Gaumukh is the snout of the Gangotri glacier located at an altitude of 4000 m in the Himalayas from where the snow and glacier-fed Bhagirathi River emerges. Snow, ice, river discharge and rainfall samples were collected for stable isotope (? 18O) analysis along with other hydrometeorological data during the ablation period (May to October) in 2004 and 2005 at Bhojwasa, 3 km downstream of Gaumukh. The variation in river isotopic composition (? 18O) with time shows the varied percentage of snow, glacier and rain contribution in the flow of the Bhagirathi River during the ablation period. The discharge of the Bhagirathi River shows positive correlation with temperature and negative correlation with the rain event.