Recently, a Vietnamese friend of mine told me about the ongoing problems facing the Vietnamese agriculturally due to the damming of the Mekong river. It came up in a discussion on a Chess Forum and I was delighted by the information that I was able to find pertaining to this.
How dams starve the Mekong River Delta of vital sediment
But as Chinese-built hydroelectric dams have mushroomed upriver, much of that sediment is being blocked, an analysis of satellite data by Germany-based aquatic remote sensing company EOMAP and Reuters shows. The analysis reinforces an estimate by the Mekong River Commission, set up in 1995 by countries bordering the river, that in 2020 only about a third of those river-borne soils would reach the Vietnamese floodplains, and at the current rate of decline, less than five million tonnes of sediment will be reaching the delta each year by 2040.
… The amount of sediment going downstream in 2007 was 143 million tons!
Stretching nearly 5,000 kilometres from the Plateau of Tibet to the South China Sea, the Mekong is a farming and fishing lifeline for tens of millions as it swirls through China, Laos, Myanmar, Thailand and Cambodia before reaching Vietnam.
“The river is not bringing sediment, the soil is salinised,” said Cung, 60, who has grown rice at his family’s 10-hectare farm for more than 40 years.
“Without sediment, we are done,” he said. His steadily diminishing harvest now brings in barely half of the 250 million dong ($10,636) annually that he earned just a few years ago, and his two children and several neighbours have left the area to seek more stable and lucrative work elsewhere.
Terrible!
And check this out:
Tidal amplification and salt intrusion in the Mekong Delta driven by anthropogenic sediment starvation
While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects.
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The earlier estimates28 of total sediment transported by MR were c. 160 Mt yr−1, but recent estimates27,36,62,63 range between 40–110 Mt yr−1. While only 13.5% of total VMD discharge springs from upper Mekong basin (Chinese Lancang-Mekong River), 40–60% of total Mekong sediment load originates from that stretch of the basin5,60. Estimates of sediment reduction rate from Chinese catchments due to 6 mainstream dams range between 40–90%5,36,64,65. Considering the hydropower development in Mekong tributaries, the aggregate sediment trapping of the dams are predicted to reduce sediment supply by 36%(moderate)−95%(worst case)66 and a recent study67 measured 75% reduction in sediment supply to the VMD. Furthermore, although sand export was banned in Cambodia and Vietnam, domestic consumption persisted. Projections to 204068 show more than 1500 Mm3 demand within the VMD for infrastructure development. Figure 1 shows the spatial distribution of sand mining within the VMD and upstream in Cambodia. The issued mining licences in Vietnam, in 2015, summed up to 28 Mm3 yr−1 (40–50 Mt yr−1), which is four times the previous estimates10. Considering that this does not account for sand mining in Cambodia, Laos and possible illegal sand mining, only in Vietnam, this amount is likely to be close to 100% (or more) of the total fluvial sediment supply.
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While sea level is rising at nearly 3 mm yr−169 and consequently, M2 tidal amplitude over the Mekong continental shelf is increasing by 1.2–2 mm yr−149, the M2 tidal amplification within the delta is c. 2 cm yr−1 (e.g., at My Thuan), and salinity is rapidly increasing in the delta by 0.2–0.5 PSU yr−1, notwithstanding the increased upstream discharge due to CC and hydropower operations. The way SLR [Sea Level Rise] affects salt intrusion is a) by increasing channel depths, b) increasing offshore tidal amplitudes and c) increasing tidal amplitudes within the estuarine channels. Furthermore, the anthropogenic land subsidence11,12 possibly also contributes to deeper channels by c. 1–4 cm yr−1. On the other hand, in physical terms, sediment deficit has similar effects as relative SLR; i.e., it results in deeper channels and larger tidal amplitudes which lead to increased SWI [Salt Water Intrusion]. However, relative SLR (including land subsidence) in the past 15 years would perhaps sum up to c. 20–30 cm, while our presented evidence in this study, backed by previous research, indicates bed level incisions in the order of 2–3 m. Therefore, it is inevitable to conclude that anthropogenic trends outpace climate change trends by an order of magnitude and pose larger threats in the short term while exacerbating the medium to long term climate change impacts.
