In the postwar era, the preferred approach to harnessing rivers was through integrated river basin planning (IRBP), which required a holistic focus on the river basin for multipurpose development. While the river basin is taken for granted as the natural unit of development in the definitions of the IRBP concept, this article problematizes the river basin idea and reveals the politics behind what has been deemed natural (scientific), with a specific focus on Turkey’s experience with IRBP. It explores geopolitical and national motivations and challenges in the context of the scaling of the Euphrates-Tigris basin. By approaching IRBP as a process of scale-making, it draws from discussions of the politics of scale in the literature on political ecology, but also incorporates a historical dimension to these discussions with attention to the political and environmental histories of Southeastern Turkey, which became home to Turkey’s first and most extensive IRBP project, the Southeast Anatolia Project (GAP).The article stretches the chronological boundaries of GAP to the decades prior to the 1970s, when the project was initiated, by analyzing archival materials, including the proceedings of the Grand National Assembly of Turkey, the archives of a daily newspaper, and the expert reports on GAP. The analysis highlights the politics of scale as a powerful constituent of the politics of technological development, and shows the significance of historical analysis to delineate the politics of river basin planning into different layers, including the level of geopolitics, territorial disputes, and international conflicts.