The Mekong runs about 4,350 km from the Tibetan Plateau through China, Myanmar, Laos, Thailand, Cambodia and Vietnam to the South China Sea. Every monsoon it pushes water so hard into Cambodia that a tributary reverses direction and the lake it drains swells to several times its dry-season size — a reversal that drives the largest inland fishery in the world.
One river, two halves, six countries
The Mekong rises on the Tibetan Plateau and runs south-east through Yunnan in a deep, steep gorge, where it is called the Lancang and runs for a time within a few dozen kilometres of both the Yangtze and the Salween — three great rivers in parallel trenches, a landscape inscribed by UNESCO as the Three Parallel Rivers.
Below China the character changes. It briefly divides Myanmar from Laos, then forms much of the Laos–Thailand border, crosses Laos and Cambodia, and fans into a delta in southern Vietnam. The upper half is a mountain river with a steep gradient and high hydropower potential; the lower half is a broad tropical river whose value is biological rather than gravitational. That division maps almost exactly onto the politics: China holds the fall, and everyone else holds the fish.
Names change along the way too — Lancang in China, Mae Nam Khong in Thai and Lao, Tonle Thom in Khmer, and Cửu Long, the nine dragons, in Vietnamese, after the delta's distributary mouths.
The reversal at Tonlé Sap
The most unusual hydrology on the river happens in Cambodia. The Tonlé Sap river runs from the lake of the same name south-east to join the Mekong at Phnom Penh. For most of the year it flows that way.
During the monsoon the Mekong rises so far and so fast that its level at the confluence exceeds the lake's, and the tributary reverses, pushing water back up into the lake. Tonlé Sap lake then expands from roughly 2,500 km² to something over 12,000 km² and deepens several times over, flooding the surrounding forest. When the Mekong falls again in the dry season, the flow reverses back and the lake drains.
The ecological consequence is enormous. The flooded forest becomes spawning and nursery habitat on a scale nothing else in the basin matches, and the fish produced there support a catch that makes the Mekong basin the most productive inland fishery on the planet — on the order of two million tonnes a year, and the main protein source for a very large number of people. Cambodia's dependence is particularly acute, with inland fish supplying a large majority of animal protein in the national diet.
Angkor sat where it did because of this system: the Khmer capital's water management worked with the lake's annual rise, and the fishery fed a population large enough to build it.
Rapids, falls and a river that resists boats
Despite its length, the Mekong has never been a great navigation corridor, and the reason is a single obstacle. The Khone Falls, on the Laos–Cambodia border, are a broad complex of cascades and rapids spanning nearly 10 km of river — the widest waterfall in the world by that measure — and they block through-passage completely.
French colonial administrators, who had annexed Indochina partly in the hope of finding a river route into southern China, discovered this on the Mekong Expedition of 1866–68 and were badly disappointed. Their response was to build a short portage railway on the Khone islands to lift boats past the falls; the rails are still there. The realisation that the Mekong was not navigable to Yunnan is a significant part of why French attention shifted to the Red River and Hanoi.
The falls are also the last refuge of the Irrawaddy dolphin in the river, a population reduced to a handful of animals in deep pools nearby.
The dams, and who bears the cost
China has built a cascade of large dams on the upper Lancang, and Laos — which has pursued a policy of becoming the region's electricity exporter — has built or planned many more on the mainstream and tributaries below, including Xayaburi and Don Sahong.
Three downstream effects concern the lower basin. First, timing: dams smooth the flow, raising dry-season levels and lowering wet-season peaks, which weakens the flood pulse that Tonlé Sap's reversal depends on. Second, sediment: reservoirs trap it, and the delta needs that sediment to keep building against subsidence and sea-level rise. Third, migration: many of the basin's commercially important fish species move long distances to spawn, and barriers fragment those runs regardless of how well fish passes are designed.
The Mekong River Commission, which includes Laos, Thailand, Cambodia and Vietnam, provides a forum but no veto — and China, holding the headwaters, is a dialogue partner rather than a member. It is the same structural weakness as on the Brahmaputra: the upstream state has the leverage and the least incentive to accept binding constraints.
The river of giants
The Mekong holds more very large freshwater fish species than any other river, and the roll-call is extraordinary: the Mekong giant catfish, which can exceed 250 kg and is among the largest purely freshwater fish known; the giant barb, Cambodia's national fish; the giant freshwater stingray, recorded at comparable weights; and the giant pangasius. In 2022 a stingray caught and released in Cambodia at around 300 kg was recorded as the largest freshwater fish ever documented.
Size is a liability under pressure. Large fish mature late, breed slowly, and are conspicuous to fishers, so they are the first to disappear when catch effort rises or migration routes are blocked. Most of these species are now critically endangered, and the giant catfish in particular has declined precipitously — its spawning migration ran far up the river past the very reaches now being dammed.
Overall the basin's biodiversity is exceptional for a temperate-to-tropical river: well over a thousand fish species are described and new ones are found regularly. The Mekong is second only to the Amazon in freshwater fish diversity, and unlike the Amazon a substantial part of that diversity is directly harvested as food by tens of millions of people, which makes its decline an immediate nutritional problem rather than only a conservation one.
The delta, sinking and salting
The Mekong Delta covers roughly 40,000 km² of southern Vietnam and produces a very large share of the country's rice, along with an enormous aquaculture industry. It is also barely above sea level — much of it under two metres — and it is in trouble from several directions at once.
Groundwater extraction for agriculture and drinking water is causing the land to subside, in places faster than the sea is rising. Sediment starvation from upstream dams and from in-channel sand mining means the delta is no longer accreting enough to compensate. And reduced dry-season flow lets saltwater push further inland each year, damaging rice paddies that cannot tolerate it. Severe intrusion events have reached tens of kilometres upstream of anything historically normal.
The Vietnamese response has shifted from resisting salinity to accommodating it — converting some rice land to brackish aquaculture rather than defending it with sluices. It is an early example of a major agricultural region planning around a change it cannot prevent, because the causes lie in five other countries.
Where the borders meet
At the point where Myanmar, Laos and Thailand converge on the Mekong sits the area long known as the Golden Triangle — for decades one of the world's principal opium-producing regions, and a place where the river's role as a border made enforcement close to impossible. Production has shifted substantially toward Afghanistan and toward synthetic drugs since, but the river remains a trafficking corridor and the borders remain porous.
The same stretch has become the focus of Chinese commercial navigation upriver, with rapids blasted in the 2000s to allow larger vessels down from Yunnan — a programme halted after objections from Thailand and Laos about the effects on fisheries and on the river's channel.
Against the Ganges: two Asian rivers, two kinds of pressure
Comparison with the Ganges is instructive because both are enormous, densely populated, deltaic Asian river systems under strain — but the strains differ:
- The Ganges' central problem is what goes in: untreated sewage, industrial effluent and the sheer weight of population along its banks.
- The Mekong's is what is taken out and held back: water stored behind dams, sediment trapped, sand mined, flood pulses flattened.
- Both deltas are subsiding and salting, and both have lost sediment supply to upstream infrastructure.
- The Ganges basin is dominated by one country; the Mekong's is genuinely shared, which makes coordination harder but also means no single government can fix it.
What the Mekong still has, and what is most at risk, is its productivity. A river that feeds tens of millions of people directly from wild fish is rare in the modern world, and that output rests on one seasonal reversal in one Cambodian lake.
