A woman walks with her child at the fishermen's village of Kbal Chroy on the Mekong River bank near Phnom Penh, Cambodia, Thursday, May 17, 2018. A Chinese-backed plan for Cambodia to build the Mekong River's biggest dam would destroy fisheries that feed millions and worsen tensions with Vietnam, the downstream country with most to lose from dams on the waterway, according to a three-year study commissioned by the Cambodian government. (AP Photo/Heng Sinith)
A woman walks with her child at the fishermen's village of Kbal Chroy on the Mekong River bank near Phnom Penh, Cambodia, Thursday, May 17, 2018. A Chinese-backed plan for Cambodia to build the Mekong River's biggest dam would destroy fisheries that feed millions and worsen tensions with Vietnam, the downstream country with most to lose from dams on the waterway, according to a three-year study commissioned by the Cambodian government. (AP Photo/Heng Sinith) Credit: Heng Sinith

Bangkok — A Chinese-backed plan for Cambodia to build the Mekong River’s biggest dam would destroy fisheries that feed millions and worsen tensions with Vietnam, the downstream country with most to lose from dams on the waterway, according to a three-year study commissioned by the Cambodian government. The report, posted this month on the website of the U.S.-based organization that conducted the study, said the Sambor dam would “generate large power benefits to Cambodia, but at the probable cost of the destruction of the Mekong fishery, and the certain enmity of Vietnam.”

It said the dam designed by China Southern Power Grid Co. would have a 239-square-mile reservoir and dwarf the biggest dam currently being built on the Mekong, the Xayaburi dam in Laos, which was bitterly opposed by environmentalists for years. The experts at the Natural Heritage Institute who authored the report, submitted to the Cambodian government late last year, recommended it defer the project while studying “better” alternatives such as using solar power to augment existing hydroelectric dams.

Alternative sites upstream where the Mekong separates into several channels are possible but either financially unfeasible or only marginally less destructive than the site currently envisaged for the 2,600 megawatt dam, the study said. Possible mitigation measures are either unproven or have a poor track record, it said.

The dam would block fish migration from the giant Tonle Sap Lake in Cambodia, which is crucial for reproduction and replenishing what scientists say is the world’s most productive fresh-water fishery. It would also prevent riverbed sediment that fertilizes the Mekong Delta rice bowl from moving down river, a particular problem for Vietnam where delta farmlands are being destroyed by saltwater incursion from the sea.