Radiological contamination of river systems threatens public health and ecological integrity, particularly in South Africa, where freshwater scarcity is exacerbated by intensive anthropogenic activities such as mining. Naturally occurring radionuclides, radium-226 (226Ra), thorium-232 (232Th), and potassium-40 (40K), can accumulate in surface waters, potentially exceeding safety thresholds. This study evaluated the radiological quality of four South African rivers (Breë, Klein-Brak, Bakens, and uMngeni) by collecting twenty (20) surface water samples using stratified grab sampling at hydrologically significant points. Samples were acidified and analyzed using high-resolution gamma spectrometry with a High-Purity Germanium (HPGe) detector, and radiological health indices were calculated following international standards. Radionuclide concentrations varied spatially, with Klein-Brak River recording the highest mean values (226Ra: 4.80 Bq L−1; 232Th: 7.79 Bq L–1; 40K: 205.12 Bq L−1), exceeding WHO guidelines. The uMngeni River had the lowest concentrations. While external exposure indicators such as radium equivalent and absorbed dose rates were within safe limits, internal exposure indices, including annual effective ingestion dose and lifetime fatal cancer risk, exceeded global reference levels in the Klein-Brak and Bakens rivers, indicating potential health risks. The findings reveal that geological and anthropogenic factors significantly influence radionuclide distribution in South African river systems. The elevated ingestion-related risks call for integrating radiological assessments into national water quality monitoring frameworks, improved regulatory oversight, and increased public awareness. Aligning these efforts with sustainable development goals (SDGs 3, 6, 12, 13, and 14) will support the development of safe, equitable, and environmentally resilient water management policies.