The Mekong or Mekong River is a trans-boundary river in East Asia and Southeast Asia. It is the world's twelfth longest river and the third longest in Asia. Its estimated length is 4,909 km (3,050 mi), and it drains an area of 795,000 km² (307,000 sq mi), discharging 475 km³ (114 cu mi) of water annually. From the Tibetan Plateau the river runs through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The extreme seasonal variations in flow and the presence of rapids and waterfalls in the Mekong make navigation difficult. Even so, the river is a major trade route between western China and Southeast Asia. The construction of hydroelectric dams along the Mekong in recent decades causes serious problems for the river's ecosystem, including the exacerbation of drought.