Abstract
Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was < +1 km2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels.
Highlights
Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction
The muddy coast along the Camau Peninsula, which forms the bulk of the large Mekong delta protrusion into the East Sea, has been eroding since the 1970s13,14, with historical maps indicating that erosion may have started prior to 194015
Long-term shoreline changes of the Mekong delta have only been constrained in the river mouth area[16] and relevant data have been missing in the southwestern delta protuberance, which has led previous studies to make assumptions on delta growth/erosion in this area[17,18]
Summary
Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. Recent anthropogenic catchment disturbances such as upstream hydropower dam construction[4,5] and fluvial sand mining[6,7,8,9], possibly along with climate change-induced discharge reduction[10], are considered to have caused severe decline in the sediment discharge to the coast, compounding concerns raised by exacerbated subsidence due to accelerated groundwater extraction[11] and leading to alarms being raised regarding the increasing vulnerability of this populous megadelta This declining sediment supply has led to enhanced coastal erosion with high-resolution satellite images from 1993 to 2013 indicating a substantial increase in the last 10 years[12]. This segregation is recorded in the relict deltaic landforms as sandy beach ridges only occur in the river mouth area[16], whereas the Camau Peninsula consists almost entirely of a muddy delta plain
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