Mekong Delta
Location:
8°33'-10°55'N, 104°30'-106°50'E; the delta region of the Mekong River from the border with Kampuchea to the sea, including the provinces of Long An, Tien Giang, Dong Thap, Ben Tre, Cuu Long, Hau Giang, An Giang, Kien Giang, Minh Hal and Tay Ninh, Ho Chi Minh City and the southern parts of Song Be and Dong Nai provinces.
Area:
Approximately 3,900,000 ha in Vietnamese territory.
Altitude
Sea level to 5m.
Biogeographical Province:
4.5.1.
Wetland type:
02, 05, 06, 07, 08, 09, 10, 11, 13, 14, 15, 18, 19 & 21.
Description of site:

The Mekong River is one of the great rivers of Asia, ranking twelfth in the list of longest rivers of the world, and sixth in terms of mean annual discharge. It rises at about 5,000m in the Tanghla Shan Mountains, on the northeast rim of the great Tibetan Plateau, and flows for 4,160 km through or along the borders of six countries: China, Burma, Laos, Thailand, Kampuchea and Vietnam. The lower Mekong exhibits pronounced seasonal variations in flow, reflecting rainfall patterns. The river starts to rise shortly after the onset of the monsoon rains in late May, and attains its maximum level in September or October. It then falls rapidly until December and slowly thereafter to reach its lowest level in April and early May.

The Mekong Delta is generally regarded as beginning at Phnom Penh in Kampuchea, where the river divides into its two main distributaries, the Mekong (Tien) and the Bassac (Song Hau Giang). The Mekong (Tien) subsequently divides into six main channels and the Bassac into three to form the nine "dragons" of the outer delta in Vietnam. The delta comprises a vast triangular plain of approximately 5.5 million ha, almost entirely below 5m above sea level. It extends for about 270 km from its apex at Phnom Penh to the the coast, and has a coastline of about 600 km. Approximately 1,600,000 ha of the inner delta lie within Kampuchea; the remaining 3,900,000 ha constitute the southern tip of Vietnam. The delta is the result of sedimentation and erosion, the sediments varying in depth from at least 500m near the river mouths to only 30m at some places in the inner delta. At the nine mouths of the Bassac and Tien branches, the combined action of river deposition and the sea has produced a coastal belt of slightly higher elevation. Deposition in the delta continues to extend the Ca Mau Peninsula south and west at a rate of l50m per year in some places.

A diurnal tide is dominant in the Gulf of Thailand, whilst a semi-diurnal tide is dominant in the East Sea. Generally there are two high waters and two low waters per day, but the two low waters are sometimes very different in level. The average daily tidal range varies between 3.5m and 4.5m in the East Sea and between 0.5m and 0.8m in the Gulf of Thailand. The tidal effects extend throughout the delta area in Vietnam, and about 500,000 ha of land are affected by sea water intrusion during the dry season. Salinity penetration lengths into various branches of the Mekong vary from 20 to 65 km. Because of the large inflow of fresh water from the Mekong, salinities along the eastern coast of the delta are very low, particularly during the flood season. The maximum salinity of 4.0 p.p.t. occurs at the end of the dry season, in April. Towards the end of the rainy season in September and October, the combination of floodwaters from the rivers, local rainfall and tidal inundation can result in the flooding of 3,400,000 ha in the Vietnamese portion of the delta.

The principal soil types in the Vietnamese portion of the delta are as follows:

  1. Alluvium soils: these soils are found along the Tien and Hau rivers; they cover an area of 1,110,000 ha (28% of the Vietnamese portion of the delta). The alluvium soils are only slightly acidic (pH values of 4.5-6.5), and are suitable for the cultivation of rice.
  2. Sulphate soils: these cover an area of 1,590,000 ha, mainly in the Dong Thap Muoi and Long Xuyen quandrangle (Long An). This soil type can be subdivided as follows:

    - Salty sulphate: found in Ca Mau and along the shores of the Gulf of Thailand. This subtype covers an area of 1,080,236 ha (28% of the delta).

    - Sulphate: found in the Dong Thap Muoi and Long Xuyen quadrangle, in low-lying areas along the Hau River and in parts of the lowlands between the Tien and Hau rivers. This covers an area of 510,027 ha (13% of the delta). These soils have very high concentrations of suiphates and low pH values ranging from 2.26 to 3.54.

  3. Salty soils: these are found along the coast from Genh Hao (Minh Hai Province) to Go Cong (Tien Giang Province), Can Duoc and Can Giuoc (Long An Province). They cover an area of 808,749 ha (21% of the delta).

