The Mekong River is Southeast Asia's longest and most economically important river, running about 4,350 km from its source on the Tibetan Plateau through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam to its great delta on the South China Sea. Its basin covers 795,000 km² and supports about 70 million people, including the entire population of the Mekong Delta in Vietnam (one of the most densely populated agricultural regions on Earth) and the Tonlé Sap floodplain in Cambodia (the largest freshwater lake in Southeast Asia and the world's most productive inland fishery). The river is also at the centre of one of the most consequential water-management debates in Asia, with rapid hydropower development on the upper Mekong (called the Lancang in China) and on the lower Mekong's tributaries reshaping the basin's hydrology, sediment balance, and fishery productivity.
Introduction
The Mekong is the lifeline of mainland Southeast Asia. It supports more than 1,200 fish species — second only to the Amazon — and produces an annual freshwater fish catch of about 2.6 million tonnes, more than the entire Atlantic cod fishery at its historic peak. The river's seasonal flood pulse drives the Tonlé Sap reverse-flow phenomenon (the river that flows backwards), one of the most distinctive hydrological events on Earth. The Mekong Delta in Vietnam produces about half of Vietnamese rice and a quarter of Vietnam's GDP. And the rapid construction of dams on the upper Mekong in China and on Laotian and Cambodian tributaries is now reshaping basin-wide ecology in ways that scientists and downstream governments are scrambling to understand.
Geography & Extent
The Mekong rises on the Tibetan Plateau in Qinghai province at about 5,200 m elevation and flows through southern China as the Lancang Jiang. After leaving China at the Burmese-Laotian border, the river forms much of the Laos-Myanmar and Laos-Thailand borders before entering Cambodia. The river divides at Phnom Penh into the Mekong proper and the Bassac, and these distributaries form the great Mekong Delta in southern Vietnam — about 40,000 km² of intensively cultivated alluvial land. Major tributaries include the Nam Ou, Nam Ngum, Nam Tha (Laos), Songkhram (Thailand), Mun and Chi (Thailand), Sekong, Sesan, Srepok (the "3S" tributaries from the Cambodian-Vietnamese highlands), and the Tonlé Sap (which seasonally reverses flow). The Mekong Basin includes 25% of Cambodia's land area, 86% of Laos, 36% of Thailand, and 20% of Vietnam.
Hydrology
The Mekong's average discharge is about 16,000 m³/s at its mouth, ranking 12th in the world by volume. Discharge is strongly monsoonal, with peak flows in August-September of around 50,000 m³/s and dry-season lows around 2,000 m³/s. The Tonlé Sap is fed by the Mekong's flood — when the Mekong rises, water flows up the Tonlé Sap River into the lake, which expands from about 2,500 km² in the dry season to 16,000 km² at peak flood. As the Mekong subsides, the lake drains back, releasing nutrient-rich water that sustains the basin's downstream productivity. Sediment delivery to the delta has historically been about 160 million tonnes per year, but recent measurements suggest a 70-80% reduction since 2000 due to upstream dam trapping, with major implications for delta subsidence and saline intrusion.
Geology & Formation
The Mekong follows a major continental fault zone — the Mae Ping fault and related structures — for much of its course, which is why the river is unusually narrow and gorge-bound rather than meandering. The upper Mekong runs through the Three Parallel Rivers UNESCO area in Yunnan, where it flows in a deep gorge alongside the upper Yangtze (Jinsha) and Salween, separated by sharp ridges only a few kilometres apart. The lower basin opens into the Cambodian floodplain — a Mesozoic-Cenozoic sedimentary basin — and finally the Holocene delta. The Tonlé Sap basin is a rapidly subsiding tectonic depression. The Mekong Delta has built outward over the past 6,000 years and is now actively prograding, although recent sediment starvation has reversed this trend.
