The unprecedented rise in Lake Victoria’s water levels during 2019–2020 caused significant regional disruptions, yet its impacts on interconnected surface and groundwater systems remain unquantified due to: (1) sparse in situ monitoring, (2) inconsistent hydrometeorological records, and (3) restricted governmental data access. This chapter pioneers a multi-satellite assimilation approach, integrating GRACE-FO terrestrial water storage anomalies, ERA5 reanalysis, and radar altimetry data within a land surface modeling framework. We simultaneously address two critical knowledge gaps: first, diagnosing the climatic and anthropogenic drivers of the lake level surge, and second, quantifying its cascading effects on all water storage compartments (surface, soil, and groundwater). Our methodology demonstrates how next-generation remote sensing can overcome data scarcity challenges in large transboundary basins, providing actionable insights for water resource management during extreme hydrological events.