Nam Ngum Reservoir
Location:
18°26'-18°47'N, l02°30'-102°55'E; on the Narn Ngum River, 60 km north of Vientiane,
Muong Keo Udom and Muong Horn, Vientiane Province.
Area:
45,000 ha at maximum water level; 23,000 ha at minimum water level.
Altitude:
196-212m.
Biogeographical Province:
4.10.4.
Wetland type:
17.
Description of site:
Nam Ngum Reservoir is the largest water impoundment in Laos; it was created in 1971 by the construction of a dam 75m in height across the Nam Ngum River, about three km upstream from its confluence with the Nam Lik. The reservoir was conceived primarily for the production of hydro-electric power and flood control. It is fed by five rivers, the Nam Ngum, Nam Sane, Nam Ke, Nam Pat and Nam Xi, and many smaller streams rising in the surrounding hills. It overflows into the Nam Ngum River which joins the Mekong River about 60 km to the southeast. The impoundment created numerous small islands in five major island groups varying in size from about 75 ha to 500 ha. The timber was not removed from the basin before the closure of the dam and so the entire water surface is dotted with emergent dead trees. These are expected to persist for more than 100 years. Most of the shoreline is steep. The average depth at high water level is l9m, the level reaching its maximum in September or October and minimum in April to July. The average drawdown is about 10m and the maximum l6m. Some 4,000 ha of lake bed are exposed for six months of the year, and 11,000 ha during the three driest months.
Surface water temperatures range from about 24°C to 30°C or more; a low of 19°C has been recorded at a depth of 47m dear the dam. The water is soft and generally has low turbidity, with an average Secchi disc reading of 4.7m. The pH exceeds 7.0 in the surface waters, and decreases to 6.5 with depth.
A pH of 9.0 has been recorded in sheltered areas with an abundant grow~5h of aquatic vegetation.
Climatic conditions:
Tropical monsoonal climate, dominated by the wet southwest monsoon from mid-May to early October and the cool, dry northeast monsoon from early November to mid-March. The annual rainfall is 2,000-2,600 mm, over 83% of which falls during the five months from May to September. The mean daily temperature for the whole year is about 25°C; the maximum at the hottest time of the year in March and April averages 33°C and the minimum in the cool season about 17°C.
Principal vegetation:
The great seasonal fluctuations in water level prevent extensive growth of aquatic macrophytes, although there are occasional dense growths of floating vegetation such as Azolla, Pfstia, Eichhornia and Salvinia. The submerged aquatic plant Hydrilla verticillata grows abundantly in shallow waters three to five metres from the shore and to a depth of four metres. Grasses appear on the exposed shoreline at low water levels. Significant changes have occurred in the phytoplankton since completion of the dam; by 1981, the phytoplankton was dominated by Desmidiacaea, Dinophyta and Chrysophyta (Mekong Committee, 1984c).
Some islands support good tropical lowland forest dominated by bamboos and species of Dipterocarpus, Anisoptern, Lagerstroemia. Irvingia, Dialiurn and Walsura. Surrounding areas were once similarly covered in forest but much of this has now been degraded by logging, wood-cutting and charcoal production, or cleared for agricultural purposes.
Land tenure:
No information.
Conservation measures taken:
No legal protection. Logging on the islands is subject to concession.
Conservation measures proposed:
No Information.
Land use:
Generation of electricity, flood control, water supply for irrigation and fishing. Since the completion of the reservoir in 1971, a number of people have immigrated into the area to look for new occupational opportunities. In 1981, the reservoir area had a population of 9,560 people in 31 villages (Mekong Committee, l982c). Eight of these are primarily fishing villages and have formed their own fishing cooperatives, while the others are both fishing and farming communities. In 1980/81, there were some 2,350 fishermen using the reservoir (Mekong Committee, 1982c). Human settlements have been established on a few of the larger islands located in the central part of the reservoir, while others are used for temporary housing for fishermen and for charcoal production. Because the shorelines are steep and the soils are poor, drawdown agriculture is not extensive, The major crops grown in surrounding areas are rice, corn, vegetables and fruit.
Possible Changes in Land use:
There are plans to increase the height of the dam in Phase 3 of the Nam Ngum Project.
Disturbances and threats:
Eutrophication from decomposition of the flooded vegetation caused serious problems for fisheries until about 1980. A considerable amount of unauthorized logging occurs around the lake shore. The forested islands will be flooded in Phase 3 of the Nam Ngum Project, as also will a part of the Phou Khao Khouay Reserved Forest in the catchment area. The catchment area is affected by serious erosion problems, due to deforestation and land reclamation by inappropriate agricultural practices. The estimated rate of soil erosion in this area is 16 tons/ha/year.
The eroded soil is transported by rivers and surface runoff to the reservoir, causing a serious siltation problem and endangering the viability of the reservoir, not only for hydropower generation but also for fisheries production.
Economic an social values:
The hydro-electric power station has an installed capacity of 110 MW. Power from the dam supplies the city of Vientiane and the surrounding countryside. Surplus energy is exported to Thailand and is the country's largest foreign exchange earner. The total fish catch was estimated at 1,583 metric tonnes in 1980 (Mekong Committee, 1982c) and 1,470 metric tonnes in 1982, the latter representing a production of 37 kg/ha/year (Mekong Committee, 1984c). In 1986, the yield was estimated at 1,800 metric tons, worth approximately US$1.4 million. However, the lake has only limited possibilites for recreational use (Sayer, 1983a).
Fauna:
At least sixty-six species of fishes have been recorded in the commercial catches (Mekong Committee, l984c). The most important are Channa micropeltes. Hampala macrolepidota, Mystus nemurus, Notopterus notopterus, Pristolepis fasciatus, Oxphronemus gorami, Puntioplites proctozysron. Osteochilus melanopleura, 0. hasselti, Morulius chrysophekadion and Puntius schwanenfeldii. Several exotic species have been introduced.Waterfowl are said to visit the area, but no details are available. Some large mammals were present on the islands after dam closure, but most have since been killed by hunters, and the area now seems to have little value for wildlife (Sayer, 1983a).
Special floral values:
No information.
Research and facilities:
A considerable amount of fisheries research has been carried out at the reservoir. A fishery development and management project was undertaken between 1978 and 1983 with financial assistance from the Government of the Netherlands.
References:
McNeely (1975); Mekong Committee (1970, 1976, 1981, 1982c, 1984a & 1984c); Pantulu (1986b); Sayer (1983a).
Criteria for Inclusion:
1e.
Source:
See references.