Its estimated length is 4,909 km, and it drains an area of 795,000 km2, discharging 475 km3 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.

Mekong is originally called Mae Nam Khong from a contracted form of Tai shortened to Mae Khong. In Thai and Lao, Mae Nam (“Mother of Water[s]”) is used for large rivers and Khong is the proper name referred to as “River Khong”. To the early European traders, the Mekong River was also known as Mekon River, May-Kiang River and Cambodia River. In Vietnam, it is called Cuu Long river (Nine Dragons River).

The Mekong rises as the Za Qu and soon becomes known as the Lancang. It originates in the “three rivers source area” on the Tibetan Plateau in the Sanjiangyuan National Nature Reserve. The reserve protects the headwaters of, from north to south, the Yellow (Huang He), the Yangtze, and the Mekong Rivers. It flows through the Tibetan Autonomous Region and then southeast into Yunnan Province, and then the Three Parallel Rivers Area in the Hengduan Mountains, along with the Yangtze to its east and the Salween River (Nu Jiang in Chinese) to its west.

Then the Mekong meets the China–Myanmar border and flows about 10 kilometres along that border until it reaches the tripoint of China, Myanmar and Laos. From there it flows southwest and forms the border of Myanmar and Laos for about 100 kilometres until it arrives at the tripoint of Myanmar, Laos, and Thailand. This is also the point of confluence between the Ruak River (which follows the Thai–Myanmar border) and the Mekong. Then, the Mekong turns southeast to briefly form the border of Laos with Thailand.

Khon Pi Long is a series of rapids along a 1.6-kilometre section of the Mekong River dividing Chiang Rai and Bokeo Province in Laos. The name of the rapids means ‘where the ghost lost its way’. It then turns east into the interior of Laos, flowing first east and then south for some 400 kilometres before meeting the border with Thailand again. Once more, it defines the Laos-Thailand border for some 850 kilometres as it flows first east, passing the capital of Laos, Vientiane, then turns south. A second time, the river leaves the border and flows east into Laos soon passing the city of Pakse. Thereafter, it turns and runs more or less directly south, crossing into Cambodia.

At Phnom Penh, the river is joined on the right bank by the river and lake system the Tonlé Sap. When the Mekong is low, the Tonle Sap is a tributary: water flows from the lake and river into the Mekong. When the Mekong floods, the flow reverses: the floodwaters of the Mekong flow up the Tonle Sap.

Immediately after the Sap River joins the Mekong by Phnom Penh, the Bassac River branches off the right (west) bank. The Bassac River is the first and main distributary of the Mekong. This is the beginning of the Mekong Delta. The two rivers, the Bassac to the west and the Mekong to the east, enter Vietnam shortly after this. In Vietnam, the Bassac is called the Hau River; the main, eastern, branch of the Mekong is called the Tien River. In Vietnam, distributaries of the eastern (main, Mekong) branch include the My Tho River, the Ba Lai River, the Ham Luong River, and the Co Chien River.

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