SBR- Content.pmd - INBO

3 - Water in the Mekong BasinConcerns have been raised at various times about the rate at which the Tonle Sap Great Lake isfilling with sediment. This concern arises from perceptions that forest clearing in the basin is leadingto increased erosion rates, and thus to increased stream transport of sediment. Since 1992, when therecords began, MRC data do not demonstrate an increase in sediment concentrations entering TonleSap Lake. A number of new studies of sedimentation in Tonle Sap Lake are underway or are planned,but the only available data 8 indicate that the lake is filling at a rate of about 0.1 mm a year and sowould take more than a thousand years to fill, rather than decades.In Northeast Thailand, there has been a conspicuous improvement in water quality, with nutrients,salinity and TSS all showing improvement. The reason is not obvious and will require furtherinvestigation. It is possible, but not likely, that this results from improved land management. Amore likely possibility is that it results from the operation of increasing numbers of small-scalereservoirs (see Figure 9) which are increasing dry season flows and thus lowering concentrations ofsalt and nutrients at this time. Such reservoirs would also trap sediment, lowering TSS levels.Within the delta region, the two conspicuous patterns in water quality are the decrease in conductivityand the increase in nutrients. The drop in salinity is probably due to a decrease in the intrusion ofsaline water from the sea, either through barriers or increased flushing with river water. The trend innutrient level concentrations gives cause for concern. Levels of Total P at Chau Doc in Viet Nam,for example, are now approaching those usually associated with eutrophication and associated algalblooms in lakes (Figure13). 9 Flushing by the river may be sufficient to reduce the likelihood ofsevere algal blooms in mainstream sites, but should the water velocity be reduced by barriers toprevent saltwater intrusion, the problems could become far more severe.Figure 13. Total Nitrogen concentrations in the Mekong River at Chau Doc, Viet Nam from 1988to 2000 with the linear regression line fitted showing the significant (p < 0.05) increaseover time.27