Morovati et al. (2026) Dam-regulated flow fragmentation and sand-mining-driven threshold shifts shorten the Mekong–Tonle Sap reverse-flow season

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-11-05
• Authors: Khosro Morovati, Hongling Zhao, Fuqiang Tian
• DOI: 10.5194/hess-30-6821-2026

Research Groups

• Department of Hydraulic Engineering & State Key Laboratory of Hydro-science and Engineering, Tsinghua University, Beijing, China (Khosro Morovati, Fuqiang Tian)
• College of Hydraulic Science & Engineering, Northeast Agricultural University, Harbin, China (Hongling Zhao)

Short Summary

This study assesses how dam regulation, climate variability, and sand mining have altered the Mekong River's hydrological dynamics and its coupling with the Tonle Sap Lake. It finds that these combined factors have significantly increased the discharge thresholds for reverse flow initiation and cessation, shortening the Tonle Sap reverse-flow season by approximately 28 days.

Objective

• To provide 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), and to link these altered flow characteristics to the onset, termination, and duration thresholds of Tonle Sap's reverse flow.

Study Configuration

• Spatial Scale: Mekong River basin, including eight mainstream hydrological stations (Chiang Saen to Kratie), the Tonle Sap Lake and River system, Cambodian floodplain, and Vietnamese delta.
• Temporal Scale: Daily discharge data from 1976–2024, classified into pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024) periods. Sub-daily water level data from 2018 onwards.

Methodology and Data

• Models used:
  • Tsinghua Representative Elementary Watershed (THREW) model: Spatially distributed, physically based hydrological model for tributary inflows and naturalized discharge simulations.
  • Delft3D-Flow: Three-dimensional (3D), grid-based hydrodynamic model for flow propagation, water surface dynamics, and reverse flow simulation in the lower Mekong River-Tonle Sap-Delta system.
  • Response time model: Eulerian framework with a virtual passive tracer to quantify travel time from Kratie to the Mekong-Tonle Sap confluence.
• Data sources:
  • Daily discharge data (1976–2024) from the Mekong River Commission (MRC) Data Portal.
  • Sub-daily (15 min interval) water level data (2018 onwards) from the MRC portal.
  • Lake water levels, reverse flow periods, and discharge from the MRC and Procedures for the Maintenance of Flows on the Mainstream (PMFM) online platform.
  • Station-based meteorological observations (precipitation, temperature, wind speed, humidity, radiation/sunshine duration) from MRC and the China Meteorological Administration (CMA).
  • Land-surface datasets: Harmonized World Soil Database (soil properties), Moderate Resolution Imaging Spectroradiometer (MODIS)-based surface-condition products (NDVI, LAI, snow cover).
  • Surveyed riverbed cross-sectional bathymetry from MRC and other hydrological agencies.
  • Forest And Buildings removed Copernicus DEM (FABDEM) data (30 m spatial resolution) for floodplain topography.
  • Canal networks extracted using a machine learning-based remote sensing model and manual digitization from high-resolution satellite imagery.

Main Results

• Flow Regime Alteration: During the post-dam period, flow memory (Hurst exponent) increased in the upper mainstream (e.g., Chiang Saen: +0.11) but decreased slightly downstream (e.g., Kratie: -0.06), indicating growing hydrological fragmentation. Daily flashiness (Richards-Baker Flashiness Index, RBI) increased modestly upstream but declined downstream. Amplitude variability (coefficient of variation, CV) declined substantially at upstream sites (e.g., Chiang Saen: ~0.77 pre-dam to ~0.52 post-dam).
• Annual Discharge Extremes: Median annual peak discharge declined by ~40% at Chiang Saen (from ~10,850 m³/s to ~6,490 m³/s) and by 9.4% at Kratie (from ~38,300 m³/s to ~34,700 m³/s) in the post-dam period. Conversely, annual minimum discharges consistently increased across all stations, most notably at Kratie, where it surged by 117.2% (from ~1,326 m³/s to ~2,880 m³/s).
• Timing of Extremes: Peak flow timing shifted approximately 10–20 days later at most stations (e.g., Kratie: late August to mid-September). Annual minimum flow timing consistently advanced by 3–4 weeks across all stations (e.g., Kratie: early April to mid-March).
• Sub-daily Water Level Variability (2018–2025): Sub-daily water level amplitude and peak counts (hourly rises ≥5 cm/h) exhibited wet-season intensification, particularly at upstream and some mid-reach stations, showing spatially heterogeneous and episodic patterns.
• Tonle Sap Reverse Flow Thresholds: The lag-adjusted Kratie discharge required for reverse-flow onset increased significantly from a median of ~3,180 m³/s (pre-dam) to ~4,300 m³/s (post-dam fixed-bed scenario, +35%) and further to ~7,210 m³/s (post-dam + sand mining scenario, +127%). Cessation thresholds rose materially only under the sand-mining scenario (to ~34,520 m³/s, +21% vs pre-dam).
• Reverse Flow Season Duration: The combined effects of altered mainstream flow and sand-mining-driven riverbed lowering shortened the Tonle Sap reverse-flow season by approximately 28 days relative to the pre-dam baseline, driven by a delayed onset (~16 days later) and an earlier cessation (~13 days earlier).

Contributions

• Provides an integrated, multi-scale assessment of flow regime shifts in the Mekong River, quantifying changes in flow memory, flashiness, and amplitude across daily and sub-daily scales.
• Introduces a hydrodynamic response-time model to empirically derive and quantify shifts in the lag-adjusted discharge thresholds for Tonle Sap reverse-flow onset and cessation.
• Mechanistically separates the hydrological effects of dam regulation and climate variability from the geomorphic effects of sand-mining-driven riverbed incision on the Tonle Sap reverse flow.
• Highlights the increasing hydrological fragmentation and loss of spatial coherence along the Mekong mainstream, with locally distinct dynamics replacing the previously unified flood pulse.
• Offers updated system-wide insights for transboundary water governance, emphasizing the critical importance of flow timing and variability, not just volume, for sustaining downstream ecosystems and livelihoods.

Funding

• Beijing Natural Science Foundation (grant no. IS25095)
• National Natural Science Foundation of China (NSFC; grant nos. W2633143 and 52609030)
• Shuimu Tsinghua Scholar Program

Citation

@article{Morovati2026Damregulated,
  author = {Morovati, Khosro and Zhao, Hongling and Tian, Fuqiang},
  title = {Dam-regulated flow fragmentation and sand-mining-driven threshold shifts shorten the Mekong–Tonle Sap reverse-flow season},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-6821-2026},
  url = {https://doi.org/10.5194/hess-30-6821-2026}
}