Our ability to figure out how old stream water is relies almost entirely on environmental tracers. But stable water isotopes rarely tell us anything about water parcels older than 1–2 years, and even tritium often doesn’t push that limit much further. This is what Raphaël found in the first paper of his Ph.D: https://dx.doi.org/10.1029/2025WR040718. It implies that it is almost impossible to estimate reliably the mean streamwater age (which is the most popular water age statistic… ). Many scientists knew this before, but this paper provide a first quantitative framework to estimate the “limiting” age that can be estimated reliably with typical environmental tracers.