California's arid Central Valley relies on groundwater pumped from deep aquifers and surface water transported from the Sierra Nevada to produce a quarter of the United States' food demand. The natural recharge to deep aquifers is thought to be regulated by the adjacent high Sierra Nevada mountains, but the underlying mechanisms remain elusive. We investigate large sets of geodetic remote sensing, hydrologic, and climate data and employ process-based models at annual time scales to investigate possible recharge mechanism. Peak annual groundwater storage in the Central Valley lags several months behind that of groundwater levels, which suggests a longer transmission time for water flow than pressure propagation. We further find that peak groundwater levels lag the Sierra Nevada snowmelt by about one month, consistent with an ideal fluid pressure diffusion time in the Sierra's fractured crystalline body. This suggests that Sierra Nevada snowpack changes likely impact freshwater availability in the Central Valley aquifers. Our datasets, analysis and process-based models link the current precipitation and meltwater in the high mountain Sierra to deep Central Valley aquifers through the mountain block recharge process. We call for new hydroclimate models to account for the role of the Sierra in California's water cycle and for revision of the current management and drought resiliency plans.