Currently, there is minimal information relating to temporal variability of water source contributions in alpine glacierized basins or the influence of glacier meltwater in a basin-wide context. This study adopts an end-member mixing approach to understand basin-scale water source dynamics in a French Pyrenean, alpine glacierized river system (Taillon-Gabiétous). Major ion and Si data were collected for snow, groundwater tributaries, and four mainstream sites during the 2002/2003 melt seasons. Three conceptual water sources were identified: “quick flow” (dilute, rapidly routed meltwater), “distributed” (SO42− enriched, slow routed subglacial waters), and “groundwater” (Si-enriched groundwater). Water source contributions at nested spatial and temporal scales were determined using end-member mixing and uncertainty analysis. Changes in stream hydrochemistry indicated marked meltwater-groundwater mixing. Quick flow contributions typically decreased over the melt season; groundwater contributions were highest at the beginning of the melt seasons following recharge by snowmelt but also later in the 2002 melt season following prolonged precipitation. Overall, the results suggest an alternative alpine basin melt season hydrological progression compared with previous models (i.e., simple snowmelt to glacier melt to groundwater domination) and emphasize the need to understand water source dynamics to inform related water resource availability, water quality, and stream ecology studies within alpine basins.