This historical analysis examines the forces that shaped the collection and use of geographical data in Tanzania’s Rufiji Basin. Hoping to develop irrigated agriculture, colonial engineers surveyed the basin’s social and environmental landscapes and weighed the costs of damming. Following World War II, international experts interested in hydropower development conducted more geographical studies. While their studies offered limited information on stream flow, political and economic pressures led to their acceptance over those of colonial engineers. By illustrating how international institutions select what is accepted as knowledge and how such knowledge is used, the case highlights the politics of hydropower development in Africa. Based on archival work and fieldwork conducted in Tanzania and Sweden, this article argues that the shifting of the setting of knowledge construction from the basin to distant planning offices did not lead to projects based on better scientific knowledge, but set the stage for Tanzania’s current electricity problems.