In the Upper Mahanadi basin, base flow estimation is essential for understanding the hydrological cycle and eco-hydrology of that region. As the Mahanadi River is a perennial river, this calculation plays an important role in water resource management, especially in the Upper Mahanadi basin. This study was conducted with help of hydrograph separation methods to analyse the behaviours of the base flow and base flow index of the Upper Mahanadi basin. The observed stream flow discharges along the basin were used to evaluate the base flow trends during the period from 1998 to 2018. This result gives an idea about the average base flow and base flow index (BFI) in the Upper Mahanadi basin, which has drastically decreasing trends. The annual base flow ranged between 0 and 189.075 cumecs for the Upper Mahanadi basin. The BFI varied from 0 to 0.4808 with an average of 0.2404. This study represents that, on average, 24.04% of the long-term stream, flow is likely to depend on groundwater discharge and shallow subsurface flow. This study helps policymakers, engineers, and government officials to understand base flow behaviours and improves awareness among local people, which can help in future strategies to manage river patterns and water quality in this region.