Disrupted Flow Memory and Synchrony in the Mekong River under Dam Regulation and Climate Change: Implications for Tonle Sap Reverse Flow

Abstract. Dam construction and climate change have profoundly disrupted the hydrological dynamics of the Mekong River and its floodplain–lake system. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with Tonle Sap Lake for three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight stations, we quantify changes in long-term flow memory, short-term variability, and flow amplitude. These analyses are complemented by a hydrodynamic response-time model that simulates how upstream regime shifts reshape the Tonle Sap–Mekong flow exchange.

We show that dam regulation and climate change have fragmented both spatial synchrony and temporal persistence, disrupting the Mekong’s behavior as a coherent hydrological continuum. Upstream stations, particularly Chiang Saen, exhibit a 33.3 % increase in minimum flows and a 40 % decrease in maximum flows during the post-dam period compared to the pre-dam period, alongside higher flow memory (+0.13), reflecting flow smoothing by reservoir operations. Downstream variability remains more pronounced due to the influence of monsoonal tributaries. At the most downstream station–Kratie, a key control point for Tonle Sap inflow–maximum flows decreased by 9.4 %, while minimum flows increased by 117 % in the post-dam period relative to the pre-dam baseline, also delaying the seasonal timing of peak discharge by two weeks.

Critically, these regime shifts–driven by the compounding impacts of dam regulations and climate change, together with observed riverbed lowering from sand mining, caused the discharge threshold for initiating reverse flow to rise significantly: the median onset flow increased from ~3,000 m³  s⁻¹ (pre-dam) to ~7,000 m³  s⁻¹ (post-dam), marking a >130 % increase. Collectively, these alterations have shortened the reverse flow period by 24 days compared to the historical baseline. Our findings demonstrate that cascading dam operations across the mainstream and tributaries, amplified by climate change and other anthropogenic stressors, have triggered multi-scalar hydrological fragmentation in one of the world's most ecologically productive river–lake systems. Preserving the Mekong's natural flood pulse and its interaction with Tonle Sap lake will require basin-wide hydrological monitoring and transboundary governance frameworks that account not only for volume but also for flow timing, variability, and ecological function.