This study documents the age of water that contributes to rivers in upper catchments using the radioactive tracer tritium. River water in the upper Ovens Valley (Australia) is several years to decades old and water from different parts of the catchment (e.g., soil, regolith, and groundwater) is mobilised at different flow conditions. The results indicate that these rivers are buffered against short term climate variability but are susceptible to longer-term climate and land use changes