Assessment of Environmental Flow Requirement and Morphological Changes of Mahananda River    

The Mahananda River is a significant transboundary tributary of the Ganges, supporting agriculture, fisheries, and local livelihoods in Bangladesh. The present study analyzes its environmental flow requirements (EFR) with three hydrological methods and assesses morphological change from 1994 to 2024. Flow data from Nawabganj Station (SW211.5) and eight cross-sections were analyzed, and seasonal variability was found to be considerable. The Tennant method recommends 72.18 m³/s (low flow) and 144.35 m³/s (high flow) for good habitat quality, and 36.09 m³/s (low flow) and 108.27 m³/s (high flow) for fair habitat quality. The flow duration curve (FDC) and constant yield methods also reported dry-season shortages, despite sufficient monsoon high-flow. Morphological surveys (2003-2023) identified major erosion and deposition: thalweg migration and channel deepening at river morphological monitoring areas (RMMAs). Overall, the river reach exhibits a dominant trend of channel deepening/erosion (RMMAs 5, 7, 9, 11, 12), while RMMAs 6, 8, and 10 display complex, mixed changes, characterized by both significant erosion in the channel and localized sedimentation on the banks or bars. These geomorphological responses show the river's response to altered flows and sediment dynamics. Declining water levels post-2007 further indicate vulnerability. To promote ecological sustainability, measures such as augmenting dry-season flow (e.g., rubber dams) and erosion control are unavoidable. The necessity of integrated river management for guaranteeing the ecological and socioeconomic functions of the Mahananda is emphasized in this research.