Ministers are expected to pass judgment on the most controversial dam ever proposed for the vast waterway
Laos hopes to built a 1,285 MW hydroelectric plant at Xayaburi that would supply Thailand with electricity and open the door to a host of other proposed dams on the Mekong. But until now its plans have been fiercely opposed by Cambodia and Vietnam, which fear the blockage would sharply reduce the water needed for downstream fisheries and irrigation.
Ministers from the four nations meet in Siem Reap, Cambodia, tomorrow to rule on the Xayaburi plan amid demonstrations and petitions by conservationists who warn that the dam could kill off some of the world's most remarkable freshwater species. Arguments between the four member states of the Mekong River Commision have held up the plan, on which a decision had been expected in April.
Laos - one of Asia's poorest nations - is desperate to start the $3.5bn project to generate foreign exchange and accelerate economic development. To meet the concerns of its neighbours, it promised a new study of the environmental impacts. But the study - produced by the Swiss firm, Poyry Energy - has been criticised by conservationists for approving construction even though it acknowledges key information is missing.
"It is astounding that Poyry affirms there are serious data gaps and weaknesses with the project and still gives it the all clear," said Dr Jian-hua Meng, WWF's Sustainable Hydropower Specialist. "Poyry recommends dealing with the critical knowledge gaps during the construction phase. Playing roulette with the livelihoods of over 60 million people would not be acceptable in Europe so why is it different in Asia?"
A coalition of 39 environmental groups have organised protests and run full-page advertisements in Thailand to oppose the Xayaburi, which - they say - threatens the livelihoods of 60m downstream dwellers in addition to some of the world's most remarkable creatures, including catfish the length of cars and stingrays that weigh more than tigers are threatened by the proposed 800m barrier.
It is the second big dam controversy of the year in southeast Asia after Burma halted construction of a hydroelectric plant on the Irrawaddy at Myitsone, which would have supplied power to China.
China also wants to build four dams on the Mekong - which would have a greater chance of approval if the Xayaburi goes ahead. It only has observer status at the commission meeting this week. The outcome remains far from clear. Vietnam has called for a 10-year moratorium on all dams on the Mekong until further detailed assessments of the entire basin. A further postponment of Xayaburi is likely.
Ame Trandem, of the US-based NGO, International Rivers, said another delay of a year or two would not send a strong enough signal. "All the evidence collected so far suggests this dam would have a devastating environmental impact and people's don't want it," she said. "They should be cancelling this project."
• 8 December 2011 update: Ministers from the four countries voted on Thursday to delay a decision on the Xayaburi dam – no timescale was given.