The Himalayan glaciers are one of the significant sources of Indian river systems. In recent times, a major question is being raised about the melting of glaciers and how it will impact river flows in the downstream regions. The Gangotri glacier has also attracted global attention due to the rapid retreat rate. Because of this, the present study has been carried out on the Gangotri glacier, which is the longest glacier located in the Bhagirathi river basin in north India, to understand the dynamics of the glacier and its melting contribution to the river water. In the present study, we collected water samples from the snow/ice, river, and rainfall for isotopic analysis from 2016 to 2018. We observed seasonality in δ¹⁸O and δ²H values of rainfall and river water in the Bhagirathi river basin. The isotopic values of rainfall range from -11.6 ‰ to 2.7 ‰ for δ¹⁸O, and from -94.8 ‰ to 38.5 ‰ for δ²H during pre-monsoon (April to June), and from -20.6‰ to -4.2 ‰ for δ¹⁸O, and from -155.6‰ to -21.9 ‰ for δ²H during monsoon (July to September) season. The river water ranges from -12.8 ‰ to -10.8 ‰ for δ¹⁸O, and from -86.3 ‰ to -72.9 ‰ for δ²H during the pre-monsoon, and from -19.0 ‰ to -12.4 ‰ for δ¹⁸O, and from -141.9 ‰ to -86.9 ‰ for δ²H during monsoon season. We used a tracer-based hydrograph separation approach to estimate the relative contribution of various river discharge sources. Our results suggest seasonality in relative contribution. The estimated snowmelt contribution is about ~58%, ice melt is ~36%, and surface runoff is ~6%, due to the variability in local climate and temperature in the Himalayan region.