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<p>Mountain snowpack has been declining and more precipitation has fallen as rainfall than snowfall, particularly in the western US. Isotopic composition in stream water, springs, groundwater, and precipitation was examined to understand the impact of declining snowpack on hydrologic processes in the mid Merced River catchment (1873 km<span class="inline-formula">²</span>), Sierra Nevada, California. Mean isotopic values in small tributaries (catchment area <span class="inline-formula"><i>&lt;</i></span> 122 km<span class="inline-formula">²</span>), rock glacier outflows, and groundwater from 2005 to 2008 were strongly correlated with mean catchment elevation (<span class="inline-formula"><i>R</i>²=</span> 0.96 for <span class="inline-formula"><i>δ</i>²</span>H, <span class="inline-formula"><i>n</i>=</span> 16, <span class="inline-formula"><i>p</i></span> <span class="inline-formula"><i>&lt;</i></span> 0.001), with an average isotopic lapse rate of <span class="inline-formula">−</span>1.9 ‰ per 100 m for <span class="inline-formula"><i>δ</i>²</span>H and <span class="inline-formula">−</span>0.22 ‰ per 100 m for <span class="inline-formula"><i>δ</i>¹⁸</span>O in meteoric water. The lapse rate did not change much over the seasons and was not strongly affected by isotopic fractionation. A catchment-characteristic isotopic value, representing the catchment arithmetic mean isotopic signature in meteoric water, was thus established for each sub-catchment based on the lapse rate to elucidate hydrometeorologic and hydrologic processes such as the duration and the magnitude of snowmelt events and elevational water sources of streamflow and groundwater for ungauged catchments. Compared to Tenaya Creek without water falls, the flow and flow duration of Yosemite Creek appear to be much more sensitive to seasonal temperature increases during the baseflow period due to a strong evaporation effect caused by waterfalls, suggesting a possible prolonged dry-up period of Yosemite Falls in the future. Groundwater in Yosemite Valley (<span class="inline-formula">∼</span> 900–1200 m) was recharged primarily from the upper snow–rain transition zone (2000–2500 m), suggesting its strong vulnerability to shifts in the snow–rain ratio. The information gained from this study helps advance our understanding of hydrologic responses to climate change in snowmelt-fed river systems.</p>