaddy River, and
on measurements from near the margins of the Mar-
taban Canyon. Since 1925, the progradation rate has
been, on average, about 10% of the Chhibber (1934)
estimate for 1860–1910.
From our average calculation of 3.6 m/year we infer
that less than 5% of the sediment flux from the
Irrawaddy River has contributed to sub-aerial delta
progradation over this period. The sub-aerial delta
plain sediments at the coast form a series of coast
parallel beach ridges of sand grain sizes (Stamp 1940)
and are probably derived from river bedload which
has never been measured in the Irrawaddy River.
Therefore a component of the total sediment flux has
not been quantified and it is possible that much of the
suspended load of the river is exported into the Gulf of
Martaban, consistent with previous qualitative obser-
vations that little of this sediment is deposited within
the delta itself (Stamp 1940; Volker 1966; Woodroffe
2000). Based on this analysis, it would appear likely
that the Irrawaddy delta section of the coastline is
stable, protected in its current position by the bedrock
high that comprises Purian Point and the offshore
Alguada Reef at its western end. It is more or less in
equilibrium, with an unknown amount of sedimenta-
tion balancing subsidence and sea level rise in the
delta. Progradation is also limited by efficient sedi-
ment transport into the gulf region due to the strong
offshore tidal currents and surface water currents on
the shelf. There has been significant sediment redistri-
bution in the shallow prodelta. Several areas south-
west of the main Irrawaddy distributary channel,
which are marked as shoals in 1850, are no longer
evident. In addition, some small, partly vegetated
sandbars closer inshore that are now emergent came
into existence between 1925 and 1973.
A comparison of each time slice for the region east
of Elephant Point into the head of the Gulf of Marta-
ban and down to the mouth of the Salween River at
Kyaikkami (formerly Amhurst) suggests a considerably
more dynamic coastline. The coastline immediately
east of the Rangoon River has advanced since the
1920s, with a mix of major advances and retreats of
several kilometres in the region at the head of the Gulf
(Sittoung River mouth) and at the mouth of the
Salween River (Figure 3). The majority of riverine sedi-
ment delivered to the Gulf of Martaban occurs during
the monsoon between late May and September (Rob-
inson et al. 2007). Most of the sediment passes
through the Irrawaddy distributary channel and the
Salween River, with only a minor contribution from
the Sittoung and Tavoy Rivers (Figure 1 and 2). It is
then transported by tidal currents and reversing ocean
currents (cyclonic in spring–summer and anti-
cyclonic from November to May) into the Sittoung
estuary and across the shelf in the Gulf of Martaban
(Ramaswamy et al. 2008). This Sittoung mouth section
of the delta is essentially replenished with sediment
due to longshore drift processes and from marine
sources, rather than by direct replenishment through
its river mouth.
Table 2 and Figure 4 suggest the total increase in
land across the entire study area between 1925 and
2006 amounts to 340 km2
at a long-term average rate
of 4.2 km2
/year, but this masks a more rapid rate of
advance of 8.7 km2
/year from 1925 to 1989, followed
by net erosion at a rate of 13 km2
/year up to 2006.
There may be a link between the increased frequency
of Indian Ocean tropical cyclones since the mid
1990s and increased erosion (Figure 1B), but only two
cyclone events are on record as impacting the coast of
Burma (Fritz et al. 2009) and the effects of the Indian
Ocean tsunami on coastline erosion were small (Swe
et al. 2006). It is very probable that sediment
discharge to the Irrawaddy delta has been reduced
since the mid 1980s due to the increased construction

Figure 3 Areas of coastline gain (dark green) and loss (light red) for the period between 1925 and 2006. Areas of
unvegetated sediment in 2006 are also shown. 134 ¥ 280 mm (400 ¥ 400 dpi)

rate of small–medium sized dams and irrigation works
on tributaries of the Irrawaddy and Chindwin. Storage
capacity has increased from 2.34 to over 18 km3
since
1988 (Myanmar Irrigation Works Department 2004).
Modern sediment discharge measurements quantify-
ing the annual Irrawaddy and Salween Rivers is
currently work in progress.
The accumulation rate for the entire area is of the
same order as recent progradation rates of 5.5–
16 km2
/year measured on the Meghna delta (Allison
et al. 2003) and historical rates of 1.2 km2
/year in part
of the Mekong delta for the period between 1885 and
1985 (Nguyen et al. 1999). The spatially averaged pro-
gradation rate of 3.4 m/year for the last ~100 years is
low in contrast to average rates of 8–15 m/year in the
Mekong delta and 50 m/year over the last 4000–
6000 years for the Yangtze delta (Tanabe et al. 2003b;
Saito 2001). However, although the mega-deltas of
Asia were in a constructional phase during the last
several thousand years, the Yellow (Huanghe), Yangtze
(Changjiang), Red (Song Hong), Mekong and Chao
Phraya have all experienced net losses of land and the
retreat of their deltas over the last decades due to
dam-related sediment discharge losses to the delta,
subsidence due to resource extraction, and erosion
(Saito et al. 2007; Syvitski and Milliman 2007; Wang
et al. 2007; Yang et al. 2006).
Assuming that the sediment ﬂuxes of the Irrawaddy,
Salween and Sittoung Rivers all contribute to land
building in the study area, a maximum of 9% (370 Mt/
Figure 3 Continued
Figure 4 Total area of net gain or loss for all time slices of
vegetated coastline and unvegetated sediments. The values
are calculated relative to 1925 (solid lines) and 1973
(dashed lines) coastlines. 769 ¥ 499 mm (72 ¥ 72 dpi)

