Persistent turbidity in a tropical river: shifting controls from hydrology to artisanal gold mining

This study examines temporal turbidity dynamics in the Kahayan River (Central Kalimantan, Indonesia) and evaluates the relative contributions of hydrological variability and artisanal and small-scale gold mining (ASGM). Monthly observations were conducted from July 2024 to July 2025 at three sampling points, where turbidity, total suspended solids (TSS), physicochemical parameters, and chlorophyll-a were measured. Results showed that turbidity and TSS remained consistently high throughout the study period, exceeding 100 NTU and 50 mg L−¹, respectively, despite seasonal water-level fluctuations. The strong positive correlation between turbidity and TSS (ρ = 0.96, p < 0.001) confirms that suspended sediments are the primary drivers of reduced water clarity. Generalized Additive Models (GAMs) indicated that temporal variation significantly influenced turbidity and TSS, whereas water depth showed no significant effect. These patterns suggest that natural hydrological controls are increasingly overshadowed by sustained anthropogenic sediment inputs associated with ASGM, including direct dredging and mining-induced riverbank erosion. The findings indicate a shift from predominantly hydrological regulation toward a persistent anthropogenic sediment baseline characterized by continuous sediment loading. Effective river management will therefore require integrated sediment-control measures, riparian protection, and realistic regulation of ASGM activities to support water-quality improvement and ecosystem recovery.