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Irrawaddy Delta

Location:
16°55'-18°15'N, 94°15'-96°20'E; the delta system of the Irrawaddy River from the region of Myanaung, 70 km north of Henzada, to the outer islands along the coast.

Area:
3,500,000 ha.

Altitude:
0-15m.

Biogeographical Province:
4.4.1.

Wetland type:
02, 03, 05, 06, 07, 08, 11, 13, 15, 18 & 19

Description of site:
The delta system of the Irrawaddy River extends in a great alluvial fan from the limit of tidal influence near Myanaung (18°15'N) to the Bay of Bengal and Andaman Sea, 290 km to the south. This alluvial plain is bounded to the west by the southern Arakan Yoma range and to the east by the Pegu Yoma. The city of Rangoon, situated on the southermost spur of the Pegu Yoma, lies at the southeastern edge of the delta. The entire area is overlain by a thick layer of recent alluvium brought down by the Irrawaddy. Three main types of soil have developed: meadow gleyey clay soils, meadow swampy soils and saline gleyey soils. The flow in the Irrawaddy is at its lowest in February and March there is a sharp rise in level in April-May as a result of melting snow in the upper catchment, and a further steep rise in May-June with the onset of the monsoon. The maximum flow occurs in July or August. Most waterways are natural water courses, and there is no extensive system of dredged canals, the only major canal being the Twante canal which links Rangoon with the western part of the delta.

The upper and central portions of the delta are almost entirely under cultivation, principally for rice. Until about 1850, much of this region comprised a complex of permanent and seasonal lakes, swamps and marshes, and vast areas of seasonally inundated plains and swamp forest. However, following the rush of settlers from Upper to Lower Burma in the late 19th Century, the construction of embankments and reclamation of land for agriculture has kept pace with the increase in population. Dyke building was initiated by the Government as early as 1861, and many embankments were constructed around 1880 and 1920. At present, there are some 1,300 km of major embankments in the delta, protecting over 600,000 ha of rice paddy. The system of embankments provides a unique example of partial flood protection. The major dykes form horseshoes around the areas between the main rivers, with the downstream ends left open. In the event of extreme flooding, the lower parts act as flood basins, thus slightly reducing the flood peak. The old embankments have been maintained, and projects are contemplated to extend the system even further. Despite these reclamation schemes, there still remain large tracts of land that are deeply flooded during the monsoon and retain water even during the dry season. In addition, there are numerous permanent oxbow lakes and associated marshes, particularly along the Irrawaddy between Myanaung and Henzada, along the Myitmaka, and along the upper Bassein and Daga rivers.

The lower, seaward third of the delta, stretching 130 km from east to west, is completely flat with no local relief. About 520,000 ha of land are below high spring tide level and subject to tidal inundation. Much of this area is covered by mangrove forest, and cultivation is limited to the higher patches of ground. Sandy ridges, such as old beaches and sand banks, provide refuges for wildlife during the highest tides. Although the mangrove vegetation has been exploited for a very long time, there are some relatively intact stands remaining. The area is dissected into a number of islands and peninsulas by a series of large, southerly flowing rivers and a complex of smaller, interconnecting water courses, all of which are at least intermittently saline due to tidal intrusion. Drainage is directly into the Bay of Bengal through nine major river mouths, the Bassein, Thetkethaung, Ywe, Pyamalaw, Irrawaddy, Bogale, Pyapon, China Bakir and Rangoon. These rivers carry a heavy silt load, and their waters are very turbid. The delta is actively accreting seawards, and as a result the sea is very shallow for some distance Out to sea. Water depths are less than 5.5m across the whole coastline fronting the delta and up to 28 km offshore in the east. The present rate of advance of the delta is estimated at 5-6 km per 100 years, equivalent to about 1,000 ha per year. Several small islands, some of which are visible only at low tide, have developed offshore. These include Kain Thaung Kyun off the mouth of the Irrawaddy River, and Kadonlay Kyun and Gayedgyi Kyun off the mouth of the Bogale River. Tides are semi-diurnal, and have a range of 2.0-2.5m along the outer coast. At Rangoon, 72 km from the open sea, the tidal range is 3.5-5.lm. Sea dykes have been constructed in some areas to prevent tidal inundation, and the Government has recently carried out several polderization schemes in the outer delta.

