Hydrological modelling of sediment dynamics and sand mining impacts in a Himalayan foreland river basin

River sand mining has become a major anthropogenic disturbance in the Himalayan foreland, significantly affecting sediment dynamics, channel morphology, and geomorphic stability. However, quantitative assessments integrating hydrological modelling with geomorphic evidence remain limited in data-scarce basins. This study evaluates the spatio-temporal variability of hydrology, streamflow and sediment yield dynamics using the SWAT hydrological model, while soil loss, sediment delivery ratio (SDR), sediment hotspot distribution, and reach-scale cross-sectional analysis are used to characterize sediment connectivity and channel morphological conditions in the Raidak-II river basin. The ArcSWAT model was developed using SRTM DEM, ESRI Sentinel-2 land-use/land-cover data, FAO soil data, NASA POWER climatic data, and observed streamflow records for 2000-2025, with 2000-2002 used as the warm-up period and 2003-2025 used for model calibration. The watershed comprised 9 sub-basins and 57 hydrological response units, and model performance was evaluated. The calibrated model showed very good performance (R² = 0.905, NSE = 0.887, RSR = 0.22, PBIAS = 8.32%). Mean annual streamflow was 35.86 m³ s⁻¹ and sediment yield averaged 22.425 t ha⁻¹, with high variability (SD = 34.317 t ha⁻¹; CV = 153.02%), indicating substantial variability in sediment yield across the study basin. Upper mining-influenced sub-basins (SB4, SB6, and SB1), characterised by steep topography, intensive sediment extraction, and monsoonal runoff, recorded the highest sediment yield and soil loss. SDR analysis revealed spatial variations in sediment delivery across the sub-basins, with higher sediment delivery in upstream areas and lower delivery downstream, suggesting differences in sediment transport and storage processes. Sediment hotspot mapping indicated progressive downstream reduction of depositional bars, while cross-sectional analysis revealed channel incision, bed lowering, bar fragmentation, reduced depositional bars, and increased bank instability, demonstrating disrupted sediment connectivity, reduced channel resilience, and the need for sub-basin-scale sediment management and river restoration.