Long-term sediment decline in the Mekong River along Thailand's border under hydropower development

The Mekong River, one of the most sediment-rich rivers in Southeast Asia, has experienced substantial alterations in sediment transport and discharge patterns from 1962 to 2022. This study investigates long-term trends in suspended sediment loads and runoff at five strategic stations using cumulative sediment load curves, the Mann-Kendall trend test, Z-score normalization, and sediment-discharge correlation analysis. The results indicate a pronounced decline in sediment loads, up to 75 % post-1990, despite stable or slightly increasing runoff at most locations. This decoupling underscores the dominant role of upstream hydropower dams in sediment retention, particularly evident at Chiang Saen and Nong Khai, where cumulative sediment curves markedly plateaued following dam construction. Correlation coefficients between sediment and discharge decreased significantly (r < 0.5) after 2000, signaling weakened sediment-flow linkages. Seasonal analysis reveals suppressed sediment peaks during the wet season, consistent with regulated flow regimes and sediment entrapment by reservoirs. Z-score anomalies highlight abrupt declines in sediment fluxes coinciding with the operational timeline of major dams along the Lancang-Mekong mainstream. Secondary influences from land use changes and interannual climate variability are also evident but less pronounced. The downstream consequences of sediment reduction are critical, particularly for the Mekong Delta, where sediment starvation contributes to coastal erosion, land subsidence, and diminished agricultural productivity. These findings emphasize the necessity for integrated, transboundary sediment management strategies, including sediment flushing, adaptive reservoir operations, and policy harmonization across riparian states. This study provides robust empirical evidence of anthropogenic disruption of sediment-water connectivity in a large tropical river basin and offers a framework to inform sustainable river basin management under increasing hydropower development and climate variability.