South of Sambor village in Kratie province, the Mekong runs wide and unhurried, splitting around wooded islands and shelving over rock, doing nothing that looks like history. This is the site of the largest dam Cambodia has ever designed. Nothing stands here. Engineers have been drawing a barrage across this reach since the 1950s, and in the fullest version of the plan a wall eighteen kilometers long would have closed the entire river - the lowest of all the proposed Mekong mainstream dams, the final gate above the delta. In March 2020 the Cambodian government stopped it, and everything else proposed for the Mekong mainstream, for a decade.
The Sambor project has outlived several political systems. First floated in the 1950s, it reappeared in 1994 when the Mekong Secretariat proposed a 3,300 megawatt structure blocking the whole river. Chinese interest followed: the power company known as CSG carried out a geological study of the site before withdrawing in 2011, explaining the decision with the observation that it was "a responsible company." The idea persisted regardless. In October 2016 Cambodia's Council of Ministers authorized the Royal Group, the conglomerate that would later build the Lower Sesan 2, to sign a memorandum of understanding with the Ministry of Mines and Energy to "thoroughly conduct" pre-feasibility, feasibility, and social and environmental impact studies for three dams at once - Stung Treng, the Sekong, and Sambor. That is where the paperwork stood when the government changed its mind.
Two configurations were on the table, and the larger one was very large indeed. It combined a concrete gravity dam with an earth rockfill embankment, running 18,002 meters end to end at a height of 56 meters, and would have carried an installed capacity of 2,600 MW - Cambodia's biggest power plant by a wide margin. The build was costed at US$4.947 billion, with a further US$312.9 million for the transmission lines to move the electricity to where the buyers were. Very few of those buyers were Cambodian. The government expected to sell 70 percent of the output to Vietnam and 10 percent to Thailand, leaving the remainder for the domestic market. Later studies put the number of people who would have to be moved at 19,034.
What made Sambor different from an ordinary large dam is where it sat in the plumbing of a country. Every monsoon the Tonle Sap river reverses direction, driven backward by the swelling Mekong until it floods the Tonle Sap lake to several times its dry-season size, and the fish that follow that pulse account for roughly 60 percent of Cambodia's annual catch. A Mekong River Commission evaluation published in 1994 warned that Sambor would block fish migration in both directions. A later policy brief from the WorldFish Center and the Cambodian National Mekong Committee widened the concern to Laos, Thailand and Vietnam, describing what mainstream dams do to river flows and, in a phrase that stuck, to the 'flattening out' of the flood pulse itself. Flatten the pulse and the lake stops breathing.
The stretch of Mekong the dam would have occupied is also the last home of the river's Irrawaddy dolphins. Roughly 105 animals survive in a 190-kilometer reach of the Cambodian Mekong centered on Kratie - the entire remaining population, after the species disappeared from Laos in 2022. Assessments of the Sambor proposal listed the dolphins among the things it threatened, which is a modest way of saying that a wall across their river would have divided a population already too small to absorb being divided.
In March 2020, citing ecological concerns, Cambodia halted all hydropower development on the Mekong mainstream until 2030. Sambor was pushed back along with its upstream neighbor, the Stung Treng dam. It is worth being precise about what that means: a moratorium is a decision to wait, and it has an expiry date. The underlying interest has not evaporated - the Royal Group secured approval in 2022 to study three possible locations for the Stung Treng project - and Sambor's studies, drawings and cost estimates all still exist. For now the reach below Sambor village stays as it is: shallow, braided, full of migrating fish, and inhabited by the 19,034 people whom the later studies counted as standing in the way.
The Sambor Dam site lies at 12.7833N, 105.9500E on the Mekong mainstream in Kratie province, Cambodia, roughly 35 km north of Kratie town. There is no structure to spot - this is a place identified by its absence. From 8,000-15,000 ft the reach reads as an unusually wide, pale, island-braided section of the Mekong, several kilometers across in places, with sandbars and rock shelves exposed in the dry season and the deep pools that hold the river's last Irrawaddy dolphins lying just downstream toward Kratie. Prey Lang forest is the dark green mass to the west. Phnom Penh International (VDPP) is about 200 km southwest and is the practical base; Stung Treng lies upriver to the northeast. December through March gives the best light and the widest exposed sandbars; the June-to-October wet season raises the river several meters, hides the braid pattern, and brings heavy afternoon buildups over the Mekong plain.