<p>Understanding the interactions among terrain characteristics, land-use dynamics, and hydrological processes is essential for sustainable watershed management, particularly in mountainous river basins experiencing strong hydro-climatic variability and increasing anthropogenic pressure. The Upper Jhelum River Basin (UJRB), a major sub-basin of the Upper Indus Basin (UIB), is highly susceptible to soil erosion, seasonal runoff variability, and land-use changes, necessitating an integrated modeling approach for effective assessment. This study integrated Geographic Information System (GIS)-based terrain analysis with the Soil and Water Assessment Tool (SWAT) to evaluate watershed hydrological dynamics. Terrain indices, including slope, aspect, Stream Power Index (SPI), and Topographic Wetness Index (TWI), were derived from a 30-m Shuttle Radar Topography Mission (SRTM) Digital Elevation Model. The SWAT model was calibrated and validated using observed monthly streamflow data, and model performance was evaluated using the Nash–Sutcliffe efficiency (NSE) and coefficient of determination (R²). Spatial and statistical analyses were conducted to examine runoff and sediment yield responses at sub-basin and hydrological response unit (HRU) levels in relation to terrain attributes and land-use/land-cover (LULC) patterns. The SWAT model demonstrated good performance, achieving an NSE of 0.81 and R² of 0.84 during calibration, and an NSE of 0.78 and R² of 0.81 during validation, indicating satisfactory simulation of monthly streamflow. The results revealed pronounced seasonal variability in streamflow, characterized by a primary peak during the monsoon season and a secondary peak associated with spring snowmelt. Sediment yield analysis identified northern sub-basins as erosion hotspots, largely due to steep slopes and elevated SPI values. Statistical analyses confirmed that slope, SPI, and LULC were the dominant factors controlling hydrological response and erosion processes across the basin. The integrated GIS–SWAT framework effectively captured the complex interactions between terrain features, land-use patterns, and hydrological processes in the Upper Jhelum River Basin. The findings underscore the need for targeted soil conservation measures, reforestation initiatives, and land-use planning in erosion-prone sub-basins. The originality of this study lies in the systematic linkage of multiple terrain indices with SWAT outputs at sub-basin and HRU scales, providing a robust, spatially explicit basis for informed watershed management and policy development in data-scarce mountainous regions.</p>