The Nam Ngum reservoir in Laos

Many dead tree trunks can be seen in most parts of the Nam Ngum reservoir.

The Nam Ngum reservoir was created in 1971 and is by far the biggest in Laos. The pre-impoundment area consisted mainly of red-yellow podzolic soils of silty and clayey sand. The area was almost entirely covered with tropical lowland forest, mainly consisting of bamboos. Prior to the inundation, the trees and bamboos were not felled. Trunks and branches just under the water surface are not visible, as are the presently emergent trees, and are therefore a severe impediment to navigation. The large number of submerged trees and islands reduce the extent of wind induced mixing of the reservoir water, and the flooded vegetation has decayed. This has resulted in a shallow mixing layer near the surface, and stagnant, anoxic water at depth has permitted the buildup of hydrogen sulfide, which has an unpleasant odor and is toxic to aquatic life.

Location of core samples in the Nam Ngum reservoir, and mean annual rate of sediment accumulation during the period 1971-1991 at the sampling stations. From Axelsson 1992 (Sedimentation in the Nam Ngum reservoir, Lao PDR. AB Hydroconsult, Uppsala).

In April 1991, 32 sediment cores were sampled in the Nam Ngum reservoir and X-rayed in the Fishery Laboratory at the reservoir. The study of these cores yielded some information about the oxygen conditions in the reservoir. A smell of H₂S was noticed from the bottom water and from several sediment cores, sampled in the drowned forest, especially from those sampled in the southern part of the reservoir. However, in most cores, sampled along the former river channels, the topmost sediment layers were bioturbated, and contained a rather dense population of bottom fauna, mainly tubificids.

The sediment cores were sampled in transparent, rectangular coring tubes, measuring 30x60 mm along the inside walls. The coring tubes, including the bottom water layer and the core, were transported, stored, and X-rayed in an upright position.

Upper part of core G7 from the Nam Ngum reservoir in Laos. The lead letters O and B are used for marking corresponding levels on the partly overlapping radiographs.

Mean annual sedimentation rate 1971-1991 at this sampling station: 37 mm and 16.3 kg of solids/m². For location, see the map above.

Partly overlapping radiographs of core G22 from the Nam Ngum reservoir. The upper sediment layers are somewhat disturbed due to the formation, expansion, and release of gas bubbles. The harder (lighter) layers are probably formed during the given rainy seasons.

In general, peaks of the film-density curves corresponding to minimum values in bulk density (dry season deposits) were used to determine the border between the sedimentological years. Mean annual sedimentation rate 1971-1991 at this sampling station: 13 mm and 4.4 kg of solids/m².

The structure of the X-rayed sediment cores indicates that the rate of sediment accumulation sometimes is about 10 times higher along the former river channels than on the surrounding bottom areas due to deposition from turbidity underflows. Most sediment cores from the former river channels showed a rhytmic variation between harder and softer layers, deposited during the rainy and the dry seasons respectively. The downstream distribution of the seasonal layers along the former Nam Ngum river channel could partly be followed by inter-core correlation. In some cores it was possible to distinguish 20 couplets with the help of stereoradiographs, representing the period of reservoir operation (1971 - 1991).

Probable mean monthly inflow of suspended, inorganic sediment to the Nam Ngum reservoir during the operational period 1972 - 1989.

The only station for sediment monitoring within the catchment was situated in the Nam Ngum river at Ban Na Luang. The probable mean values of inorganic, suspended sediment inflow to the reservoir, shown by the figure above, are calculated by assuming the relation of monthly mean sediment yield to monthly mean run-off in the Nam Ngum river at Ban Na Luang for the period 1987 - 1990 to be valid also for the reservoir and for the period 1972 - 1989. According to the calculations the annual mean inflow of suspended inorganic sediments to the reservoir during the given operational period amounted to about 1.2 million tonnes, corresponding to a sediment yield of about 140 tonnes/km² and year. These figures, although probably of the right order, are very uncertain, since they are based on many assumptions. The morphology of the river mouths and the composition of the bottom deposits indicated that the inflow of bed load + suspended organic sediments amounted to about 15 % of the total sediment inflow. The total mean annual sediment inflow to the Nam Ngum reservoir during the operational period 1972 - 1989 therefore probably was of the order of 1.4 million tonnes.

The results of the sedimentological studies showed that the rate of sediment accumulation in the Nam Ngum reservoir was low and that it will take a very long time before the larger part of its storage has been taken up by sediment. If the conditions of sediment inflow will be the same in the future, it will probably take more than 1000 years to fill 50 % of the reservoir storage with sediment.

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