Dam-regulated flow fragmentation and sand-mining-driven threshold shifts shorten the Mekong–Tonle Sap reverse-flow season
- 1Tsinghua University (Beijing, China)
Abstract
Sand mining, dam construction, and climate change have profoundly altered the hydrological dynamics of the Mekong River-Tonle Sap Lake floodplain system, one of the world's most ecologically productive river-lake complexes. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with the Tonle Sap Lake across three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight mainstream stations, we quantify shifts in long-term flow memory, short-term variability, and flow amplitude. We use a hydrodynamic response-time model to link upstream regime change to the Mekong–Tonle Sap flow exchange. During the post-dam period, flow memory increased in the upper mainstream but decreased slightly downstream, indicating growing upstream–downstream divergence in flow-regime characteristics and increasing hydrological fragmentation. 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 (+0.11), reflecting flow smoothing by reservoir operations. At the most downstream station at Kratie, a key control point for Tonle Sap inflow, maximum flows decreased by 9.4 %, while minimum flows increased by 117 % relative to the pre-dam baseline; the seasonal timing of peak discharge was also delayed by approximately two to three weeks. Critically, the combined effects of the altered mainstream flow regime and sand-mining-driven riverbed lowering have shifted the conditions under which Tonle Sap flow reversal initiates. Using the lag-adjusted Kratie discharge at onset as a proxy for the Mekong-Tonle Sap confluence stage/head condition, we find that the median onset discharge increased from ∼ 3200 m ³ s ⁻¹ (pre-dam) to ∼ 4300 m ³ s ⁻¹ under the post-dam fixed-bed scenario (2010–2024 hydrology with reference morphology), and to ∼ 7200 m ³ s ⁻¹ once sand-mining-driven channel incision is added. This indicates that post-2010 hydrological alteration reduced the probability of exceeding the discharge threshold for initiating reverse flow, while sand-mining-driven riverbed lowering substantially amplified this shift by modifying local stage–discharge relationships. Together, these interacting drivers shorten the reverse-flow season by approximately 28 d relative to the pre-dam baseline. Sustaining the Mekong–Tonle Sap flood pulse will therefore require basin-wide monitoring and transboundary governance that address flow timing and variability, not volume alone.
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BibTeX
@article{morovati2026damregulated,
title = {{Dam-regulated flow fragmentation and sand-mining-driven threshold shifts shorten the Mekong–Tonle Sap reverse-flow season}},
author = {Khosro Morovati and Hongling Zhao and Fuqiang Tian},
journal = {Hydrology and Earth System Sciences},
year = {2026},
month = nov,
doi = {10.5194/hess-30-6821-2026},
url = {https://doi.org/10.5194/hess-30-6821-2026},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Dam-regulated flow fragmentation and sand-mining-driven threshold shifts shorten the Mekong–Tonle Sap reverse-flow season
AU - Khosro Morovati
AU - Hongling Zhao
AU - Fuqiang Tian
PY - 2026
DA - 2026/11/05
JO - Hydrology and Earth System Sciences
DO - 10.5194/hess-30-6821-2026
UR - https://doi.org/10.5194/hess-30-6821-2026
AB - Sand mining, dam construction, and climate change have profoundly altered the hydrological dynamics of the Mekong River-Tonle Sap Lake floodplain system, one of the world's most ecologically productive river-lake complexes. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with the Tonle Sap Lake across three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight mainstream stations, we quantify shifts in long-term flow memory, short-term variability, and flow amplitude. We use a hydrodynamic response-time model to link upstream regime change to the Mekong–Tonle Sap flow exchange. During the post-dam period, flow memory increased in the upper mainstream but decreased slightly downstream, indicating growing upstream–downstream divergence in flow-regime characteristics and increasing hydrological fragmentation. 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 (+0.11), reflecting flow smoothing by reservoir operations. At the most downstream station at Kratie, a key control point for Tonle Sap inflow, maximum flows decreased by 9.4 %, while minimum flows increased by 117 % relative to the pre-dam baseline; the seasonal timing of peak discharge was also delayed by approximately two to three weeks. Critically, the combined effects of the altered mainstream flow regime and sand-mining-driven riverbed lowering have shifted the conditions under which Tonle Sap flow reversal initiates. Using the lag-adjusted Kratie discharge at onset as a proxy for the Mekong-Tonle Sap confluence stage/head condition, we find that the median onset discharge increased from ∼ 3200 m ³ s ⁻¹ (pre-dam) to ∼ 4300 m ³ s ⁻¹ under the post-dam fixed-bed scenario (2010–2024 hydrology with reference morphology), and to ∼ 7200 m ³ s ⁻¹ once sand-mining-driven channel incision is added. This indicates that post-2010 hydrological alteration reduced the probability of exceeding the discharge threshold for initiating reverse flow, while sand-mining-driven riverbed lowering substantially amplified this shift by modifying local stage–discharge relationships. Together, these interacting drivers shorten the reverse-flow season by approximately 28 d relative to the pre-dam baseline. Sustaining the Mekong–Tonle Sap flood pulse will therefore require basin-wide monitoring and transboundary governance that address flow timing and variability, not volume alone.
SN - 1607-7938
ER -