Chapter 12 - Water Quality of the Lower Mekong River

Abstract

The hydrology of the Mekong basin is dominated by monsoon rains that occur in the period June–November and by the dry period from December to May. China contributes only 16% of the mean annual discharge, whereas Laos contributes some 35% and up to 60% during the wet season. The hydrology of the lower Mekong basin is particularly complex in the area from Kratie (Cambodia) downstream due to the extremely low gradients. During the dry season, the Mekong drains through the Mekong and Bassac distributary channels through the Mekong Delta to the South China Sea. The tropical Tonle Sap Lake (the Great Lake of Cambodia) and the Tonle Sap River are a unique lacustrine and wetland complex. The water quality of this lake has been monitoring by the Mekong River Commission since 1993 and as part of a special study on nutrient and sediment fluxes as part of the Water Utilization Project. In the lower Mekong basin, pollution sources contributing directly to the Mekong main stem are few. Thailand's contribution to pollution in the Mekong is mainly limited to salt leaching from the Korat Plateau—part of the Nam Mun watershed. The two largest urban areas, Vientiane in Laos and Phnom Penh in Cambodia, are of potential concern as they lie on the banks of the Mekong.