COSMOGENIC TRACERS REVEAL GROUNDWATER CONTRIBUTIONS TO MAJOR SIERRA NEVADA RIVERS

Mountain runoff provides a large portion of freshwater resources, not only to California, but also globally. Uncertainty in storage and timing of release from mountain headwaters to major rivers leads to challenges in water management. Complex topography and lack of instrumentation present research challenges for mass balance approaches to close this knowledge gap. We investigated the age of major river runoff in California. Firstly, we estimated the portion of runoff composed of recent water year’s snowpack and, secondly, we estimated the contribution of older sources stored in the subsurface. To do so, we measured sulfur-35 (half life = 87 days) and tritium (half life 12.32 years) in 17 major Sierra Nevada rivers. We sampled these two cosmogenic tracers during spring runoff in the wettest year on record, 2017, and again following the summer dry season, typical of this Mediterranean climate. We observed that all rivers had similar mean residence times between 8 and 14 years with little seasonal variation. Meanwhile, the portion of river runoff made up of the most recent winter’s snowpack varied considerably from river to river and season to season, with some catchments releasing large amounts of young water in spring and other river catchments showing a lag in snow melt contributions. The presented approach is applicable in other mountain ranges with complex topography and limited instrumentation. This is the first time sulfur-35 has been measured at this low level and the first time cosmogenic tracers have been measured in rivers across the entire Sierra Nevada. The amount of yearly snowpack has previously been the focus of determining river runoff, however, these results highlight the importance of subsurface storage in headwater catchments that provide water to major rivers years after snow has melted.