Mekong River
Location:
21°34'N, 101°09'E to 13°51'N, 105°59'E; the Mekong River from the border with China and
Burma in the extreme northwest of the country to the border with Kampuchea in the extreme south.
Area:
c.1,700 km of mainstream river.
Altitude:
c.250m in the north down to c.65m in the south.
Biogeographical Province:
4.10.4.
Wetland type:
11, 12, 13, 18 & 19.
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).
Almost 88% of Laos (207,400 sq.km) is drained by the Mekong River and its tributaries. The river itself flows for about 1,700 km along the Lao border or through Lao territory from the Chinese and Burmese borders in the north to the Kampuchean border in the south. For most of its course north of Vientiane, the river flows through a rather narrow valley in hilly and mountainous country. In the region of Vientiane, two large tributaries, the Nam Ngum and Nam Lik, form a broad alluvial plain along the north bank of the Mekong at the edge of the Korat Plateau. For the next 560 km, the river flows placidly through a wide valley. Natural levees begin in this section, and several major tributaries, notably the Nam Theun, Se Bang Fai and So Bang Hieng, enter from the east. These rivers rise in the Annamite Range on the Vietnamese border, and all form broad alluvial valleys in the Korat Plateau. South of its confluence with the Se Bang Hieng, the Mekong enters the Khemarat Rapids where, in the monsoon season, it reaches speeds of 5m per second and can rise more than 20m. The river continues on through a rocky gorge for a further 160 km before emerging onto the plains above Pakse, at about l00m above sea level. It then wanders south across the lowlands of southern Laos to Khone Falls, where it plunges onto the lower Mekong plain at the Kampuchean border. At the top of the falls, the river is spread broadly over a rocky stream bed some 16 km wide.
The northern section of the Mekong is relatively clear and fast-flowing, with many rock outcrops and rapids. Sandy-gravel islands and near-bank shoals are prominent where the river banks are low. The water tends to be neutral, with a pH of 6.9 ranging to 8.2. Dissolved oxygen concentrations frequently exceed saturation, and the nutrient level is low. From the region of Vientiane to the Kampuchean border, the river is turbid, particularly during the rainy season when bank erosion is at its most severe. Suspended lateritic soil gives the water an apparent rusty-tan colour. River temperatures fluctuate between 21.1°C and 27.8°C, and the pH varies from 6.2 to 6.5. The mainstream habitats range from sandy-gravel bars to deep pools up to 100m deep and several kilometres long, interspersed with rocky rapids (Pantulu, 1986a).
Snowmelt produces a rather uniform river flow in the upper Mekong, while the lower Mekong exhibits pronounced seasonal variations in flow, reflecting rainfall patterns. The river starts to rise with the onset of the monsoon in May or June, and attains its maximum level in August or September. It then falls rapidly until December and slowly thereafter to reach its lowest level in April. Inundation zones occur where the Mekong overtops its banks at the height of the rainy season, but these are localized and nowhere very extensive.
Climatic conditions:
Tropical monsoonal climate dominated by the wet southwest monsoon and dry northeast monsoon. The average annual rainfall ranges from about 1,400 mm near Khemarat in the south to 3,000 mm near Pak Sane, east of Vientiane. In most regions, about 80% of the rainfall occurs during the southwest monsoon in May-October. Both diurnal and annual temperature ranges are relatively high. Cold air from Siberia and China occasionally penetrates the basin, lowering air temperatures to near zero in the north.
Principal vegetation:
No information is available on the aquatic vegetation. Throughout most of the regions bordering the Mekong, the lower areas are covered with dry evergreen forests, while the valley floors are invariably cultivated, mainly for rice. Above 1,000m elevation, the climax vegetation is usually hill evergreen forest. However, much of this has been greatly disturbed by destructive shifting agriculture, and now supports unproductive Imperata grasslands and broad-leaved Eupatrium.
Land tenure:
No information.
Conservation measures taken:
None.
Conservation measures proposed:
McNeely (1975) suggested that at least one reserve should be established to protect a
typical example of the Mekong flood plains. Such a reserve would have to be located as far away from any dams as possible. He tentatively suggested a stretch of about 50 km of river on the Thai border between Ban Pak Se Novan and Ban Keng Yapeut, at about 15°50'N, 105°25'E. McNeely also recommended the establishment of a protected area in a remote region on the upper Mekong River in Houa Khong Province, on the Burmese border at about 21°10'N, l00°45'E. He suggested that this protected area might best be considered as a "Remote Reserve", where humans should be allowed to continue their traditional activities. Hunting would be restricted to the local people, and only minimal protection would be required. Sayer (1983a) has recommended that the area around Pak On, at the confluence of the Mekong and Nam Ou in Luang Prabang Province (20°03'N, 102°13'E), be protected as a cultural park or national monument. The area has impressive limestone cliffs and caves containing many Buddha images of religious and historical significance, and the excursion by boat to Pak Ou is popular with visitors to Luang Prabang. The region has little value for nature conservation, as most of the natural vegetation has been destroyed by shifting cultivators and little wildlife remains. However, it is likely that valuable conservation sites exist further upstream on both rivers. Sayer (l983a) therefore recommends that the upper course of the Nam Ou, Mekong and their tributaries be surveyed in order to identify sites for development as national parks and/or wildlife reserves. McNeely (1975) has pointed out that some of the forested islands which will be left in the proposed Pa Mong Reservoir will be quite large. The largest one, at 4,164 ha, is forest-covered with steep topography and would make an ideal game reserve.
