My dissertation centers on the story of the connections between the largest developmental megaproject of the western hemisphere – the building of the Itaipú Dam in the late twentieth century – and its unintended effects in the making of a new transnational region: the Upper Paraná in the borderlands of Brazil and Paraguay. As an interdisciplinary environmental historian I draw on classic social theory in conjunction with environmental and development studies to explain the impact of the Itaipú Dam. I address two major conceptual questions. First, I explain a crucial developmental experience in the Southern Cone through the lens of the theory of the unanticipated consequences of social action. Secondly, I seek to grasp the role of megaprojects in the making of transnational regions through its most sensitive political, social and environmental effects. The scholarship on the Itaipú Dam has focused on the intended effects of the dam: energy, development, modernization, state-building, and environmental costs. However, I argue that the most significant effects of the construction of the Itaipú Dam were unanticipated: informal modernization; unplanned urbanization; the unexpected effects of scientific management of the environment; and the rise of unruly borderlands. I pursue five lines of research: modernization; urbanization; environmental history; transnational studies; and democratization. My research method is multidisciplinary and combines archival work with interviews in United States, Brazil and Paraguay. The time period of this study spans three and a half decades from the foundation of the border town of Ciudad del Este in 1957 to the creation of MERCOSUR (the Southern Common Market) in 1992, a year after the completion of the Itaipú Dam. The region under study cuts approximately ninety-thousands square miles across the Brazilian border state of Parana, and the Paraguayan border states of Alto Paraná, and Canendiyú, all connected by the Paraná River waters.