Some 60 million people along the Mekong's 4,800 kilometer path depend on the river for transportation and food. The Mekong originates on the high Tibetan plateau, winding through China, Burma, Laos, Thailand, Cambodia, and Vietnam before reaching the South China Sea.

Recently, water levels in several parts have dropped to record lows and the fish catch has fallen.

The findings are causing controversy about the causes, with many conservationists saying the damage is due to hydroelectric dams, especially China's two huge dams in the upper part of the Mekong.

Marc Goichot, coordinator of the Mekong river project of the conservation group World Wildlife Fund in Cambodia, explains that upstream dams can change the main qualities of the river, such as its temperature, rate of flow, and the amount of oxygen in the water. It also affects the migration of fish.

"The reservoir is trapping the sediment and the sediment is very important for the stability dynamics of the river downstream. If the water has less sediment, it is more erosive," says Mr. Goichot. "So the shape of the riverbed downstream will be affected."

China started building large dams on the Upper Mekong, in southwestern Yunnan province, near the border with Laos, in the mid-1980s. The government plans to build eight of them, and has already completed two dams, Manwan and Dachaosan. Work on a third one (Xiaowan) is in progress.

China needs the dams to create enough electricity to feed its voracious economic growth.

Experts say the deep gorges of the Upper Mekong, known in China as Lancang Jiang, are geographically well suited for hydroelectric power generation.

The Mekong River Commission or M.R.C., an organization backed by all Mekong countries except China and Burma, says the completion of the China's Manwan Dam in 1993 is linked to irregular water levels detected in Chiang Saen, Thailand, 200 kilometers from the Chinese border.

Environmental specialists from the M.R.C. say the variation in water levels has spoiled aquatic life and decreased sediment. In addition, the nutrient-rich sediments spread by annual floods fertilize farmland in the Lower Mekong delta. But the M.R.C. says the recent low water levels in the Mekong were the result of drought, not China's dams.

Chinese officials defend their dams, saying they help prevent flooding in the Mekong delta during the wet season.

Feng Yan, a professor at the Asian International Rivers Center of Yunnan University, says China's upstream dams trap water in reservoirs during the wet season, which can then be used during the dry season. Downstream countries then get water in the dry season, and China gets electricity when demand is highest. "Maybe in the future other dams will be built and there will be more water downstream in the Mekong River."

Conservationists say it is not only China's dams that are the problem; several large dams in Mekong tributaries are in the planning stage and could further harm the river system.

Laos is building a large hydropower dam, Nam Theun II. Much of the power generated from this project will be sold to Thailand. Environmental activists warn that Nam Theun Two will change the natural flow and ecology of the Xe Bang Fai River, which flows into the Mekong in Laos.

Earlier this month, the Vietnamese government approved the construction of the Ninh Binh Two hydroelectric plant project in northern Vietnam.

Witoon Permponsacharoen is director of the Thai environmental activist group, TERRA. He says that in the past, economists in the Mekong Delta promoted a model of regional development that relied on hydroelectric dams. He says that since that time, the damage to the Mekong has become apparent.

"The model that believed that economic growth will be a regional phenomenon," he says. "The dam industry is looking at the Mekong as a very attractive business area."

Conservationists say halting the building of dams will not solve the problem if countries replace hydroelectric power with something less environmentally friendly. They say countries should pursue alternative power sources if they want to preserve their river of life.