Reverse flow control on suspended-sediment exchange between Tonle Sap Lake and the Mekong River under historical and mega-dam regimes

The Mekong–Tonle Sap Lake–Delta system supports highly productive floodplain ecosystems and regional food security, in part through the delivery of nutrient-rich suspended sediment. However, this sediment pathway is strongly modulated by the flood-pulse–driven “reverse flow” at the Mekong–Tonle Sap confluence: during the wet season, high Mekong stages drive net inflow from the mainstream into Tonle Sap Lake, whereas during the dry season the lake releases stored water back to the mainstream and onward to the delta. How this bidirectional exchange reshapes sediment delivery—specifically whether Tonle Sap acts as a net sink or a net source of suspended sediment for the downstream Mekong—remains poorly quantified. Most existing assessments approximate delta sediment supply using upstream stations and do not resolve the river–lake exchange, largely because near-confluence discharge and continuous sediment observations are limited.

Here we develop an integrated modelling framework that couples a physically based, spatially distributed hydrological model with Delft3D-Flow hydrodynamics to reconstruct daily discharge and river–lake exchange over the last ~35 years, including the reversal period. We then estimate suspended sediment fluxes using seasonally stratified, hysteresis-aware rating curves that account for distinct sediment–discharge relationships on rising versus falling limbs of the hydrograph. Combining daily exchange discharge with the corresponding rating-curve sediment concentrations enables a bidirectional suspended-sediment budget across the Tonle Sap River, separating wet-season import to the lake from dry-season export back to the mainstream.

During the historical baseline (1980–2000), we estimate that the Mekong mainstream delivers ~4 Mt yr⁻¹ of suspended sediment into the lake on average, and a comparable magnitude is returned to the mainstream during the dry season, indicating that Tonle Sap primarily acts as a transient store rather than a sustained additional sediment source to the downstream system. In the mega-dam period (2010–2025), despite substantially reduced upstream sediment supply, the river–lake exchange continues to route similar volumes of water into the lake, but the suspended-sediment contribution released from the lake does not compensate for the mainstream deficit. These results suggest that reverse-flow dynamics alone do not sustain suspended-sediment delivery to the Mekong Delta under contemporary sediment scarcity, with implications for nutrient replenishment, recession agriculture, and floodplain productivity.