Climate & Ecology
The basin is dominated by the Asian monsoon system, with about 80% of annual rainfall falling between May and October. Rainfall ranges from over 3,000 mm/year in the central Laotian-Vietnamese highlands to under 1,000 mm/year in the upper Tibetan headwaters and parts of the Khorat Plateau in northeastern Thailand. The basin contains exceptional aquatic biodiversity — over 1,200 documented fish species (and possibly several hundred more undescribed), including the Mekong giant catfish (the largest freshwater fish in the world, up to 3 m and 300 kg) and the Irrawaddy dolphin (a critically endangered freshwater population in Cambodia). The Tonlé Sap supports the world's largest inland fishery, with about 500,000 tonnes per year in normal years. Wetland ecosystems include the seasonally flooded forest of the Tonlé Sap, the Plain of Reeds in Cambodia and Vietnam, and the mangroves of the southern delta.
Cultural & Historical Significance
The Mekong has been the cultural heart of mainland Southeast Asia for at least 2,000 years. The Khmer Empire (9th-15th centuries CE), centred at Angkor on the Tonlé Sap, was the largest pre-industrial civilisation in Southeast Asia and built one of the most extensive hydraulic systems in the world. Lao, Thai, and Vietnamese civilisations all subsequently developed along the Mekong and its tributaries. French colonial occupation (1862-1954) mapped the basin and built extensive infrastructure; the term "Indochina" referred to the Mekong-centred French colonial possessions. The river was central to the wars of decolonisation and Cold War — the Vietnam War (1955-75) saw extensive combat in the Mekong Delta and along Laotian Mekong tributaries. Today the basin is split between communist Vietnam and Laos, semi-monarchical Cambodia, semi-democratic Thailand, military-ruled Myanmar, and authoritarian China — a complex political geography that shapes basin water management.
Modern Issues
The Mekong is the test case of 21st-century basin politics. China has built 11 mainstream dams on the upper Lancang, with combined storage exceeding 47 km³ — enough to substantially regulate downstream flow. Laos has built a series of mainstream dams (Xayaburi, Don Sahong, others under construction) and many tributary dams, positioning itself as the "battery of Southeast Asia." Cambodia paused mainstream dam construction in 2020 amid environmental concerns. The Mekong River Commission (MRC), founded 1995, includes only the four lower-basin states (Cambodia, Laos, Thailand, Vietnam) — China and Myanmar are not members — significantly limiting its effectiveness. The Mekong Delta in Vietnam is sinking due to groundwater extraction and sediment starvation, with sea-level rise threatening up to half the delta by century's end. Tonlé Sap fishery productivity has collapsed in recent dry years, with 2019-20 producing some of the lowest Tonlé Sap flood pulses on record.
Frequently Asked Questions
How long is the Mekong River?
About 4,350 km, making it the longest river in Southeast Asia and the 12th-longest in the world. It flows through six countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam.
What is the Tonlé Sap flow reversal?
The Tonlé Sap River connects Cambodia's Tonlé Sap lake to the Mekong at Phnom Penh. During the May-October monsoon flood, Mekong water flows up the Tonlé Sap into the lake, which expands sixfold; in the dry season, the flow reverses and water drains back to the Mekong. This flood pulse drives the most productive inland fishery in the world.
Why are dams on the Mekong controversial?
Mainstream and tributary dams trap sediment, alter flow timing, and block migratory fish. Up to 70% of the basin's sediment delivery to the delta has been lost since 2000, contributing to delta subsidence and saline intrusion in Vietnam. The basin's unique fish fauna depends on long-distance migrations that dams interrupt.
How many fish species does the Mekong support?
Over 1,200 documented species, with new species described every year. The Mekong is second only to the Amazon among the world's rivers in fish diversity, and supports the largest inland fishery on Earth (about 2.6 million tonnes per year basin-wide).
Final Thoughts
The Mekong is the most consequential river basin in Southeast Asia and one of the most rapidly changing in the world. As upstream dams reshape its sediment and flow regimes, as the Vietnamese delta sinks and salinises, and as the Tonlé Sap flood pulse weakens, the Mekong is becoming the test case of how heavily contested transboundary basins can — or cannot — be managed in the climate era.