Climatic conditions:
Monsoonal climate, with an average annual rainfall of about 1,500-2,000 mm in the north increasing to 2,500 mm in the southeast and 3,500 mm in the southwest. Over 90% of the rain falls between mid May and mid November. During the monsoon season, the maximum and minimum temperatures in the coastal zone are about 37°C and 22°C, respectively. The seas may be very rough, and there are often strong winds from the south and southwest. The period from mid October to mid Febuary is generally dry and cool. Temperatures rise after Febuary, and April and early May are characterized by hot, variable weather with premonsoon squalls.

Principal vegetation:
The natural vegetation of the lower, tidal delta is mangrove forest, but this has been heavily exploited and most of the remaining forest is in various stages of regrowth. Four types of forest are recognized (Salter, 1982):

1. low mangrove forest, colonizing soft mud submerged at every tide; characterized by species of Ceriops, Avicennia, Kandelia and Bruguiera;
2. tree mangrove forest, developing on mud banks inland of low mangrove forest and at the edges of tidal streams; dominated by species of Rhizophoraceae;
3. saltwater Heritiera forest, on the landward side of the above two types, but still flooded at every tide; dominated by Heritiera tomes;
4. freshwater Heritiera forest, a closed evergreen high forest, flooded at high tide by only moderately brackish water; comprised mainly of Bruguiera and Heritiera.
Virtually all areas not within Reserved Forests are used for growing rice paddy or other crops.

Land tenure:
No information.

Conservation measures taken:
Almost all of the southern third of the delta is included within Reserved Forests. Most of these were established during the late 1800s and early 1900s, and have been managed primarily for fueiwood and, less commonly, charcoal production. Other areas are unprotected.

Conservation measures proposed:
Three areas in the outer delta have been proposed for designation as Wildlife Sanctuaries: Meinmahla Kyun (site 12a), Kadonlay Kyun (site 12b) and the Letkokkon islands (site 12c). Salter (1982) has proposed that, in order to provide a further measure of protection to mangrove fauna and flora, Pyinalan, Kadokani and Pyindane Reserved Forests continue to be managed for sustained production of fuelwood, and that further encroachment by agriculture be prevented. He also recommends that crocodiles of breeding size and all species of sea turtle be given completely protected status under new wildlife legislation. A review of the present system of turtle egg collection should be undertaken, and measures instituted to ensure that this valuable resource is managed on a sustained yield basis. In addition, Blower (1983) has recommended that full protection be given to the endangered River Terrapin Batagur baska and its eggs, and that the possibility of establishing a hatchery on Kadonlay Kyun or elsewhere be considered.

Land use:
With a total population of about 3.5 million people and a population density of 100 per sq.km, the Irrawaddy Delta is one of the most densely populated parts in the country. Virtually all land not designated as Reserved Forest has been converted to intensive agriculture. The predominant form of cultivation is single rice cropping during the wet monsoon, and this accounts for two-thirds of the total area under cultivation. The rice yield is about 2,000-2,500 kg per hectare.

Numerous towns and villages are scattered throughout the delta and particularly along the larger rivers. In addition, temporary camps are established by forest workers and fishermen. All major rivers and streams are used by the local commercial fisheries, which operate mainly from small boats and fixed fishing frames. Prawns are a major product of the area. Every important sea turtle nesting beach is commercially exploited for eggs by local cooperatives. All these beaches fall within Reserved Forests, and are leased out by the Forest Department on a year to year basis. Throughout the delta, communication is easiest by water; virtually every householder owns a boat, and major population centres in the southern delta, such as Bogale, Moulmeingyun and Myaungmya, are served by steamer. The Irrawaddy itself is a very important artery for trade throughout a large part of Burma.

Disturbances and threats:
The Irrawaddy is one of the most heavily silted rivers in the world, not only because of deforestation and serious erosion in the watershed, but also because of a long history of intensive agriculture along the river banks. Virtually all land outside the Reserved Forests has already been converted into agricultural land, and the mangrove forests within the Reserved Forests are now disappearing at a rapid rate. Large parts of the Kyagan Kwinbauk, Kakayen and Pyinland Reserved Forests have already been deforested and converted into agricultural land. A comparison between the situation in 1977 and the situation in 1986 indicates that if the present rate of destruction is maintained, all the mangrove forest will disappear in 50 years (Harald Sutter, pers. comm.).

