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Observations from a granitic watershed within a Mediterranean climate reveal the hydrologic and critical zone functioning of a perennial stream headwater and its upslope contributing area within a meadow system in the Sierra Nevada, California. Chemical analysis (diagnostic tools of mixing models, end member mixing analysis, tritium, etc.) and physical data (stream stage, piezometers, soil water, snowpack, etc.) indicate there are two primary pathways of water input into a headwater stream sourced from a mountain meadow. One input is a shallower and younger subsurface pathway with water that resembles snowpack chemistry, and the other a deeper and older subsurface pathway with water that reflects the chemistry of the groundwater derived from the contributing hillslopes. Multi‐year observations reveal that regardless of snowpack amount, during the period of peak hillslope infiltration, shallow and deep pathways in the hillslope behave similarly to initiate headwater streams. However, during summer dry periods, similarities in active pathways within the meadow center are not maintained between high and low snowpack years. With less snow, perennial groundwater discharge within the meadow center is eliminated, becoming only a seasonal source at the meadow's outlet. At the meadow's edge, geophysically observed downslope thinning in saprolite thickness creates reduced lateral transmissivity and initiation points for headwater streams via enhanced groundwater discharge of upslope water. Combined, these findings suggest how loss of snowpack and critical zone structure can together mediate hydrologic function in a wet meadow system in a Mediterranean climate. Creating new understanding about the stability of hydraulic functioning in headwater wet‐meadow systems under a changing climate. Plain Language Summary: A new combination of data sets collected over several years in the Sierra Nevada Mountains of California reveal how smaller wet meadows sustain headwater streamflow, or not, during years of higher or lower snowpack or drought. The location of streamflow generation at these sites may be strongly tied to the shape and characteristics of the weathered bedrock in the shallow subsurface. Findings from this study show how underground flow pathways of water can function to promote year‐round headwater stream sources in mountain environments with wet‐winter, dry‐summer climates, when large snowpacks are present. However, long‐term the prevalence of seasonal surface flooding in these headwater wet meadows may decrease as climate warms and snowpacks continue to decline and become more variable from year to year. Key Points: During low snow years, perennial surface saturation is lost within the meadow, but streamflow is sustained year‐round at the meadow's baseExistence of these groundwater‐fed meadows is related to regular surface saturation caused by downslope thinning of saproliteTogether, this suggests that under continued snowpack decline, regular surface saturation may be lost in mountain meadow systems [ABSTRACT FROM AUTHOR]