Environmental flow assessment (EFA) of a river denotes the estimation of minimum volume of discharge, which is necessary for the survival of the native species of the aquatic habitat. The present research deals with the assessment of reach-specific environmental flow of Mahananda river at Gulma and Siliguri station of the Sukna Geomorphic Region (SGR) under the Darjeeling district of West Bengal, India. Discharge-wetted perimeter curve method has been used by using the RL data extracted from field to measure the e-Flow at this ungauged site of Mahananda river, whereas the volume of environmental flow at Siliguri stretch has been estimated by using the daily discharge data extracted from the Geoglows-Hydroviewer data repository. Vertical mixing in flow column has been estimated through VMF model, which is found dominant in the monsoon months with temporal variation in last four decades in Mahananda at Siliguri stretch. A minimum stream discharge of 26 m3s−1 in Mahananda river at Gulma may necessary to meet the sustainable ecological demand of the native riverine ecosystem as per discharge-wetted perimeter method, whereas 40, 80, and 15 m3s−1 flow are required as environmental flow of Mahananda river at Siliguri reach as per Tennant, Tessman and Smakhtin model respectively in the span of 1979 to 2022. The outcome of the present research certainly provides insight to the local administration that 80 m3s−1 flow should be assured in the Siliguri stretch of Mahananda river despite of any human interference throughout the year for the sustainability of this river.