The populations of Estuarine Crocodile Crocodylus porosus, sea turtles and River Terrapin Batagur baska have been drastically reduced by commercial exploitation and are now seriously threatened. The crocodile population continues to be exploited by the People's Pearl and Fisheries Corporation, which removed an average of 465 hatchlings per year during the period 1978-83 for rearing at a crocodile farm in Rangoon. Other threats to the remaining crocodiles include habitat destruction, collection of eggs for food, hunting of adults and juveniles, and entanglement in fishing nets. The number of sea-turtle eggs collected annually in the Irrawaddy Delta declined from 1,500,000 in 1911 to an average of 400,000 during the period 1978/79 to 1981/82. This decline in egg harvests has been reflected in a decline in adult turtles and the abandonment of some former nesting beaches. Mature turtles are taken by fishermen and caught by trawlers in their nets throughout the delta, and Hawksbill Turtles Eretmochelys imbricata are hunted for their "tortoise shell". At the turn of the century, approximately 70,000 eggs of the River Terrapin Batagur baska were collected annually in the delta. Although the species is now on the verge of extinction in the delta, the adult terrapins and their eggs continue to be taken wherever they are found.

Economic and social values:
The northern and central parts of the delta are major rice-growing areas, producing 40% of the national total. The delta supports a very important fishery, especially for prawns, and has traditionally provided a large annual harvest of sea turtle eggs. The extensive mangrove forests have provided a valuable source of fuelwood and timber for construction purposes. However, the mangrove and sea turtle resources have now been seriously depleted, and as further mangrove areas are cleared for agriculture, the important prawn fishery is at risk. The delta has some potential for tourism based on interest in the wildlife, although the shallowness of the sea around many of the outer islands tends to hinder access.

Fauna:
One of the major commercial fish species in Burma, Hilsa ilishka, is abundant in the delta.

The delta is still of great importance for a wide variety of both resident and migratory waterfowl, although populations of many species have declined dramatically since the end of the 19th Century. Resident species and monsoon visitors listed as common by Smythies (1953) and probably still at least fairly common include Tachybaptus ruficollis, Phalacrocorax fuscicollis, P. niger, Anhinga me!anogaster, Ixobrychus sinensis, I. cinnamomeus, NyctIcorax nycticorax, Ardeola grayii, Bubulcus ibis, Egretta sacra, E. garzetta, E. intermedia, E. alba, Ardea purpurea, A. cinerea, Ardea sumatrana (confined to the tidal estuaries and creeks), Ciconia episcopus, Threskiornis melanocephalus, Dendrocygna javanica, Sarkidiornis melanotos, Nettapus coromandelianus, Anas poecilorhyncha, Rallus striatus, Amaurornis phoenicurus, Gallicrex cinerea, Porphyrio porphyrio, Heliopais personata, Hydrophasianus chirurgus, Metopidius indicus, Rostratula bengha!ensis, Esacus recurvirostris, Glareola rnaldivarum (abundant), G. lactea, Vanellus duvaucelii, Charadrius dubius, Gelochelidon nilotica, Sterna aurantia, S. melanogasler, S. albifrons and Rhynchops albicollis (along the Irrawaddy north of Henzada).

Winter visitors and passage migrants include Phalacrocorax carbo (formerly bred), a wide variety of Anatidae, Fulica atra, about 30 species of migratory shorebirds, Chlidonias hybrida, Hydroprogne caspia and Larus brunnicephalus (very common). The most numerous wintering shorebird is probably Charadrius mongolus, which occurs in flocks of many thousands along the outer coast of the delta. Tringa glareola is also abundant. Other common shorebirds include Vanellus cinereus, Pluvialis dominica, P. squat arola, Charadrius alexandrinus, C. leschenaultii, Limosa limosa, Numenius phaeopus, N. arquata. Tringa totanus, T. nebularia, T. ochropus, Xenus cinereus, Actitis hypoleucos, Gallinago gallinago, G. stenura, Calidris ruficollis, C. temminckii, C. subminuta, C. ferruginea and Limicola falcinellus. Eurynorhynchus pygmeus was regarded by Smythies (1953) as a regular winter visitor in very small numbers, and Limnodromus semipalmatus has been recorded.

