Riverine systems in eastern India are increasingly threatened by anthropogenic activities, particularly unregulated sand mining and rapid urban expansion. This study examines the spatial dynamics of riverbank erosion and urban–peri-urban expansion in the Ajay–Mayurakshi basin of South-West Bengal using geospatial techniques. Multi-temporal satellite data (2000–2020), GIS-based analysis, and field observations were employed to assess changes in channel morphology, erosion rates, and land use patterns. The results indicate that intensified sand mining has significantly altered channel planform, reduced braiding, and increased erosion rates. Simultaneously, built-up areas have expanded rapidly, replacing agricultural and vegetative land. The study highlights the interconnected nature of sand mining, erosion, and spatial expansion, emphasizing the need for sustainable river basin management.