Asia’s third-longest river starts as Tibet’s Za Qu
The Mekong ranks as the world’s twelfth-longest river and Asia’s third, running an estimated 4,909 kilometres from the Tibetan Plateau through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. Its basin covers about 795,000 square kilometres and dumps roughly 475 cubic kilometres of water a year.
Headwaters rise as the Za Qu in Sanjiangyuan’s “three rivers source” country, the same reserve that guards the springs of the Yellow River and the Yangtze, before the Chinese stretch is called the Lancang—“turbulent green river,” echoing the Lao kingdom of Lan Xang. Thai and Lao Mae Nam Khong contracts older Kra-Dai forms; Khmer speaks of Tônlé Thum, the Great River, or Mékôngk; Vietnamese uses Sông Mê Kông or Sông Cửu Long, Nine Dragons. Historian William Dalrymple suggests the name may even descend from a Khmer phrase meaning Mother Ganges.
After meeting the China–Myanmar border, the river traces about 100 kilometres of Myanmar–Laos frontier to the Golden Triangle tripoint with Thailand where the Ruak joins. From there it forms some 920 kilometres of Lao–Thai border past Khon Pi Long rapids—“where the ghost lost its way”—before swinging into the Lao interior, rejoining the frontier for another 850 kilometres past Vientiane. In the upper basin, a run of about 2,200 kilometres, the channel falls some 4,500 metres through narrow Yunnan gorges.
That steep upper quarter of the catchment yields only 15 to 20 percent of the flow but roughly half the sediment. Downstream, Thailand's Mun and Chi rivers drain 15 percent of the basin yet add just 6 percent of the water, because the Khorat Plateau loses so much to evaporation; its forest cover shrank from 42 percent in 1961 to 13 percent by 1993. By the time the river enters Cambodia, more than 95 percent of its water has arrived.
At Phnom Penh the Tonlé Sap performs a famous reversal: in the dry season it feeds the Mekong, but when floods rise the current runs backwards up into the lake. Just south, the Bassac branches off and the delta begins, and in Vietnam the main stem becomes the Tiền. Dams built from the 2000s onward have damaged the ecosystem and worsened drought, and the heavily farmed delta, with under 10 percent forest left, is widely seen as the reach most exposed to upstream change.
Source: Mekong