The Vietnamese portion of the Mekong Delta can be subdivided into five main regions: (a) the floodplains of the Tien and Hau rivers; (b) the Thap Muoi closed floodplain system; (c) the ha Tien open floodplain system; (d) the U Minh Melaleuca forests; and (e) the tidal floodplain.

a) The floodplains along the banks of the Tien and Hau rivers cover an area of 1,201,861 ha (31% of the delta). The region consists of recent alluvial soils, and is the most suitable area for agriculture.
b) The Thap Muoi closed floodplain system covers an area of 414,398 ha (10.6% of the delta). Agricultural potential is very low, the limiting factors being the high concentrations of sulphates, deep and prolonged inundation during the rainy season, and insufficient fresh water during the dry season. In the higher parts of this region, one or two rice harvests can be produced each year, but this entails a considerable amount of drainage and irrigation. The most appropriate forms of land use would be retaining the wetlands in their natural state for nature conservation, management of the Melaleuca forests for timber production, and management for fisheries production.
c) The ha Tien open floodplain system covers an area of 217,508 ha (5.6% of the delta). The area is relatively well drained, but is not very suitable for agriculture because of the shortage of fresh water during the dry season and presence of salt and sulphates in the soil. This region should also be maintained in its natural state.
d) The U Minh Melaleuca forests cover an area of 189,358 ha (4.9% of the delta). The main soil types are peat and sulphate, and agricultural potential is low. Large tracts of the forest were destroyed by toxic chemicals and napalming during the war, and forest fires continue to destroy large areas; fires during the dry season in 1982-83 and in 1987 were particularly damaging. Reafforestation with Melaleuca should be undertaken to restore the forests and maintain fisheries production.
e) The tidal floodplain covers an area of 215,974 ha (5.5% of the delta). Large areas of land subject to tidal inundation occur along the coasts of Long An, Ben Tre, Cuu Long, Hau Giang and Minh Hai provinces. Most of intertidal zone was formerly covered in mangrove forest, but large parts of this were destroyed by toxic chemicals during the war. Almost all of the mangroves in the Dong Nai river system in the northeast were destroyed during the period 1965-1970, along with about 40% of the mangrove forests in the south of the delta. Land use should now be directed towards reafforestation with mangrove species, management of the forest resources and development of fish and shrimp culture.

Seven of the most important wetland areas for nature conservation are described in detail. These are Dong Thap Muoi, Tien River estuary, Minh Hai Melaleuca forest, Bac Lieu coastal marshes and bird colony, Dam Doi bird colony, Cai Nuoc bird colony and Nam Can mangrove forest.

