Shipping on the Mekong river between China and Thailand has stopped because of a sharp drop in water levels, Thai businessmen and officials say.
The Mekong is one of the great rivers of Asia
Hundreds of boats have been stranded in Chiang Saen port in northern Thailand, they add - and blame China for damming the river upstream.
The Mekong runs 4,500 kms (2,800 miles) though China, Burma, Thailand, Laos, Cambodia and Vietnam.
More than 60 million live on its banks - many are dependent on fishing.
"The water levels began to drop in late February and now the Mekong is only 90 cm to one metre deep in parts (three feet)," Sermchai Kittiratanapaiboon, chairman of the Chiang Rai chamber of commerce, told AFP news agency.
"We need at least 1.5 meters of water (five feet) for ships to be able to operate," he said.
Mr Sermchai said a severe drought has compounded the problem this year - but that the main factor was the damming of the river in its northern reaches in China.
In some sections, the once-mighty waterway has been reduced to a trickle, the agency says.
Television footage has shown several boats that had run aground on sandbanks.
Port official Krisada Worawasu says that although China is releasing water occasionally, allowing some small ships to operate, traffic on the river has largely ground to a halt.
The Mekong River Commission (MRC) - an intergovernmental body based in Cambodia - has said sections of the Mekong will reach their lowest level in a decade this year.
The MRC is not blaming China directly for the crisis - saying current drought conditions are responsible.
However the commission says China is contributing to the river's erratic fluctuations in depth.
Water levels have been fluctuating in the past decade
"After the completion of the Manwan dam in 1992, water levels downstream became more variable at Chiang Saen," MRC environment expert Ian Campbell said.
Surachai Sasisuwan, who heads Thailand's Water Resources Department, has said he would take Beijing to task on the issue.
"We would like China to tell us frankly about its dam project and let us know whether or not the dams are the cause of the disappearance of the water," he told the Bangkok Post.
However China has no intention of changing its policy. Two more dams on the Mekong are under construction.