The Mendoza River streamflow, South America (∼32 °S), derives almost exclusively from winter snow precipitation falling in the Andes. Almost 70% of the water feeding the river originates in the Cordillera Principal geological province. In addition to the snow that precipitates in this area, there are 951 cryoforms providing meltwater to the upper catchment. Given the high inter-annual variability of snowfall and the megadrought affecting the region since 2010, it is crucial to quantify the contribution from different water sources buffering the Mendoza River runoff. Combining instrumental records of streamflow from glaciers and rivers, meteorological data, remote sensing of snow-covered areas and ionic and stable isotope analysis of different water sources, this study attempts to understand the hydrological contribution of different water sources to the basin. We demonstrated for the first time the relevance of different water sources in addition to snow in a dry period. During the melting season, 65% of the streamwaters originated from the glaciers (i.e. 50 and 15% from glaciers and rock glaciers, respectively), representing a higher proportion compared to snowmelt (17%). Groundwater input showed relatively large contributions, averaging 18%. This work offers information to develop adaptation strategies for future climate change scenarios in the region.