The damming of the Mekong: major blow to an epic river

The water has to go somewhere, and it backs up into the Tonle Sap. The tributary flows back upstream for some 201km into its lake, which expands hugely, flooding surrounding forests. At the height of the monsoon season, this reverse flow swallows a fifth of the Mekong’s raging waters, making the tiny Tonle Sap for a while one of the world’s biggest rivers, albeit flowing backwards.

During this flood, the submerged forest around the lake becomes the nursery for the Mekong fishery. In the silty water among the tree roots, billions of fish fry grow into fat adults. The flooded forest of the Tonle Sap is one of the most productive ecosystems on Earth. And each November, as the Mekong flood abates, the Tonle Sap turns, the lake empties, and the fish swim out. To mark this annual event, Cambodia has held a huge water festival in Phnom Penh since the 12th century. Over the ensuing months, the fish will migrate for hundreds of kilometres up and down the Mekong - filling nets that feed tens of millions of people. Two-thirds of the fish in the Mekong begin their life in the Tonle Sap.

The most extraordinary product of this fishery is the giant catfish. The protected species grows up to three meters long and can weigh a third of a ton. Its numbers are declining, but it still lurks in huge hollowed-out pools on the river bed, some of them 400 feet deep, and occasionally turns up in the nets that fishermen put across the Tonle Sap.

This riverine cornucopia happens under the eye of the great temples of Angkor Wat. These remains of an ancient civilisation that prospered here a thousand years ago sit on the lake’s north shore, on the fringe of the flooded forest. It was the Mekong fishery, centred on the lake, that sustained this empire.

The Mekong is a reminder of how the world’s rivers used to be before the dam-builders got to work. Two-thirds of our rivers today, including most of the largest, have dams holding back their natural flood pulses on their main channels. China has already dammed the other major Asian rivers that flow out of Tibet, including the Yellow River and the Yangtze, which is now stopped by the Three Gorges Dam.

They are tamed, but much less productive as a result. The Mekong is the exception. No river on Earth has such variation in flow. Only the Amazon has greater biodiversity. Only the Amazon produces more fish.

There is scarcely a mile of riverbank along the Mekong where nobody is taking fish. And yet the fishery keeps providing. Some 60 million people eat or draw their income directly from the river. They include three-quarters of the population of Cambodia where, according to Oxfam, “river fisheries make a bigger contribution to economic well-being and food security than in any other country”.

The precise effect of China’s dams will depend on how they are operated. But according to operating instructions seen by Western hydrologists, the dams are intended to cut flood-season flow on the lower Mekong by a quarter - enough to halve the flood pulse in Phnom Penh. Hydrologists are divided about whether this is enough to end the reverse flow of the Tonle Sap. What is certain is that it will drastically reduce the amount of water backing up into the flooded forest, which will dry out - with severe consequences for the Mekong fishery.

The construction of the dams is a political as well as an ecological travesty. China is blocking the river without any prior consultation with its neighbours. In 1995 Vietnam, Cambodia, Laos, and Thailand - the four downstream nations on the river - formed the Mekong River Commission as a forum to consult on the river’s future. China never joined. It still refuses to. And it has never even discussed its dam plans with the commission. It is a vivid example of China’s disregard for its neighbours, who are too scared of Big Brother’s clout to speak out.

China is not alone as an upstream bully on the world’s great international rivers. Ask the Iraqis about Turkey’s dam construction on the Tigris and Euphrates, or the Mexicans about the US on the Colorado. We badly need international law to protect downstream nations - something the UN agreed was necessary a decade ago but has never acted on. But even more, we need international initiatives to protect the ecological integrity of the world’s last great wild rivers. Starting with the Mekong.