Figures from the Mekong River Commission (MRC), a joint Lao, Cambodian, Thai and Vietnamese body set up to oversee the health of the 3,000 mile waterway, confirm that in some places the river is flowing close to rock bottom.

Many of the MRC's monitoring stations on the Mekong's long journey from the glaciers of Tibet to the 'rice bowl' of southern Vietnam reveal it is well below levels recorded in 1993, itself the lowest year in living memory.

"We are very concerned," said Pech Sokhem, an MRC director in the Cambodian capital, Phnom Penh. "It may be good for flood control, but it is very bad for agriculture and fishing. If the water doesn't flow properly, the fish will not spawn or migrate."

A low 2003 wet season rainfall was partly to blame, he said, but China must also take some responsibility - it was no coincidence the 1993 'dry up' coincided with the completion and filling of its first major hydroelectric dam, at Manwan in the southwestern province of Yunnan, he said.

China has since completed another dam and, according to the MRC's 2003 report, has plans for six more, all of which are likely to have implications for the 60 million people living in the Mekong basin, an area the size of France and Germany.

The impact is likely to be highest in deeply impoverished Cambodia, where the river's annual floods create the world's fourth largest catch of freshwater fish and where nearly 1.5 million people are involved in fishing.

It could also sound the final death knell for animals such as the critically endangered giant Mekong catfish, the world's largest freshwater fish and one of the roughly 1,500 different species unique to the southeast Asian river system.

Besides affecting aquatic habitats, huge and previously unseen sand-banks are emerging from the murky waters along the river's lower stretches, making navigation increasingly hazardous for humans.

"The river has been getting shallower for many years now," said Yang Yara, a 44-year-old ferryman near Phnom Penh. "It makes my life hard because my boat is always getting stuck on islands and mud-banks."

(Additional reporting by Lach Chantha)