Mekong River
Location:
13°55'N, 105°58'E to 10°55'N, 105°08'E; the Mekong River from the border with Laos in the
north to the border with Vietnam in the south.
Area:
486 km of mainstream river; over 2,000,000 ha of floodplain wetlands including 1,600,000 ha of the
Mekong Delta.
Altitude:
c.65m in the north down to 3m in the south.
Biogeographical Province:
4.10.4./4.5.1.
Wetland type:
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. It rises at about 5,000m in the Tanghla Shan Mountains, on the northeast rim of
the great Tibetan Plateau, and flows for 4,200 km through or along the borders of six countries: China, Burma, Laos, Thailand, Kampuchea and Vietnam. In terms of mean annual discharge, the Mekong ranks sixth in the world. The total drainage basin of 783,000 sq.km includes 160,000 sq.km in China, 12,000 sq.km in Burma, and 611,000 sq.km in Laos, Kampuchea, Thailand and Vietnam (the lower Mekong basin).
Approximately nine-tenths of Kampuchea (154,730 sq.km) lie within the lower Mekong basin. The river itself flows for 486 km across Kampuchean territory from the Lao border In the north to the Vietnamese border in the south. Leaving the southeast edge of the Korat Plateau in southern Laos and eastern Thailand, the Mekong plunges over the Khone Falls at the Laos-Kampuchea border and reaches the lowlands of northern Kampuchea after crossing a series of rapids. Below Kompong Cham, the river forms a fluviatile lowland landscape with high natural levees, broad floodplains and extensive backwater swamps, many of which remain flooded throughout the dry season. Beyond the backswamps are parallel belts of paddy fields.
The mainstream habitats range from sandy-gravel bars to deep pools up to loom deep and several kilometres long, interspersed with rocky rapids. in several places between the Lao border and Phnom Penh, the river divides into two or more channels, creating large islands and extensive sand banks. The Mekong receives the waters of its last major tributary, the Tools Sap, at Phnom Penh. Immediately below this confluence, the Mekong divides to form the Mekong and the Bassac (Song Hau Giang), the two major channels of the delta. The triangular delta, with its apex at Phnom Penh, forms a vast fertile plain covering 49,520 sq.km. Some 16,000 sq.km of this delta are within Kampuchea.
The lower Mekong exhibits pronounced seasonal variations in flow, reflecting rainfall patterns~ Water levels are lowest in April and May; at this time, many water bodies in the inundation zone are isolated, and the smaller tributaries tend to dry out. With the onset of the monsoon rains in late May, almost the entire region is transformed into a sheet of muddy water. Rivers, tributaries and numerous small ponds, oxbows and ditches are engulfed. The river starts to rise shortly after the onset of the monsoon, 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. Extensive flooding takes place on both banks, but the by-pass flow on the western bank escapes towards the Great Lake (site 2). The main channel also adds its flow to the waters of the Great Lake through the Tonle Sap River which reverses direction during the rising flood. The Great Lake thus acts as a natural flood retention basin, without which the delta would be flooded to even greater depths. Towards the end of the rainy season in September and October, some seven million ha of lowlands in the delta and around the Great Lake are covered with water at various depths of flooding.
Between Kompong Cham and Phnom Penh, the plains adjacent to the river are flooded to a depth of l.0-1.5m for the four months from August to November. Further inland, the water level is higher (up to 3m) and the flooding persists for five, six or sometimes seven months of the year. In the Trans-Bassac area, there is a very large floodplain wetland between the Stung Takeo and Chau Doc rivers which is flooded to a depth of l.5-3.0m for five to eight months of the year. In the Cis-Bassac area, flooding occurs to a depth of 2.5-4.5m for five to seven months. Between the Mekong and the Tonle Tauch river to the east, flooding can reach a depth : of 7m, while to the east of the Tonle Tauch, on the edge of the delta, only shallow flooding occurs (0.3-1 .0m), for a period of one to three months.
The sediment loads in the Mekong are relatively low compared to other major Asian rivers such as the Ganges, Irrawaddy, Yellow and Yangtze. However, the organic content of the sediments is high, which explains the considerable secondary production in the Mekong (Pantulu, l986a). River temperatures fluctuate between 25-30°C, while the pH varies between 6.2 and 6.5. The stagnant waters of ponds and rice fields generally have high temperatures (28-30°C at the surface and 26-28°C at the bottom) and are alkaline (pH 8.0-8.6). The Mekong's water is generally very soft, with low mineral content. With the onset of the floods, water temperature, turbidity and hardness increase, while pH decreases sharply (Pantulu, 1986b).
Climatic conditions:
The Kampuchean portion of the Mekong lies within the tropical wet and dry zone, with a pronounced dry season during winter (at least one month with less than 60 mm of rainfall). The average annual rainfall ranges from 1,250 to 1,875 mm, with about 80% of the rain falling during the southwest monsoon in May-October. Both diurnal and annual temperature ranges are relatively high.
Principal vegetation:
No information is available on the aquatic vegetation. There are large tracts of seasonally flooded freshwater swamp forest in some places along the river, but widespread deforestation has taken place due to the demand for agricultural land and firewood. The principal tree species in the swamp forest include Barringtonia acutangula, Hydrocarpus anthelmintica, Terminalia chabula, Homalium brevidans and Amelia asiatica.
Land tenure:
No information.
Conservation measures taken:
None.
Conservation measures proposed:
None
Land use:
Fishing and agriculture, mainly the cultivation of rice. Of the one million hectares under cultivation in the Kampuchean portion of the Mekong Delta, some 800,000-900,000 ha are utilized for rice production.
