Case study on sediment in the Mekong River Basin: Current state and future trends

The Mekong River carries with it huge loads of sediment and nutrients across its entire length, from the Upper Mekong (Lancang) River down to the Mekong Delta in Vietnam. These sediments are critical for the formation and stabilization of deltas and the ecosystems that they support.  Sediments and attached organic matter are essential for fish, plant, and soil nutrition across the wider Mekong Basin. The silt enriches and replenishes the entire basin, and performs critical ecosystem services for agriculture, fisheries, water-based plants and animals, and water quality, and in turn supports the economies of the basin countries.

The loss of nutrients could have implications for fisheries and soil fertility, and affect the livelihoods and economies of millions of people in the Mekong Basin and beyond.

The sediment transport in the Mekong is affected by existing and ongoing large hydropower development in the mainstream and its tributaries and other activities such as river bed mining, deforestation, and development both in the floodplains and the Mekong Delta. Land use change and extreme events due to climate change is expected to exacerbate the current situation. Sediment management, therefore, needs to become a key consideration in land and water resources planning and implementation to ensure sustainable development.

This study identified key issues on sustainable sediment management for the Mekong River Basin. The key findings from the study are:

• Based on previous studies and observed data, sediment carriage and deposition has been drastically reduced mainly due to large reservoir trapping and riverbed mining.
• Land use change and extreme events linked to climate change will exacerbate the current situation and increase uncertainty over sediment transport from the catchments.
• If all the dams proposed for the Lower Mekong Basin are developed, including the planned or ongoing 11 mainstream dams, it could prevent up to 94% of the river’s sediment load from being transported further downstream.
• This reduction of sediment delivery to downstream areas will have implications for the stability of the Mekong Delta and will lead to loss of nutrients that are vital for the basin’s fisheries. The reduction will result in loss of soil fertility in the floodplains along the entire basin, especially the Tonle Sap in Cambodia and the Mekong Delta in Vietnam.
• These developments have an impact on poor rural populations throughout the entire basin whose livelihoods depend on the river for irrigation, soil fertilization, fisheries or water-related natural products.
• Lack of regionally binding guidelines and standards prevents countries from committing to best practices, effective monitoring and integrated sediment load management at every stage of the hydropower life-cycle, as well as riverbed mining.

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The study aims to contribute to the implementation of the International Sediment Initiative (ISI) of UNESCO’s International Hydrological Programme (IHP). UNESCO’s International Hydrological Programme (IHP) launched the International Sediment Initiative (ISI) in 2003 to to develop a decision support framework for sediment management, to provide guidance on legislative and institutional solutions applicable to various socioeconomic and physiographic settings, and to further advance sustainable sediment management in the context of global change.