The dam was designed as a run-of-the-river barrage: a low structure that would use the natural flow of the Mun without creating a massive impoundment. The Electricity Generating Authority of Thailand, EGAT, presented it as a development project that would bring power to Ubon Ratchathani Province and the surrounding Isaan region while displacing a manageable number of families. The original environmental assessment projected the displacement of 262 households. The actual figure, once construction was complete and the reservoir filled, was 912 families displaced, with 780 more households losing all or part of their land. Around 25,000 villagers ultimately claimed to have been affected. The gap between the projection and the reality was not a small discrepancy. It became the fault line around which years of protest would organize.

The Mun River had supported fishing communities for generations. The river was rich: before the dam, 265 fish species had been documented in its waters. A report commissioned by the World Commission on Dams found that at least 50 of those species had disappeared after the dam began operating. Fish catches across the region decreased by 60 to 80 percent. The fish ladder built into the dam structure — included precisely to address this concern — was later judged largely ineffective. Families who had fished the Mun for their livelihood found themselves with neither fish nor adequate compensation. EGAT paid out 377.7 million baht in relocation compensation and 356.9 million baht for fisheries losses, with an additional 200 million baht in claims still unsettled. For the communities displaced, the numbers told a story the official reports tended to understate.

The communities who lost their livelihoods did not accept the situation quietly. For years, villagers staged protests at the dam site itself and traveled to Bangkok to demonstrate outside Government House, calling for the dam to be decommissioned entirely. The pressure was significant enough that in June 2001, the government opened the dam gates temporarily to allow the river to flow more naturally. A study by Ubon Ratchathani University subsequently recommended keeping the gates open for a further five years. Living River Siam, an environmental advocacy group, called for permanent decommissioning. The Thai Cabinet's response, in October 2002, was a compromise: the gates would be closed for eight months each year, open for four. It satisfied neither side fully.

The Thailand Development Research Institute conducted a formal case study of the dam for the World Commission on Dams, published in 2000. Its conclusion was blunt: EGAT had overstated project benefits, misrepresented the value of power production gains, and claimed irrigation benefits on grounds the analysts deemed invalid. The dam's actual power output fell short of original projections. The costs — environmental, social, and financial — exceeded them. The Pak Mun Dam became a reference case in international debates about the true cost of large hydroelectric projects, cited alongside contemporaries from India, China, and South America as evidence that development benefits and displacement harms are rarely distributed to the same people. The dam continues to operate. The Mun River continues to reach the Mekong, five and a half kilometers downstream.