The Mekong River Basin

The 4800 km Mekong (known as the Lan Tsan Chiang or Lancang in its upper reaches in Yunnan Province, China) rises at 5100 m elevation on the eastern edge of the Tibetan (Xizang) Plateau where the Yangtze (Chang Jiang) and Salween also rise. With a drainage basin covering 795 000 km2, the river ranks as the ninth largest and twelfth longest in the world and discharges some 475 billion m3 of water to the South China Sea annually. The mean annual flow at Kratié in Cambodia (where the catchment area upstream is 646 000 km2) is 14 700 m3 s−1 with a maximum of 67 000 m3 s−1 and a minimum of 1250 m3 s−1 (Committee for Coordination of Investigations of the Lower Mekong Basin 1966; Volker 1983). The river flows from the Tibetan Himalayas southward through China receiving tributaries from a small part of Myanmar. The drainage basin also encompasses nearly all of Lao PDR, northeast Thailand, most of Cambodia, and part of the Central Highland and the delta of south Viet Nam. In the heart of Cambodia, where the river is joined by the Tonlé Sap or Great Lake River, it rises from 1 or 2 m above sea level in May to 8 or 10 m above sea level in August. The Mekong Basin embraces some of the most diverse scenery in the world, with landforms ranging from deep gorges, to spectacular karst features, great lakes, and a huge delta. These varied landscapes support one of the most biologically diverse river systems in the world, surpassed only by the Amazon and possibly the Nile. The high biodiversity varies greatly across the following distinct landform and biogeographic provinces: 1. the eastern edge of the Tibetan Plateau (here termed the Chinese upper reaches); 2. the highlands of Myanmar, northern Thailand, and the northern Lao PDR; 3. the Annamite Mountains of eastern Lao PDR and western Viet Nam; 4. the plains around the central Mekong in Lao PDR, Thailand, and Cambodia; 5. the Tonlé Sap Basin; 6. the Mekong Delta and coastal mangroves (MacKinnon and MacKinnon 1986).