By 2050, one billion people will live in low-lying coastal zones and river deltas – areas that are also paramount for regional and global food security. Worldwide, coastal populations are growing and climate change and major infrastructure plans threaten the resilience of many large river deltas. 

Recent research focused on the Mekong Delta, which supports more than 17 million livelihoods and food systems of global importance, points to the need for sustainable land, water, and sediment management on delta and basin scales. While river basin and delta processes are commonly disconnected in research and management, the new findings highlight the importance of integrating assessments across scales to estimate future relative sea level rise, as well as identifying effective management levers for achieving delta resilience in a robust manner.

Up to 90% of the Mekong Delta might fall below sea level by 2100. (A) The 800,000-km2 Mekong basin can be divided into distinct geomorphic provinces (numbers 1 to 7), each with a different contribution to the sediment budget of the basin. (B) Different levels of sediment supply from the basin (rows) together with different scenarios for DSDs (columns) result in different levels of relative sea level rise (rSLR) (colours) and thus different fractions of the current delta surface remaining subaerial, i.e., above sea level (percentages). (C) The change in subaerial delta surface for each level of rSLR in B based on most recent topographic data. (D) These topographic data are used to locate areas below sea level. Note that colours in B, C, and D are corresponding.

As Schmitt et al state in their paper published in the Proceedings of the National Academy of Sciences of the United States of America, maintaining sediment supply from the river basin is “crucial for maintaining delta land and enhancing the climate resilience of the system”. 

Sediment supply will be reduced by dams, but the magnitude of reduction depends on where and how dams are built and operated. To model the joint impacts of changing sediment yields and dam siting, the authors considered 17 different scenarios of dam siting. Each of these represented a different portfolio, ie, a different set of dam sites, and is associated with one of 11 levels of increasing hydropower generation.

Current plans for expanding hydropower in the region focus on large dams in the lower Mekong tributaries and the mainstem of the Mekong in Laos and Cambodia. This would increase generation to around 194,000 GWh/yr but decrease sediment supply to 31 ± 13 Mt/yr. With more dams in the lower Mekong Basin, sediment supply to the delta becomes less sensitive to changing sediment yields from hillslope processes and sediment trapping in dams.

The authors say their analysis highlights that strategic placement of hydropower dams will be more important for maintaining sediment supply than either projected increases in sediment yields or improved sediment management at individual dams. The proliferation of dams in the basin not only reduces total sediment supply but also changes the sensitivity of sediment supply to basin-scale processes. 

“Strategic infrastructure planning is paramount for maintaining sediment supply to the Mekong Delta and for maximising its resilience,” Schmitt et all explain. “Specifically, further business-as-usual dam development is not compatible with maintaining sediment supply even at the current levels, which are strongly reduced compared to the basin’s pristine state. Only strategic dam planning across all riparian countries could produce a modest increase in hydropower generation while maintaining sediment supply close to current levels.

“Analysis highlights that strategic infrastructure planning on a portfolio level is a robust strategy to minimise the cumulative environmental impacts of dams even under major uncertainty. This finding has implications beyond the Mekong,” the authors continue, “because most of the global hydropower potential is in large and poorly studied basins, where origins of sediment and other natural values are poorly constrained and subject to change, and where the design and operation of future dams is highly uncertain.”

If dams are not developed in a strategic manner in the Mekong, the authors warn, the better design and management of individual dams is unlikely to translate into better sediment supply.

“Of course,” Schmitt et al conclude, “environmental safeguards on the scale of individual dams can alleviate local negative externalities and might seem more practical from a political perspective than a basin-scale planning process. However, our results show that only strategic planning on a portfolio level ensures that local safeguards, such as better sediment management, result in basin-scale improvements in environmental objectives.”