Until the 1980s the Mekong River flowed freely for 4,900 kilometres from its 5,100-metre high source in Tibet to the coast of Vietnam, where it finally poured into the South China Sea. The Mekong is the world's twelfth longest river, and the eighth or tenth largest, in terms of the 475 billion cubic metres of water it discharges annually. Then and now it passes through or by China, Burma (Myanmar), Laos, Thailand, Cambodia and Vietnam. It is Southeast Asia's longest river, but 44% of its course is in China, a fact of capital importance for its ecology and the problems associated with its governance.