Plans for a series of Mekong mainstream dams have been made and scrapped several times since the 1960s, but now, with oil above 100 dollars a barrel, the projects look more appealing than ever to their proponents.

The river's future will be a key issue when prime ministers of the Mekong countries meet Sunday and Monday in the Lao capital Vientiane for a summit of the Greater Mekong Subregion (GMS), with the Asian Development Bank.

The 4,800-kilometre (3,000-mile) river originates in the Tibetan plateau of China, where it is called the Lancang, before running through Yunnan province, Myanmar, Laos, Thailand, Cambodia and Vietnam to the South China Sea.

To the pro-development lobby, the Mekong is a dream of hydropower potential for an energy-hungry region. To environmentalists, it's a nightmare.

Laos, Cambodia and Thailand have all allowed Chinese, Malaysian, Thai and Vietnamese companies to study at least seven mainstream hydropower projects.

The new projects on the drawing board are "a serious threat to the river's ecology" and the millions who depend on it for water, food, income and transport, said Carl Middleton of environmental watchdog International Rivers.

"By changing the river's hydrology, blocking fish migration and affecting the river's ecology, the construction of dams on the Lower Mekong mainstream will have repercussions throughout the entire basin."

Many of its tributaries have already been dammed, including several in Laos, Southeast Asia's poorest country, which plans to ramp up hydropower exports to its more industrialised neighbours Thailand and Vietnam.

But so far most of the Mekong itself remains relatively untouched and clean, in part due to its isolation during decades of revolution and war.

Only China has so far dammed the mainstream, at Manwan and Dachaoshan in Yunnan, and is building three more dams while planning another three.

The largest Chinese Mekong dam under construction is the Xiaowan, which will be second in size only to the Yangtze's Three Gorges Dam, with a reservoir over 160 kilometres long that will displace about 35,000 people.

China's existing dams, along with the blasting of rapids to allow all-year navigation, have angered Thai and Lao villagers who claim they have suffered declining fisheries and unnatural fluctuation in water levels.

Downstream, the seven main new projects under consideration are in Laos at Don Sahong, Pak Beng, Xayabouri, Pak Lay and Luang Prabang, at Ban Koum on the Thai-Lao border, and at Sambor in Cambodia.

The Don Sahong dam site, at the most advanced stage of planning, is at the Khone Falls of Laos, a scenic area home to endangered freshwater dolphins and just upstream from Cambodia.

Mekong expert Milton Osborne says the new dams would be "almost certain to have serious effects on fish catches taken out of the river."

"These catches are vital for the populations of Laos and Cambodia but also for Vietnam," said Osborne, the Australian author of the book "The Mekong -- Turbulent Past, Uncertain Future."

In the Mekong delta, Vietnam's main rice basket, officials say upstream water extraction for farm irrigation has already caused oceanic salt water intrusion that has destroyed fields.

Osborne said there is evidence Chinese dams have already reduced fish stocks in Yunnan and warned that the Xiaowan's blocking of nutrient-rich sediments will likely reduce silt deposits over a large section of the river.

China's Assistant Foreign Minister He Yafei said Wednesday that, on Southeast Asia's largest waterway, "China, as an upstream country, will never do anything that will harm the interests of downstream countries."

The Mekong system boasts over 1,000 fish species, a biodiversity second only to the Amazon, and it feeds Cambodia's giant Tonle Sap lake, whose fish stocks are the nation's main protein source.

Some 40 million people are active in Lower Mekong fisheries, says the UN.

The economic worth of Lower Mekong fisheries has been estimated at over two billion dollars per year by the Mekong River Commission, a four-nation forum that China and Myanmar have refused to join.

"This natural resource, which is threatened by dam development, is renewable and comes for free," said Middleton, "yet its value is largely unrecognized in regional infrastructure development plans."

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