In the late 19th century, the Spot-billed Pelican Pelecanus philippensis nested in huge numbers in south Burma. One colony on the Sittang plain to the east of the delta was described in November 1877 as covering 100 square miles and containing millions of birds. Immense colonies still bred in the area in 1910, but the birds had disappeared completely by 1939. Small numbers were regularly reported in the delta in the l940s, but no breeding sites were located. No pelicans have been recorded in recent years, and it may well be that the species is now extinct in Burma. The disappearance of this once abundant species can be directly attributable to the massive conversion of floodplain lakes and marshes into monocultures of rice throughout south Burma during the latter part of the 19th century and first decades of the present century. A similar fate has befallen the Greater Adjutant Stork Leptoptilos dubius. In the late 19th century, this was an abundant breeding visitor (October to March) throughout the delta and Sittang plain, but the species had already become scarce by the I 940s, and is now a rare visitor. A single bird was reported on Meinmahla Kyun in late 1982. The Asian Openbill Stork Anastomus oscitans remains a common passage migrant and winter visitor. However, the Painted Stork Mycteria leucocephala, which was regarded as a locally common resident in the early part of this century, is now only a scarce visitor.

Several species of large mammal occur in the delta, but their populations are small and scattered, with the possible exceptions of Sambar Cervus unicolor, Hog Deer C. porcinus and Wild Boar Sus scrofa, which have been reported from all Reserved Forests. Asian Elephants Elephas maximus are said to occur in Kadonkani, Meinmahla and Pyindaye Reserved Forests, but the total population in 1982 was only about 15 individuals. Other species reported to be present include Leopard, Tiger, Wild Dog and otters (Panthera pardus, P. tigris, Cuon alpinus and Lutra sp) (Salter, 1982).

The southern delta area is the last stronghold in Burma of the Estuarine Crocodile Crocodylus porosus. The total population was estimated at about 4,000 in 1980. Some two-thirds of these were confined to the eastern delta, mainly in the Pyindaye Reserved Forest and on Meinmahla Kyun. The population has declined dramatically over the last 30 years, largely as a result of over-hunting.

Sea turtles nest on beaches along the seaward edges of Kyagan Kwinbauk Reserved Forest, Pyinzalu Kyun, Pyinalan Reserved Forest and Kadonkani Reserved Forest, and on the offshore islands of Kaingthaung, Kadoniay and Gayedgyi Kyun. Five species are known to occur: Chelonia mydas, Caretta caretta, Lepidochelys olivacea, Eretmochelys imbricala and Dermochelys coriacea. C. mydas is one of the commonest species. C. caretta is reported to be common at some beaches, but confusion has arisen over identification, and it is possible that many if not most of these are L. olivacea. E. imbricata and D. coriacea occur only in very small numbers. Almost all of the eggs laid are harvested, and many former nesting beaches have been abandoned, presumably because of this over-exploitation. It is clear that the turtle populations have declined markedly, and that two species, D. coriacea and E. imbricata, are now endangered, while the other three species must be considered seriously threatened (Blower, 1983; Salter, 1982).

The River Terrapin Batagur baska, which formerly nested in the delta in large numbers, is now reduced to a few remnant populations on some of the offshore islands and sand banks. The massive exploitation of both adults and their eggs has resulted in the near extermination of the species in Burma (Salter, 1983).

Special floral values:
Good stands of continuous mangrove forest still exist in the Kadonkani Reserved Forest and in Pyindaye Reserved Forest (Salter, 1982).

Research and facilities:
The southern delta was surveyed in November and December 1982 (Salter, 1982) and in 1983 (FAO, 1983c). The Forest Department is currently carrying out a forest inventory of the mangrove resources in the delta, to provide data for the development of a management plan. No serious ornithological investigations have been carried out for almost fifty years.

References:
Blower (1983); FAO (1980, 1983b, 1983c, 1985a & 1985b); Groombridge (1982); IUCN (in prep); Karpowicz (1985); Luthin (1984); Maxwell (1911); Salter (1982 & 1983); Smythies (1953); Stoutjesdijk (1982).

Criteria for Inclusion:
123.

Source:
See references.