Hydropower dam projects in the Lower Mekong Basin are part of long-term and interactive land and water transformations, displacement, and violence. Within these ongoing processes, dams represent intense and adverse episodes of disruption that escalate nature-society transformations. Drawing on research at Cambodia's Lower Sesan 2 Hydropower Dam (LS2 Dam), we examine how such episodes of nature-society rupture catalyze new waves of frontier-making and mobility that further intensify land and resource struggles. 1 In this ethnically diverse landscape, the abrupt hydrological changes caused by the LS2 Dam have escalated land struggles among various ethnic groups, especially migrants intent on claiming land and water resources, and Indigenous/minority groups displaced by the dam. We show how historical relations with land and socio-political marginalization by the state have produced differentiated opportunities, risks, and frictions among the four main ethnic groups present in this landscape: Indigenous Bunong, Lao, Cham, and Khmer. The LS2 Dam case shows how nature-society rupture reifies frontier dynamics by disrupting existing land/water relations, which precipitates in-migration, new resource claims, and associated conflict along ethnic lines.