Land use:
Fishing and agriculture, mainly the cultivation of rice. The human population in the northern highlands is sparse (5-14 persons per sq.km); most of the inhabitants are of hill-tribe stock. Population densities are considerably higher along the edge of the Korat Plateau and in the south, ranging from 80-150 per sq.km. Here the major crops include rice, kenaf, maize and cassava.
Possible Changes in Land use:
The Mekong Committee has considered some 230 possible development projects on the lower Mekong River and its tributaries. Three large projects have been implemented in Laos, and a further 64 projects are currently considered as feasible. Feasibility or pre-feasibility studies have been carried out for two of these, and desk studies have been made for the remainder.
Of the six mainstream projects proposed by the Mekong Committee in Laos, much the largest is the Pa Mong Multipurpose Project. This involves the construction of a dam 1 15m in height across the Mekong at Pa Mong, about 20 km upstream from Vientiane. Excess flow in the Mekong would be stored to reduce natural flooding, generate energy and provide water, particularly during the dry season, for the development of irrigated agriculture on both banks of the river. Initial surveys were carried out between 1956 and 1963; a feasibility study was completed in 1971 and numerous follow-up studies have been conducted since then. The dam would flood 360 km of the Mekong River and create a reservoir of 372,200 ha, partly in Laos and partly in Thailand; it would have an initial installed power capacity of 4,800 MW, and permit the irrigation of one million hectares of dry season crops. The dam would eliminate bank overflow of the Mekong from the dam site to the Vientiane/Nong Khai area, and significantly reduce overflow as far south as Pakse. This would enable large areas of floodplain to be reclaimed for agriculture, including about 100,000 ha between the dam site and Pakse. Supplemental flow at low water during the dry season would increase the mean depth of the river by more than threefold over a 460 km stretch from Vientiane to Savannakhet, and double the depth over the 225 km stretch from Savannakhet to Pakse. The creation of the reservoir would entail the relocation of some 100,000 people on the Lao side of the river and 300,000 on the Thai side. The possible impact of this and other 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 fioodplains 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, l986b). The Pa Mong Dam is likely to have a serious adverse effect on the fisheries in the river below the dam and in the inundation zone due to flood regulation, but it is argued that fisheries production in the new reservoir will far outweigh the loss in production downstream (Mekong Committee, 1978).
Upland mainstream fishes will probably suffer most from the construction of dams since they have small ranges, various degrees of isolation, and are without the security of a large continuous mainstream population (Mekong Committee, 1976). Migratory species of fishes which could be seriously affected by the mainstream dams include Pangasianodon gigas, Pangasius sanitwongsei, P. sutchi, P. larnaudi, Probarbus jullieni and Cirrhinus microlepis (Mekong Committee, 1983).
In the upper reaches of the river and its tributaries, water quality does not present a problem at present, except in the vicinity of urban centres. The water quality in the lower Mekong has been affected by domestic wastes and agricultural runoff carrying pesticides and fertilizers, and 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
Mekong fishery in Laos was estimated at about 16,500 metric tons in 1973, and was thought to employ some 75,000 persons (Mekong Committee, 1976).
Fauna:
The fish fauna of the the fast-flowing, upper reaches of the river includes loaches, sucker catfish, Homalopteridae and Garrinae. In the slower middle and lower reaches, the fishes are 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 (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 in Kampuchea 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 (Pantulu, 1986b).
Only one species of dolphin has been recorded in the Mekong River upstream of Khone Falls: the Irrawaddy Dolphin Orcaella brevirostris. It appears to be rare in Laos, and no recent information is available on its status and distribution. Other wetland mammals known to occur include the Smooth-coated Otter Lutra perspicillata (rare) and Fishing Cat Felis viverrina. Although the river is thought to be rich in waterbirds, no studies have been carried out in recent years and no details are available.
The endangered Siamese Crocodile Crocodylus siamensis is reported to occur in Laos, and may still occur along the Mekong.
Special floral values:
No information.
Research and facilities:
The Mekong Committee has carried out a number of investigations on the Mekong River and its fisheries since 1957. Fisheries research has been directed principally at assessment of the effects of water resource management projects on fisheries production. The Pa Mong Multipurpose Project has been studied extensively over a period of twenty years at a cost of over US$20 million.
References:
IUCN (in prep); Karpowicz (1985); McNeely (1975); Mekong Committee (1970, 1976, 1978, 1981, 1983 & 1984a); Pantulu (1975, 1986a & l986b); Sayer (1983a & 1983b).
Criteria for Inclusion:
123.
Source:
See references.