A 30-month study by Vietnamese researchers on the impact of 11 proposed hydroelectric dams in the Lower Mekong Basin (LMB) has found that the economic damage to Cambodia alone will be worth some $450 million per year.

While experts have warned about the potential dangers of dams for years, the 800-page study, parts of which were released by the Mekong River Commission (MRC) yesterday, quantifies the potential damage to for the first time.

“Hydropower development in the LMB would cause long-lasting damage to the floodplains and aquatic environment, resulting in significant reduction in the socioeconomic status of millions of residents,” the researchers say in the study’s executive summary.

According to the researchers, the Mekong River at Kratie could, at times, lose 60 per cent of its water flow while sediment and nutrient deposits there could decline by 65 per cent.

The travel routes of migratory fish would be blocked completely, leading to an extinction of 10 per cent of fish species in southern Cambodia and Vietnam, and fisheries in both countries would produce 50 per cent less yields – a problem even the best fish ladder technology available would not be able to mitigate.

“In Cambodia . . . biodiversity would be adversely impacted and fisheries, which have great national significance, would suffer very high declines in yields,” the summary said.

Salt intrusion, primarily a Vietnamese problem, would accelerate, ruining swathes of farmland in the country, and most of these changes would be “permanent”.

Vietnam’s Ministry of Natural Resources and Environment conducted the study with the help of Denmark’s DHI Group and specialists from Laos and Cambodia.

The MRC intends to add the research to its bigger flagship study on sustainable development in the region due to be published in 2017.

While the study found the dams could cost Cambodia $450 million per year, the impact on Vietnam would be even greater, at some $760 million per year.

The executive summary says that these losses could be reduced “primarily through avoidance” – not building some or all of the dams or relocating them to tributaries instead of the main river. “Fewer numbers of dams would decrease the projected impacts to varying degrees,” it says.

MRC spokesman Sopheak Meas said that most of the document was still in the review stage and could not reveal which dams would have the greatest impact. The study’s authors were not available for comment yesterday.

Marc Goichot, a water and security team leader with the World Wildlife Fund in Vietnam said yesterday that the “development space” along the Mekong is shrinking – that is, the river can support fewer and fewer development projects.

“Sediment impacts are not speculative, they are happening right now and climate change will only make things worse,” he said.