Tracing the flow of estuarine water into the coastal ocean is complicated due to intense regional mixing and hydro-morphological restrictions. Here we apply high precision 87Sr/86Sr measurements (2σ = ¿3 ppm) in estuarine waters, together with dissolved Ba and Mn, to trace freshened plumes after Typhoon Toraji. Small, but distinguishable Sr isotopic variations are detected in the Kao-ping Estuary, where Δ87Sr varied up to 50 ppm. On the basis of these geochemical constraints, two distinct sources with high Δ87Sr are identified, which subsequently mixed with the South China Sea Surface Water. The most radiogenic Sr source can be attributed to enhanced water-sediment interactions generated by the typhoon. An additional source of high 87Sr/86Sr is attributable to redox-transformation of suspended particles due to variations in river discharges.