The Mekong river, with a length of 4,200 km, is the main watercourse in South-East Asia, flowing from the north-eastern rim of the Tibetan plateau at an altitude of 5,000 m into the South China Sea near Ho-Chi-Minh Ville. Its drainage basin of nearly 800,000 km is larger in area than France and includes Kampuchea, most of Laos and parts of the other four riparian countries, i.e., the People's Republic of China, Burma, Thailand and the Socialist Republic of Vietnam. In its course it constitutes parts of the boundaries between the People's Republic of China and Burma, Burma and Laos, Laos and Thailand and Laos and Kampuchea. Its flow increases considerably during the rainy season (May-October), raising the water level at Phnom-Penh by 16 m and flooding the Central Plain of that country, thereby trebling the surface of the Tonlé-Sap (Great Lake) and the Delta in Southern Vietnam. With an annual average discharge of about 475,000 million cubic metres, it is the eighth largest river in the world.