In the sleepy northern Thai border town of Huay Luk, a community leader, Pornsawan Boontun, still remembers the day when villagers netted a Mekong giant catfish more than a decade ago. The fish weighed 615 pounds, and it surprised everyone since the elusive species has never been common in this stretch of river.

The giant catfish, among the planet’s largest freshwater fish species, is now rare throughout the entire Mekong River Basin. While overfishing and past practices, such as dynamiting or electroshocking pools in the river, contributed to the species’ decline, a much larger threat now looms for aquatic life and human populations along the Lower Mekong River: dams.

Seven dams built upstream in China and the blasting of rapids to improve navigation have already altered flows, reduced fish populations, and affected communities along portions of the Lower Mekong, which flows through Thailand, Laos, Cambodia, and Vietnam. But the impacts may soon get much worse as a new era of hydroelectric dam-building begins in the Lower Mekong Basin. Eleven major hydroelectric dams – mostly within Laos – and dozens of dams on tributary streams that feed into the Mekong have been proposed or are under construction.

Xayaburi Dam, a few hundred miles downstream from Huay Luk in Laos, has been under construction for more than a year. In Cambodia, construction on the $781 million Lower Sesan 2 Dam, financed by a Chinese utility, is expected to start in the coming weeks. Laos has vowed to begin work on the Don Sahong Dam, downstream of the Xayaburi, which scientists say would cause significant environmental damage due to the dam’s proximity to the Mekong Delta and Cambodia’s Tonle Sap Lake. Pak Beng Dam, another proposed project, would create a stagnant reservoir, raise water levels behind the dam 40 feet, and back up the river near Huay Luk, forcing the resettlement of communities.

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