Climatic conditions:
Tropical monsoonal climate with a pronounced dry season from December to March or April and a pronounced rainy season during the southwest monsoon from May to October or November. The average annual rainfall ranges from less than 1,500 mm in the central region and northwest to over 2,350 mm in the south, with some 70-80% of the precipitation concentrated into four months at the height of the rainy season. The mean annual temperature is about 26°C throughout the delta, the difference between the mean monthly minima and maxima being only about 5°C. The relative humidity remains high throughout the year.
Principal vegetation:
The delta contains about 280,000 ha of mangrove and Melaleuca forest. The mangrove forest consists of 46 plant species, 38 of which are of some economic importance. In sequence from the sea, the forest is dominated by Avicennia, Bruguiera, Rhizophora and Nypa. The Melaleuca forest consists of 77 plant species, with M. leucodendron predominating throughout. By 1980, 2,474,000 ha of the delta were under cultivation, mainly for rice. Some 40 plant species have been identified in the rice paddies.
Land tenure:
State owned.
Conservation measures taken:
Conservation measures taken: At least six reserves have been established to conserve the wetland ecosystems and their wildlife. These are: the Tram Chim Sarus Crane Reserve in Dong Thap Province (9,000 ha); the Vo Doi Protected Forest in the U Minh Melaleuca forests in Minh Hai Province (3,945 ha); the Nam Can Mangrove Reserve in Minh Hai Province (7,547 ha); and three small reserves to protect large breeding colonies of waterbirds at Bac lieu (40 ha), Cai Nuoc (20 ha) and Dam Doi (119 ha) in Minh Hai Province. A considerable amount of reafforestation work has been carried out in the delta; between 1975 and 1985, some 29,188 ha of mangrove forest and 20,734 ha of Melaleuca forest were replanted. In many areas, policies have been developed for the conservation and management of wetland resources, and considerable effort has been made in some areas to restore wetlands to their natural condition. In 1983, the Mekong Committee and UNEP sent a joint mission to the delta to formulate and initiate a programme of research and pilot development projects with a view to ensuring environmentally compatible development of water and land resources in the region. In an international agreement on cooperation in wildlife conservation signed by Vietnam and Kampuchea in January 1986, special emphasis was given to the conservation of endangered waterfowl species in the Mekong Delta.
Conservation measures proposed:
As a result of three State Programmes concerning development in the Mekong Delta, numerous recommendations have been made for the rational utilization of wetlands in the delta. The Ministry of Forestry aims to enlarge the forested areas in the delta from the present level of about 290,000 ha to between 343,000 and 400,000 ha by reafforestation of uncultivated lands. This would include the establishment of an almost continuous forested strip along the sea coast. McNeely (1975) proposed that a delta mangrove reserve be established to take in all coastal mangroves from the town of Bac Lieu southwest to the Nam Can mangroves around Cua Song Bay. He suggested that the seaward edge of the reserve should be inviolable, but certain parts of the reserve might be harvestable on a rotational basis, under strict control. Luthin (1987a & 1987b) has made a variety of recommendations for protection, research, management and public education at the breeding colonies of large waterbirds in Minh Hal Province.
Land use:
Agriculture, fishing and forestry. The human population of the delta has increased rapidly during the past century, from and estimated 1.7 million in 1880 to 4.5 million in 1930 and 13.5 million in 1980. Similarly the area under cultivation has increased dramatically, although this increase has not kept pace with the increase in population. In 1867, an estimated 215,000 ha were under cultivation; this figure had increased to 522,000 ha by 1880 and 2,234,000 ha by 1930. However, between 1930 and 1980 the area of agricultural land increased by only 240,000 ha whilst the population increased by nine million.
Possible changes in land use:
The Mekong Committee has considered some 230 possible development projects on the lower Mekong River and its tributaries. One of the primary objectives of this programme is to eliminate seasonal flooding in the delta areas of Kampuchea and Vietnam. Eliminating seasonal flooding would require the upstream storage of flood waters, primarily in the proposed Pa Mong reservoir in Laos and in the proposed Stung Treng reservoir in Kampuchea, and the diking of river banks throughout the delta region. Full implementation of these projects would result in the loss of a large proportion of the seasonally inundated wetlands, with a profound effect on wildlife and fisheries (Mekong Committee, 1976).

Stoutjesdijk (1982) has listed seven major existing reclamation schemes in the delta involving over 920,000 ha of floodplains and tidally inundated lands. These are the Cai San Project (60,000 ha), Quan Lo-Phung Hiep Project (600,000 ha) and Tiep Nhut Project (34,000 ha) on the south bank of the Bassac, the Ba Dong Project (110,000 ha) between the Bassac and the Tien, and the South Kien-Hoa Project (30,000 ha), Ba Tn Project (37,000 ha) and Go Cong Project (50,000 ha) in the Tien estuary. Further extension of the area of agricultural land in the Vietnamese portion of the delta will prove difficult because a large proportion of the land suitable for agriculture has already been developed. The best way to increase production will be to use more intensive farming methods on existing agricultural land. Despite high investment, attempts to drain marginal land in the Dong Thap Muoi area have proved uneconomic, and some of this land has since been restored to its natural state.
Disturbances and threats:

Rapid growth in the human population and intensive development of the delta lands for agricultural purposes pose a major threat to the natural wetland ecosystems and their wildlife. Various proposed developments on the lower Mekong River are likely to conflict with wildlife and fisheries interests in the delta. These include large irrigation projects, hydro-electric power projects, other industrial development and flood control projects. Dam construction upstream on the Mekong in Laos and Kampuchea will change the hydrology of the delta, reducing seasonal flow peaks and the extent of flooding. This is likely to have a disastrous effect not only on waterfowl populations but also on those fish species which utilize the floodplain wetlands for spawning. Changes in water quality and the timing of peak flows are likely to have adverse effects on fish migrations and spawning, and dams will create obvious problems for long distance longitudinal migrants. The dams will reduce sediment flow, particularly in the main channels, and thereby affect the nutrient regime in the delta (Pantulu, 1986b).

The water quality in the lower Mekong has been affected by domestic wastes and agricultural runoff carrying pesticides and fertilizers. Though localized at present, such problems are expected to increase. Industrial activities, such as pulp and paper mills, textile mills and chemical factories, are increasing within the basin, and these, together with increased waste from shipping, are likely to create a serious pollution problem in the future (Pantulu, l986a).