Possible Changes in Land use:
The Mekong Committee has considered some 230 possible development projects on the lower Mekong River and its tributaries. Twelve large projects have been implemented, and 149 projects are currently considered as feasible. Feasibility or pre-feasibility studies have been carried out for 17 of these projects, including two in Kampuchea, and desk studies have been made for the remainder, including 35 in Kampuchea. One of the largest of the possible mainstream water storage projects is the Stung Treng Dam project. This was also considered to be one of the most viable projects. The project involves the construction of a dam 77m in height across the Mekong below its confluence with the Tonle San and Tonle Kong rivers, some 40 km south of the Lao border. The dam would create a reservoir of 797,000 ha it would generate 3,600 MW of elecricity, irrigate an area of about 1,287,000 ha (in both Kampuchea and Vietnam), and lower flood peaks at Kompong Cham by 1-2 metres. Some site investigations were carried out prior to 1975, but no full reconnaissance study was made, and the project has been shelved, at least temporarily. The construction of a second mainstream hydro-electric dam, capable of producing 875 MW of electricity, was proposed at Sambor, about 100 km downstream from Stung Treng. A feasibility report was completed in 1969, but this project has also been shelved (Mekong Committee, 1976b).
One of the primary objectives of Mekong River Basin development has been the elimination of seasonal flooding in the downstream delta areas of Kampuchea and Vietnam. Lands that are now seasonally inundated would be utilized for irrigated agriculture. Eliminating seasonal flooding will require the upstream storage of flood waters, primarily in the Pa Mong reservoir in Laos and the proposed Stung Treng reservoir in Kampuchea and secondarily behind the proposed Tonle Sap barrage, as well as the diking of river banks throughout the delta region and the diking of the perimeter of the Great Lake. Full implementation of these water management projects would decrease the annual Mekong flood discharge at Phnom Penh from 40,000 cubic metres per second to 23,000 cubic metres per second, lowering the high water level at this location by three metres, and would result in the loss of as much as 92% of the entire high-water surface area of the inundation zone. The possible impact of these projects on the basin's fisheries has been discussed in a report entitled Fisheries and Integrated Mekong River Basin Development (Mekong Committee, 1976a).
Disturbances and threats:
Major developments on the Mekong which are likely to conflict with fisheries interests include various proposed irrigation projects, hydro-electric power projects, other industrial development and flood control projects. Dam construction and operation will change the hydrology of the river downstream, reducing seasonal flow peaks and minimizing access to floodplains for feeding and 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.
Impoundment will reduce sediment flow, particularly in the main channels, and thereby affect the nutrient regime downstream (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, 1986a).
Economic and social values:
The lower Mekong River supports one of the world's largest inland fisheries. The total fishery of the Mekong inundation zone from Kratie to the Vietnamese border (but excluding the Great Lake-Tonle Sap inundation zone) was estimated to be about 75,000-80,000 metric tonnes in the l970s (Mekong Committee, 1976a). The annual flooding is of vital importance in maintaining this fishery for several reasons. The inundation zone provides the spawning and nursery grounds which supply the bulk of the young fish to repopulate most water bodies in the basin, including the main river and its tributaries, the many swamps and oxbows, and the paddy fields. The floods transfer millions of tons of suspended and dissolved solids into the water from highlands to lowlands and from the surface of land which has been lying fallow for several months. The flooding also releases nutrients from the soil, vegetation and inundated organic debris. Water enriched in this manner annually supports an explosive expansion of the riverine fish populations.
Fauna:
The dry season fish fauna of the Mekong River is dominated by species of carp (Cyprinidae; 54%), catfish (Siluridae, Clariidae, Schilbeidae, Bagridae, Sisoridae and Akysidae; 19%) and murrels (Chanidae and Ophicephalidae; 8%). The remaining 19% consist of featherbacks (Notopteridae), herring (Clupeidae), climbing perch and gouramis (Anabantidae) and other miscellaneous groups. Some 85-95% of the freshwater fish populations in the Mekong basin follow the inundation spawning pattern, undertaking lateral migrations from the mainstream and tributaries into the inundation zones to spawn and rear young between July and September (Pantulu, 1986b).
One of the most remarkable of the Mekong's endemic fishes is the Giant Catfish Pangasianodon gigas. This species, which can attain a length of over three metres and a weight of 250-300 kg, is a main channel migrant, reportedly migrating over a distance of several thousand kilometres from the region of Phnom Penh to spawning grounds in the province of Yunnan in China. It was once quite common in the lower stretches of the Mekong, with several thousand being caught each year below Khone Falls. However, over-exploitation of existing stocks and environmental changes consequent on development activities in the region have contributed to a serious decline in the population, and the species is now almost extinct below Pak Sane in northern Laos (Pantulu, l986b).
Three species of dolphins have been recorded from the lower Mekong in Kampucheur the Irrawaddy Dolphin Orcaella brevirostris (the only species known to occur upstream of Khone Falls), Chinese White Dolphin Sotalia chinensis (rare) and Black Finless Porpoise Neophocaena phocanoides (uncommon). Other wetland mammals known to occur include the Smooth-coated Otter Lutra perspicillata (quite rare) and Fishing Cat Felis viverrina.Although the entire area is known to be very rich in waterbirds, no studies have been carried: out in recent years and no details are available.
Special floral values:
No information.
Research and facilities:
The Mekong Committee carried out a number of investigations on the Mekong River and its fisheries between 1957 and 1975. The fisheries research was directed principally at assessment of the effects of water resource management projects on fisheries production.
References:
FAO (1980); McNeely (1975); Mekong Committee (1970, l976a, l976b, 1978, 1981 & 1984); Pantulu
(1975, l986a & 1986b).
Criteria for Inclusion:
123.
Source:
Le Dien Duc, Tran An Phong and references.