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14 lower Mekong countries (Myanmar, Laos, Cambodia, Thailand and Vietnam) produced more than 100m tonnes of rice, around 15% of the world’s total.

The region’s fertile soil depends on nutrient-rich sediment that the Mekong carries downriver, mainly during the rainy season from June to October;

The region boasts remarkable biodiversity; only the vast basins of the Congo and the Amazon compare to or surpass it

here are more than 20,000 types of plant and nearly 2,500 animal species; freshwater dolphins and giant catfish; spiders 30 centimetres across and, in a limestone cavern in Thailand, a day-glo pink, cyanide-secreting millipede

he human diversity is striking, too: Tibetan monks pray; Burmese traders buy and sell; Cambodian fishermen cast nets; Thai farmers reap; Vietnamese markets float. The history is as rich as the soil. The Bud

a smiled while resting at the northern Lao city of Luang Prabang. Angkor Wat on the Mekong-fed Tonle Sap lake was among the biggest cities of the preindustrial world. The Khmer empire that built it dominated South-East Asia for longer than there have been Europeans in the Americas.

s its currents are rechannelled down copper conduits to power far-off cities the river itself will be trapped behind a series of concrete walls. Its fisheries, agriculture and biodiversity will suffer; the lives lived on its banks will be reshaped with scant regard for the feelings of those who lead them

t least 86,000 have been built over the past six decades, providing 282 gigawatts (GW) of installed hydroelectric capacity by 2014

y 2020 it wants an astonishing 350GW of installed hydropower capacity; in the European Union that would be enough to meet about three-quarters of total electricity needs. The dam at Wunonglong, about 300 metres long and more than 100 metres high, will provide a smidgen less than one of those extra gigawatts. The other 13 are expected to add 15.1GW mor

Downriver countries intend to build another 11 large dams on the Mekong, with dozens more planned for its tributaries

he dams will change the quality of the water in the river and the rate at which it flows. Some of this change could be for the better. Dams can prevent flooding by regulating the flow of water downstream. But some Mekong riverbank agriculture would not welcome too steady a flow

Increasing water in the dry season would shrink riverbeds, leaving less space for crops—millions of Mekong-basin dwellers grow vegetables on riverbanks. Reducing water in the rainy season produces smaller floodplains with less sediment deposited in them, impoverishing the soil.

According to International Rivers, an environmental NGO, the full cascade of dams planned for the Lancang would trap nearly all of the sediment coming from China, cutting the water’s sediment load in half.

And the dams lower down could worsen the problem; the clear, “hungry” water that flows from them in spates will carry away existing sediment in riverbanks and riverbeds. Some of that will be deposited farther still downstream; some will wash uselessly out t