Economic and social values: The Mekong Delta includes some of the most productive agricultural land in Southeast Asia. In the 1950s, the delta produced 70% of the rice grown in southern Vietnam, along with 70% of the green peas, bananas, pineapples, vegetables, duck meat and duck eggs, 60% of the chicken and hens eggs, 50% of the pork, 37% of the fish, and significant proportions of the jute, coconut, sugar cane and fruit. The annual yield of rice in the delta as a whole currently amounts to about 6.5 million tonnes, with the average yield in the Vietnamese part of the delta at about 2.3 tonnes per hectare.

The lower Mekong River and its delta support one of the largest inland fisheries in the world. During the past decade, the Vietnamese portion of the delta has yielded an annual harvest of about 400,000 metric tonnes of fish. Approximately 156,000 tonnes of this are derived from the brackish water and estuarine zone. However, fish production has been declining in recent years as a result of over-exploitation, forest destruction, drainage of wetlands for agriculture and the affects of toxic chemicals such as Agent Orange.

The mangrove and Melaleuca forests constitute an important forestry resource, potentially capable of meeting the local demand for construction materials, firewood, fodder for domestic animals and other forest products. In addition, the Melaleuca forests provide a valuable harvest of honey from wild bees' nests, amounting to five or six litres of honey per hectare per year.

The mangrove forests also play a very important role in coastal protection and land reclamation. Mangrove species not only retard erosion due to tidal action (of vital significance in a region prone to typhoons), but also tend to accumulate soil around their root systems, thereby accelerating accretion of new land.
Fauna: The fauna of the delta includes 23 species of mammals, 386 species and subspecies of birds, 35 species of reptiles, six species of amphibians and 260 species of fishes.
Five species of dolphins have been recorded. Two of these, Stenella malayana and Tursiops aduncus, are confined to the estuarine zone, but the other three, the Irrawaddy Dolphin Orcaella brevirostris, Chinese White Dolphin Sotalia chinensis and Black Finless Porpoise Neophocaena phocanoides, occur upstream into Kampuchea. Other wetland mammals known to occur include the Crab-eating Macaque Macaca fascicularis, Smooth-coated Otter Lutra perspicillata and Fishing Cat Felis viverrina. The Clawless Otter Aonyx cinerea may also occur.

The avifauna includes about 92 species of waterfowl and a variety of other species associated with wetlands, such as the raptors Pandion haliaetus, Haliastur indus, Haliaeetus leucogaster, Ichihyophaga ichthyaetus and Circus (aeruginosus) spilonotus, and about eight species of kingfishers (Alcedinidae). The delta is particularly important for its large populations of cormorants, herons, egrets, storks and ibises, which nest in huge colonies in the mangrove and Melaleuca forests. Vo Quy (1984) estimated that between 200,000 and 250,000 birds were nesting at the main colonies in the southern part of the delta in 1982. Seven large colonies are presently known, the largest being at Bac Lieu (Vinh Thanh), Dam Doi (Ngoc Hien) and Cai Nuoc (Tan Hung). A very large colony in the Minh Hai Melaleuca forests disappeared in the early 1980s, but two or three new colonies have recently become established in the Nam Can mangrove forests. The breeding species include three species of cormorants Phalacrocorax spp, Anhinga melanogaster, a wide variety of herons and egrets (notably Ardea sumatrana), seven species of storks and the ibises Threskiornis melanocephalus and Plegadis falcinellus. All seven species of storks have, however, decreased markedly in recent years. Mycteria cinerea and Leptoptilos dubius are now very rare, and no breeding sites are known. Mycteria leucocephala and Anastomus oscitans still breed in small numbers at four and three colonies, respectively, while Ciconia episcopus, Ephippiorhynchus asiaticus and Leptoptilos javanicus occur in ones and twos at several sites. The pelicans Pelecanus onocrotalus and P. philippensis were reported to be common in the 1960s (Wildash, 1968), but P. onocrotalus seems to have disappeared completely, and P. philippensis is now rare.

The Dong Thap Muoi wetlands are very important for the endangered eastern race of the Sarus Crane Grus antigone sharpii. This crane, which formerly nested in the area, disappeared during the war years but has returned in recent years as a non-breeding visitor; over 1,000 individuals were present in spring 1988. Three other endangered species of waterfowl, the White-shouldered Ibis Pseudibis davisoni, Giant Ibis Thaumatibis gigantea and White-winged Wood-Duck Cairina scutulaza, still occurred in the delta in reasonable numbers in the 1960s (Wildash, 1968). However, none of the three has been reported since about 1980, although it is believed that the two ibises may still survive in very small numbers in the wetlands along the Kampuchean border.