year of sediment flux and 1.5 tonnes/m3
sediment
density) to a minimum of 2% (600 Mt of sediment flux
and 0.5 tonnes/m3
) of the total annual sediment flux of
these rivers may have been incorporated into newly
vegetated areas since 1925; the average value is 5.5%.
The area of unvegetated sediment offshore has also
fluctuated dramatically in the head of the Gulf of
Martaban (Table 2). A quantitative interpretation of
net sediment gains or losses is not possible for the
unvegetated areas due to the large tidal range in the
region, with images acquired at different points in the
tidal cycle. However, in general terms it would appear
that since 1925, there may have been overall losses or,
at best, only relatively small gains in the area of off-
shore unvegetated sediment across the study area.
The conclusion that possibly 91% of the total sus-
pended flux delivered by rivers into the Gulf of Marta-
ban is exported offshore is supported by the perennial
presence of a high turbidity region covering most of the
gulf area (Ramaswamy et al. 2004) and by the exist-
ence of an extensive mud belt covering the floor of the
Gulf (Figure 1) (Rao et al. 2005). Ramaswamy et al.
(2008) demonstrate from carbon isotope analysis that
over 70% of the organic carbon in the surface sedi-
ments of the Gulf of Martaban is of terrestrial origin and
this percentage increases to over 90% at the edge of the
shelf in the gulf area (Figure 1).These results imply high
rates of sediment accumulation at depths below the
lowest tidal level in the Gulf.This sediment is predomi-
nantly fine grained (Ramaswamy et al. 2004 2008) and
is distributed over the maximum area of the high
turbidity zone (45 000 km2
). Assuming that it is rapidly
compacted to 1 tonne/m3
(Bird et al. 2004) implies that
the sea floor has been raised 0.6–2 m since 1925 at a
rate of 0.7–2.5 cm/year. This calculation neglects the
sediment deposited in any available accommodation
space created through subsidence and sea level rise,
probable small losses to the Bay of Bengal, and
unknown losses to the Martaban Canyon, but is never-
theless consistent with the 2 cm/year estimate of
Rodolfo (1969a).
Conclusions
The results of this study suggest that the distributary
delta section of the Irrawaddy coastline has been
stable since at least 1850, with a maximum average
progradation rate of 0.34 km per century. The most
dynamic part of the coastline is in the estuarine part of
the Sittoung River at the head of the Gulf of Martaban.
The average progradation rate is less than 10% of most
previous estimates (Chhibber 1934; Pascoe 1950;
Bender 1983; Aung 2003), although the most authori-
tative of these (6.1 km per century; Chhibber 1934),
based on the seaward displacement of bathymetric
contours between 1860 and 1910, was considered a
maximum for the delta. This is potentially a biased
record due to extensive Irrawaddy River embankment
construction in the late nineteenth century. Many of
the Asian mega-deltas are in a destructive phase due
to sediment loss associated with damming (Saito et al.
2007) and the stability of the Irrawaddy delta coast-
line, including the Salween River mouth region, may
now be anomalous in Asia because of the current low
density of dams on the mainstems of both rivers. This
is forecast to change in the next 5–10 years as the
construction of a series of dams on the Salween River
has begun and construction of dams on tributaries of
the Irrawaddy and Chindwin Rivers continues. Our
historical perspective on the apparent stability of the
Irrawaddy to Salween coastline over c. 150 years is an
important baseline on which future changes to the
coastline in the aftermath of Cyclone Nargis can be
compared, and the impact of future damming on the
Salween (and Irrawaddy) can be evaluated.
The slow rate of progradation of the coastline
appears to be partially due to the effective transport of
the bulk of the sediments carried by the Irrawaddy
River through the delta plain, as noted previously
(Stamp 1940; Volker 1966; Woodroffe 2000), the effi-
cient dispersal of these sediments by strong tidal cur-
rents (Ramaswamy et al. 2004; Rao et al. 2005), and
by cyclonic oceanic currents during the dominant
southwest monsoon season. The distributary delta
section appears to be protected at its current location
by coastal and offshore bedrock highs at its western
margin and is unable to prograde significantly
because of efficient longshore transport.
While a considerable flux of sediment is delivered
to the shelf of the Gulf of Martaban by the combined
Irrawaddy, Salween and Sittoung Rivers (Robinson
et al. 2007), 9% or less of the sediment delivered
since 1925 has contributed to land-building along the
coasts of the study area. Most of this sediment accu-
mulated prior to 1989, with erosion occurring at the
head of the Gulf since that time. The bulk of the
sediment is accumulating as a wedge of mud covering
the floor of the Gulf of Martaban (Ramaswamy et al.
2004; Rao et al. 2005). A mass balance suggests that
this sediment is likely to be accumulating offshore at
a rate of 0.7–2.5 cm/year. This rapid rate of sedimen-
tation suggests that an annually resolvable regional
record of environmental change for the last several
millennia could be preserved in the Gulf of Martaban
if there is not extensive re-working of the sediments.
Acknowledgements
Funding for this research was in part provided by a
Royal Geographical Society grant to Bird and Robin-
son. V. Ramaswamy is gratefully acknowledged for
allowing us to use his bathymetric contours in
Figure 1. The paper was improved by the detailed
comments and suggestions from four anonymous

reviewers. This publication is a contribution from the
Scottish Alliance for Geoscience, Environment, and
Society (www.sages.ac.uk).
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