Gravest threat to Mekong delta today is sediment starvation not rising seas

Posted on 10 December 2019

New research shows that the increasing vulnerability of the Mekong delta to floods, salt intrusion and erosion is caused by insufficient sediment in the river not climate-induced rise in sea levels.
Published in the journal Nature Scientific Reports, the findings of the Rise and Fall Project at Utrecht University are clear: the growing threat to the Mekong Delta – and the communities, cities, rice fields and biodiversity that depend on it – posed by higher tides and salt intrusion is almost entirely due to the loss of river sediment because of upstream dams and sand mining in the delta.
Rising tides in the delta have major ramifications for flooding in subsiding and increasingly vulnerable cities, and river bank erosion. While sea level rise and climate change have received most attention in relation to the sinking and shrinking of the Mekong delta, the research shows that in the last 20 years, they have driven less than 5% of these trends.
The primary culprit is direct human intervention in the form of sand mining and hydropower dams.
The analyses show that riverbeds are sinking by 200-300 mm per year due to sand mining and upstream dams. Meanwhile, land is subsiding by 20-30 mm per year due to excessive groundwater extraction and sea level is rising by 3mm per year due to climate change.
“The observed trends cannot be explained by sea level rise. While climate change poses grave natural hazards in the coming decades, human activities are driving short-term impacts that far outstrip current climate change effects,” said Sepehr Eslami, lead author on the paper.
“Previous studies highlighted that upstream hydropower dams impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. The recent study shows that tidal amplification and saline water intrusion in the Mekong delta are also increasing at alarming pace.”
While offshore tidal amplitude increases by 1.2-2 mm per year due to sea level rise, tidal amplitude within the delta is increasing by 2 cm per year. Salinity in some channels has doubled in the last 15 years. The research shows that this is mainly driven by riverbed incision, which is the result of lack of sediment flowing down the river to the sea.
“The Mekong delta is sinking and shrinking because countries have not factored the paramount role of natural sediment replenishment into their decisions on hydropower and sand mining,” said Marc Goichot, WWF Freshwater Lead in the Mekong.
“We are now working with partners on an ambitious Resilient Asian Deltas initiative, which aims to generate political momentum and financial investment to build with nature – including ensuring sufficient sediment reaches the deltas to stop them from sinking and shrinking,” added Goichot.

While climate change poses grave hazards in the coming decades, it is sometimes difficult to spark action – especially collaborative action involving communities, companies and authorities – since it seems too big to tackle.
However, this research shows that collective actions at local and regional levels to ensure sufficient sediment flow in the river can have a huge direct impact on the resilience of the delta – so that it can cope with today’s threats and be better able to adapt to the coming impacts of climate change.
Dam in the Mekong river basin. Hydropower dams & sand mining contribute to sinking and shrinking of Mekong Delta
© Adam Oswell / WWF-Greater Mekong
Sampans meet at early morning market in the Mekong Delta where rivers converge Vietnam
© Elizbeth Kemf / WWF
Sand mining barge in the Mekong
© Sepehr Eslami