This paper examines the intertwined social and environmental histories of the Cahora Bassa dam constructed on the Lower Zambesi River in Mozambique. A basic premise of this historical analysis is that social and ecological sustainability are necessarily linked. We trace the transformations wrought by Cahora Bassa and the devastating effects on peasant communities inudated by the dam's reservoir, on down-river communities and on the ecosystems of the Lower Zambesi. The grandiose aims of the Portuguese colonial state to 'develop' the Lower Zambesi by regulating the river contrast sharply with the exploitation of African workers recruited to build the dam. The project was also steeped in the discourse of national security, which provided a convenient rationale for burying information regarding the dam's likely consequences. Oral testimonies from the labourers who constructed the dam and peasants whose livelihoods were disrupted reveal the harsh costs of Cahora Bassa. Similarly, the regulation of the Lower Zambesi irrevocably altered the region's ecosystems contributing to a general loss of ecological integrity. The Cahora Bassa project demonstrates that questions of sustainability are linked to relations of domination and struggles over meaning. The history of Cahora Bassa is about an authoritarian colonial state willing to achieve a set of economic and strategic objectives using all the coercive power at its disposal without regard for the social and ecological consequences.