Some of the water that is stored in soils and bedrock is young, but other water is very old. Similarly, water flowing in streams has a range of ages. Visser et al.  used the radioactive isotopes of hydrogen (tritium), sodium-22, and sulfur-35 to determine water ages of streamflow for a catchment in the Sierra Nevada. The unique combination of age tracers allowed them to constrain the parameters of a hydrological model and determine the relationship between the ages of water that is stored in the catchment and water in the stream. The results show that during dry conditions streamflow consists mainly of old groundwater, but during wet conditions it includes younger water. The water that is used by plants is relatively young.
Citation: Visser, A., Thaw, M., Deinhart, A., Bibby, R., Safeeq, M., Conklin, M., Esser, B., & Van der Velde, Y. . Cosmogenic isotopes unravel the hydrochronology and water storage dynamics of the Southern Sierra Critical Zone. Water Resources Research, 55. https://doi.org/10.1029/2018WR023665
—Ilja van Meerveld, Editor, Water Resources Research