Survey & investigation | The mighty Mekong
Launched in December 2020, the Mekong Dam Monitor’s annual report provides a unique insight into the impact of dams on the natural flow of the Mekong River. The report strives to improve understanding of the river’s course from China to Vietnam, provides policy recommendations for planners and practitioners, as well as pointing to gaps in research and data in the search for improved understanding of the river system.
Below: Tonle Sap Lake in Cambodia is often called the beating heart of the Mekong Basin
USING INFORMATION DERIVED FROM satellite observations with data provided by the Mekong River Commission, the Mekong Dam Monitor’s annual report for 2022-23 provides an insight into dam operations and Mekong River flows during the 2022 wet season to the 2023 dry season. As the report states: “So much of what makes the Mekong unique, from being the world’s largest freshwater fishery to its robust agricultural production to its incredible biodiversity, relies on the extreme highs and lows of its annual flood pulse.” However, the Mekong Dam Monitor (MDM) says evidence shows that the operations of the largest storage dams in the Mekong Basin are leading to a reduction of the benefits provided by the flood pulse. Over time, MDM states, especially with the impact of new climate patterns, this flattening of the flood pulse will significantly reduce the Mekong’s fish population and the opportunity for floodplain agriculture, particularly in Cambodia and Vietnam. Improved basin-wide seasonal and monthly weather forecasting and better communication between upstream and downstream countries could mitigate the impacts of dams during times of crisis, helping to avoid exacerbating an already bad situation, it adds. To reduce the impacts that dams deliver
to seasonal flow regimes and thus maximise downstream fishery productivity and agricultural production, minimum and maximum flow thresholds for existing and planned seasonal storage dams on the mainstream and tributaries should be determined. Furthermore as climate interactions with
dam impacts and river flow are complex, and much is unknown about the interaction of these effects on the Mekong’s ecology and socio-economic outcomes, the MDM says more study is required to provide greater understanding and improved evaluation.
Hydropeaking
Between June 2022 and May 2023, the MDM team issued 20 alerts when China’s upstream dams released or restricted enough water to cause a 0.5m or higher rise or fall in river levels within a 24-hour period at Chiang Saen, Thailand. Alerts are issued to national governments, the Mekong River Commission, and the tens of thousands of people living in affected communities via social media. Local community people in Chiang Saen who received the alerts confirmed they helped protect their assets, namely boats which had previously been washed away during sudden releases of water. MDM says that smarter dam management is needed and dam operators throughout the basin should avoid sudden flow releases or restrictions which cause severe downstream fluctuations, while reducing hydropeaking practices can avoid these impacts. In cases where sudden releases and restrictions are necessary for dam safety or other reasons, dam operators should provide early warning days in advance to relevant authorities and communities downstream so that vulnerable people have time to prepare. In addition, local authorities should create clear processes for notification and require compliance by dam operators.
Tonle Sap The Tonle Sap Lake is often called the beating heart
of the Mekong Basin. During normal wet season conditions, it expands to five times its dry season area and sixty times its dry season volume of water. This expansion is mostly driven by the Mekong flood pulse bringing higher river levels throughout the Mekong mainstream, causing the famous “Tonle Sap Reversal” where the river which traditionally flows out of the Tonle Sap reverses direction. This reversal sends water from the Mekong into the Tonle Sap, and transported with that water are fish eggs and larvae which find a rich habitat inside the Tonle Sap Lake where fish feed and grow. As the lake contracts in the dry season and drains back into the Mekong system, migratory fish move upstream into tributaries in Cambodia, Laos, Vietnam, and Thailand and water and sediment flow downstream to nourish the Mekong Delta. This
36 | June 2024 |
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