The Mekong River is the longest river in Southeast Asia. The river has a length of approximately 4,900 km, flowing from its source on the Tibetan Plateau in China through Myanmar, Lao PDR, Thailand, Cambodia and Viet Nam via a large delta into the sea. The river basin drains a total land area of 795,000 km2 and has a mean annual discharge of 475 km3 , the tenth largest in the world.

The Mekong River Basin includes seven broad regions with diverse topography, drainage patterns and geomorphology. The basin is commonly divided into the Upper and Lower Mekong River Basin. The Upper Mekong River Basin comprises the Tibetan Plateau, the Three Rivers Area and Lancang Basin in China and Myanmar. The Lower Mekong River Basin is made up of the Northern Highlands, the Khorat Plateau, the Tonle Sap Basin and the Mekong Delta. It covers Thailand, Lao PDR, Cambodia and Viet Nam.

Learn more about the regions of the Mekong River Basin

River Course

Throughout its course, the Mekong River is fed by a series of tributaries. These tributaries are generally classified into two groups: the left-bank tributaries drain areas of high rainfall and contribute to wet season flows, while the right-bank tributaries drain low relief regions with lower rainfall.

In the Upper Mekong River Basin in China’s Yunnan Province, the tributaries are small. As the river widens in the Northern Highlands, large tributaries – including the Nam Ta, Nam Ou, Nam Soung and Nam Khan – enter on the Mekong River’s left bank; the Nam Mae Kok and Nam Mae Ing enter on the right bank.

Further downstream in the Khorat Plateau, the main stream is joined by the gently sloping Songkhram and Mun Rivers on the right bank and the steep Nam Ca Dinh, Se Bang Fai and Se Bang Hiang Rivers on the left bank.

The Se Kong, Se San, and Sre Pok (3S Basin) are the main tributaries entering on the left bank of the Mekong. The Tonle Sap River drains the Great Lake (or Tonle Sap Lake) into the Mekong River during the dry season and reverses its flow during the rainy season.

Near the Cambodian capital Phnom Penh, the Bassac River, the Mekong tributary, branches off. This is where the Mekong Delta begins as the Mekong and Bassac Rivers enter a large fertile plain in southern Viet Nam. In this area, known as the ‘Nine Dragons’, a series of smaller distributaries split off from the main stream of the Mekong and Bassac.