In addition to the colonial nesting waterbirds, there are many other resident waterfowl which remain fairly common, such as Tachybaptus ruficollis, Dendrocygna javanica, Nettapus coromandelianus, Anas poecilorhyncha, Rallus striatus, Gallicrex cinerea, Porphyrio porphyrio, the two jacanas (Jacanidae), and several shorebirds (e.g. Rostratula benghalensis, Esacus magnirostris, Glareola maldivarum and G. lactea). The Masked Finfoot Heliopais personata and Indian Skimmer Rhynchops albicollis are also known to occur. Common passage migrants and winter visitors include four species of ducks (notably Anas querquedula), a large number of migratory shorebirds, and several species of terns. Fischer (1983) gives some information on the migratory shorebirds. He observed large numbers of shorebirds of over 20 species in the eastern part of the delta in autumn 1980. The most abundant species were Himantopus himantopus, Charadrius dubius, C. leschenaultii, Tringa nebularia, T. ochropus, T. glareola, Act itis hypoleucos, Xenus cinereus, Gallinago stenura, G. gallinago, Calidris ruficollis, C. subminuta and C. teminckii. A flock of 22 Asian Dowitchers Limnodromus semipalmazus in Minb Hai Province was particularly noteworthy.
Reptiles include the monitor lizard Varanus salvator, the python Python reticulatus, four species of water snake Enhydris spp, the endangered River Terrapin Batagur baska and the Estuarine Crocodile Crocodylus porosus.

In the brackish water and coastal zone, the fish fauna is dominated by Clupeidae, Scombridae, Sciaenithe, Tachysauridae, Polynemidae, Tachysuridae and Cynoglossidae. Most of the species are diadromous and some, particularly species of Polynemidae and Tachysuridae, seasonally ascend the rivers to spawn in the gradient or freshwater zone of the estuaries. In the freshwater zone, the fishes are dominated by species of Cyprinidae, Siluridae, Clariidae, Schilbeidae, Bagridae, Sisoridae, Akysidae, Chanidae and Ophicephalidae. Over 200 species of fishes contribute to the commercial fishery, along with many shellfish, mussels and clams (Mollusca), and prawns and shrimps, notably Macrobrachium rosenbergii and Penaeus monodon (Pantulu, 1986b).
Special floral values: The delta still contains some tracts of mangrove and Melaleuca forest in relatively good condition.
Research and facilities: Many Vietnamese and overseas research institutions have conducted and are conducting scientific research in the Mekong Delta, and a considerable amount ol information is now available, particularly on the mangrove forests, fishery resources and agricultural potential. The Committee for Coordination of Investigations of the Lower Mekoni Basin (the Mekong Committee) has undertaken many studies during the past twenty yean aimed at full development of the Mekong Delta. One of the major studies, concerning the development of marshes and tidal lands, was carried out by the Netherlands Delta Team between 1970 and 1974 (Mekong Committee, 1977). Other studies sponsored by the Mekoni Committee have concerned the fisheries (Mekong Committee, 1976). During the perioc 1983-85, State Programmes 02-11 and 60-02 considered zoning and planning issues in the delta with a view to sustainable exploitation of the natural resources. These programmes incorporated studies of natural factors such as climate, hydrology and geomorphology as well a socio-economic surveys. During the period 1982-85, State Programme 52-02 supported considerable amount of research on the mangrove and Melaleuca forests, including research OT the breeding colonies of waterbirds.
Criteria for inclusion: 123.
References: Fischer (1983); Hoang Thi San & Phan Nguyen Hong (1984); Karpowicz (1985) Le Dien Duc (1984, 1987a & 1987b); Le Dien Duc & Le Dinh Thuy (1987); Le Dien Duc et al. (1986); Luthin (1987a, 1987b & 1988), Ly Tho (1985); McNeely (1975); Mekong Committee (1970, 1976, 1977, 1978, 1981, 1982 & 1985); Morris (1986 & l988b); Nguyen Hoang Tr (1984a & 1984b); Pantulu (1986a & l986b); Phan Nguyen Hong (1984a, 1984b, 1984c & 1984d) Phung Trung Ngan (1987); Stoutjesdijk (1982); Vo An ha & Nguyen Dinh Dien (1985); Vo Qu: (1984); Vo Quy & Le Dien Duc (1984); Vo Quy & Phan Nguyen Hong (1984); Vo Quy et al. (1984); Vu Trung Tang et al. (1981); Wildash (1968).
Source:
Le Dien Duc and references.