Hydropower has become a central pillar of energy security and low-carbon development across the global South, yet its expansion is increasingly shaped by colonial legacies, geopolitical tensions, and financing constraints. This paper examines these dynamics through a qualitative case study of the 5150-megawatt Grand Ethiopian Renaissance Dam (GERD) employing a multi-scalar analytical framework and drawing on colonial-era treaties, policy documents, secondary literature, and negotiation records. It traces the evolution of Africa's largest hydropower across interconnected local, regional, and financial domains. At the local scale, the paper documents the socioeconomic impacts of reservoir-related displacement on Gumuz communities, including livelihood disruptions, uneven compensation, and deficiencies in essential services. These outcomes illustrate how large-scale dam projects can reproduce existing patterns of marginalization when social safeguards are weak. At the regional scale, the analysis shows how the GERD challenges Egypt's longstanding downstream hydro-hegemony and unsettles the colonial-era legal framework governing the Nile Basin water relations. While this has strengthened Ethiopia's assertions of water sovereignty, it has also intensified diplomatic tensions involving Egypt and Sudan, prompting mediation efforts by the African Union and deliberations before the United Nations Security Council. At the financial level, the analysis shows how constraints in international development finance contributed to Ethiopia's reliance on domestically mobilized and diaspora-supported financing mechanisms. While focused on Ethiopia, the GERD offers insights into how large hydropower projects in Africa can reshape development trajectories, regional power relations, and infrastructure financing. The paper concludes with recommendations for future research on hydropower, financing, and transboundary water governance.