In the latter half of the twentieth century thousands of Khmer people were displaced from their homes along the freshwater rivers of Vietnam's Mekong delta. Their pattern of settlement along freshwater tidal rivers was an ecological adaptation unique in the Khmer-speaking world, of which only vestiges remain. Drawing upon oral histories and ethnographic observations of O Mon, a district in the central Mekong delta, this paper reconstructs a picture of the traditional river-based livelihoods, social structure and religious life of Khmers in this region in the 1940s. It describes how these Khmers were driven from their villages early in the First Indochina War. Experiencing ongoing dislocations in subsequent periods of war and peace, most have been prevented from returning to their former homes or reclaiming their land. Relying on testimony by elderly Khmers, who witnessed the disintegration of their riverside communities, the account challenges existing depictions of the ecology and history of the Mekong delta, offering new insights into the complexity of the Indochina wars and the severity of their consequences.