Where the Mekong breaks apart at Si Phan Don, the river stops being a river and becomes a puzzle. Channels split around sandbars, rejoin, split again, and at the southern edge of the maze the whole system drops over the Khone Falls in a barrier of white water that migrating fish cannot climb. One channel is the exception. The Hou Sahong runs between the islands of Don Sahong and Don Sadam on a gentler grade, and it has long been understood as the door - the only passage through the Khone complex that fish can use in the dry season, and the principal route they use in every other season too. In January 2020, a 22.5-meter dam began operating across the downstream end of it.
By the standards of Mekong hydropower, Don Sahong is small. It is a run-of-the-river plant rather than a reservoir project: 22.5 meters high, with four bulb turbines rated at 65 megawatts apiece for a total installed capacity of 260 MW. Mega First Corporation Berhad, a Malaysian engineering and construction firm, holds the majority stake through the Don Sahong Power Company, with Electricite du Laos taking the remainder. Sinohydro signed the engineering and construction contract on 15 October 2015, and the concession runs as a build-operate-transfer arrangement for 25 years from the start of commercial operation. Most of the electricity leaves the country. A dedicated 195 MW line to Cambodia was commissioned in early 2020, and Thailand takes much of the rest. The dam stands less than two kilometers upstream of the Cambodian border.
That last detail - less than two kilometers - is why the project consumed a decade of regional diplomacy. Laos signed a memorandum of understanding with Mega First in March 2006 and a project development agreement in February 2008. Cambodia, Thailand and Vietnam argued that a dam on the Mekong mainstream triggers Prior Consultation under the Mekong River Commission's notification procedures, which obliges the building nation not merely to inform its neighbors but to work toward consensus with them. Laos maintained that the Hou Sahong is a channel within the mainstream rather than the mainstream itself, so the requirement did not apply. In June 2014 it submitted to the consultation anyway. The process, established under the 1995 Mekong Agreement, is not binding - the Xayaburi Dam had already proceeded without consensus - and the four members could not agree. The question went up to governmental level, and construction went ahead.
The fisheries argument turned on whether fish could be persuaded to go somewhere else. The developers proposed that the smaller channels around the dam site could substitute for the Hou Sahong once fish traps were removed and the depth and bathymetry were modified, and they maintained the project would have no significant impact on the local environment. Independent fisheries scientists were unconvinced. Vietnamese representatives said flatly that it is not possible to replace the modified channels for upstream migration. Reviewers questioned the adequacy of the environmental impact assessment, described the mitigation approach as experimental and unproven, and characterized the confidence that it would work as faith based. In Cambodia, where fish supply the great majority of the protein people eat, civil society groups campaigned hard. One went so far as to call for a boycott of Angkor Beer over the brewer's financial ties to dam developers.
A short distance downstream, straddling the Lao-Cambodian border, lies the deep pool known as Veun Nyang, or Anlong Cheuteal. It held the only Irrawaddy dolphins left in Laos, a handful of animals belonging to a critically endangered Mekong population. Conservation scientists warned before construction that the project would pose a very high risk to this sub-population and predicted the dam would end the species' presence in Laos altogether. In early 2022, a 25-year-old male, 2.6 meters long and weighing 110 kilograms, was found dead on an island in Si Phan Don called Koh La Ngo. He was the last Irrawaddy dolphin in Laos. Downstream in Cambodia, roughly 105 animals remain in a 190-kilometer reach of the Mekong. That is the whole of what is left.
Khone Phapheng still thunders, but the project reduces flow into the falls and the surrounding channels, and Si Phan Don's candidacy as a Ramsar wetland of international importance was compromised by the construction. An alternative had been on the table. The Thako scheme would have diverted water without blocking a channel outright, and was promoted as the more sustainable design, though it would generate less power. Feasibility studies and impact assessments were completed for both proposals. The two are economically incompatible, since they compete for the same water, and Don Sahong is the one that got built. Fishers on both sides of the border now live inside the mitigation experiment that reviewing scientists called unproven, running at full scale on the one channel that reliably worked.
Don Sahong Dam sits at 13.9439N, 105.9563E in Champasak Province, Laos, less than 2 km north of the Cambodian border. From 6,000-10,000 ft the Si Phan Don braid is unmistakable: a 10-km-wide spread of channels and sandbar islands where the single-thread Mekong shatters, with the white line of Khone Phapheng Falls on the eastern side of the complex. The dam is the small structure at the downstream end of the narrow Hou Sahong channel, with the switchyard and the transmission corridor running south toward Cambodia. Nearest major field is Pakse International (VLPS), roughly 130 km north; Ubon Ratchathani (VTUU) in Thailand and Phnom Penh International (VDPP) serve the wider region. Dry-season months from December to April give the clearest air and the sharpest channel definition; the southwest monsoon from June to October brings haze, convective buildups and a river so swollen the island pattern partly disappears.