Landform controls on surface-water quality and dominant pollution pressures in Southern Malang Plateau, Indonesia

River water quality is declining across tropical landscapes undergoing rapid land-use change. We tested whether landform-controlled hydrological pathways explain spatial water-quality patterns in the Southern Malang Plateau (Indonesia). At 28 sites spanning tectonic, karst, alluvial, and volcanic landforms, we computed the National Sanitation Foundation Water Quality Index (NSF-WQI) and applied multivariate analyses. Alluvial rivers were most degraded, with nitrate up to 60 mg L⁻¹, COD >200 mg L⁻¹, and total coliforms up to 175 MPN 100 mL⁻¹, resulting in poor status (WQI <50). In contrast, volcanic catchments dominated by forest and agroforestry maintained good quality (WQI >75), while karst systems showed intermediate but spatially sensitive conditions. Multivariate structure indicates that nutrient enrichment and sediment-associated impairment are regulated by landform-specific erosion–sedimentation and flow-path connectivity rather than land use alone. Integrating landform controls into water governance supports SDG 6 (clean water and sanitation) and nature-based solutions.