The Mekong River and Its Delta

01 From Tibet to the South China Sea

The Mekong River begins on the Tibetan Plateau at an elevation of roughly 5,000 metres, gathering from snowmelt and monsoon rainfall in one of the planet's most remote watersheds. From there it runs 4,909 kilometres through or along the borders of six countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. It is the world's twelfth-longest river, the third-longest in Asia, and one of the most biodiverse freshwater systems on Earth.

The river's character changes dramatically along its course. In its upper reaches in Yunnan province, the Mekong — known as the Lancang in China — is a fast, narrow torrent cutting through deep gorges. As it enters the lowlands of Laos and Thailand, it widens and slows. In Cambodia, it expands into the Tonle Sap, Southeast Asia's largest freshwater lake, which swells to five times its dry-season size during monsoon floods. Finally, in Vietnam, the river fans into the Mekong Delta, a 55,000-square-kilometre alluvial plain where it deposits its sediment load before reaching the South China Sea.

02 The Delta: A Fertile Machine

The Mekong Delta is one of the most productive agricultural regions on the planet. Though it accounts for only 12 percent of Vietnam's land area, it produces more than half of the country's rice output and supplies roughly 90 percent of Vietnam's rice exports. The delta also produces over half of Vietnam's seafood and most of its fruit. An estimated 18 million people — roughly a fifth of Vietnam's population — live in the delta region, their livelihoods intertwined with the river's seasonal rhythms.

The delta's fertility depends on a simple mechanism: every monsoon season, the Mekong inundates its floodplain, depositing nutrient-rich sediment across the low-lying land. When the flood recedes, farmers plant their crops in the newly replenished soil. This cycle has sustained civilisation in the delta for at least two thousand years. But the mechanism is now breaking down. The sediment that makes the delta fertile is being trapped behind upstream dams, and the land itself is sinking and shrinking as the sea encroaches.

03 The Dam Cascade

China has built eleven major dams on the upper Mekong since the 1990s, with several more planned. These dams are designed to generate hydropower and store water for dry-season release, but they have fundamentally altered the river's hydrology. A 2020 study using satellite data and river gauges found that upstream dams held back so much water during the 2019 drought that they worsened the drought's impact on downstream nations by an estimated 70 percent.

Laos, Cambodia, and Vietnam have also built or planned dozens of dams on Mekong tributaries and the mainstream. The largest is the Xayaburi Dam in Laos, completed in 2019, the first mainstream dam below China. These dams trap sediment that would otherwise reach the delta. Estimates suggest that the sediment load reaching the delta has already declined by as much as 80 to 90 percent compared to pre-dam conditions. Without sediment, the delta cannot rebuild itself against the erosion of tides and waves — it is literally being starved to death.

04 Sand Mining and Land Subsidence

Dams are only part of the delta's problem. Sand mining — the extraction of river sand for construction aggregate — is rapidly hollowing out the riverbed. Vietnam, Cambodia, and other Southeast Asian nations have been extracting sand at rates far beyond what the river can replace. A single sand barge can remove thousands of tonnes in a day; across the delta, the extraction totals tens of millions of tonnes annually. The riverbed drops, banks collapse, and the tidal reach of the sea extends further inland each year.

Groundwater extraction compounds the problem. As delta farmers pump aquifer water for irrigation, the land subsides — in some areas at rates exceeding 2.5 centimetres per year. Combined with sea-level rise of roughly 3 to 5 millimetres per year from climate change, the effective rate of relative sea-level rise in parts of the delta may exceed 3 centimetres annually. At this rate, significant portions of the delta could be underwater by 2050, displacing millions of people and destroying some of Southeast Asia's most valuable farmland.

05 The Tonle Sap Reversal

One of the Mekong's most remarkable features is the Tonle Sap, a lake in central Cambodia whose flow reverses direction with the seasons. During the dry season, the Tonle Sap River drains the lake into the Mekong. But when the monsoon swells the Mekong in June, the increased flow reverses the Tonle Sap River's direction, pushing water and fish back into the lake and expanding it from about 2,500 square kilometres to more than 13,000 — five times its dry-season area.

This annual reversal is the biological engine of the lower Mekong. The flooded forest around the expanded lake becomes a vast fish nursery, producing an annual freshwater fish harvest of roughly 500,000 tonnes — one of the largest inland fisheries in the world. The Mekong as a whole supports an annual fish catch exceeding 2.6 million tonnes, worth an estimated $7 billion and providing the primary protein source for tens of millions of people. Dams that alter the flood pulse or block fish migration routes directly threaten this entire system.

06 A River at the Crossroads

The Mekong sits at the intersection of several colliding forces: energy demand in rapidly developing Asian economies, food security for 70 million people who depend on the river's protein, biodiversity in one of the world's richest freshwater ecosystems, and the existential threat of climate change to the delta. There is no simple resolution. Hydropower provides clean energy and economic growth, but at the cost of the sediment and seasonal flow patterns that sustain the entire downstream system.

The Mekong River Commission, an intergovernmental body of the four lower basin nations, has attempted to coordinate management and resolve disputes, but it has no binding enforcement power. China, which controls the upper basin and operates its dams without consultation with downstream nations, is a dialogue partner rather than a member. The fundamental tension — between upstream development and downstream survival — remains unresolved, and the delta continues to erode.