Abstract
Based on a new dataset of high temporal resolution of water discharge (hourly frequency) and suspended sediment concentration (twice daily frequency at ebb and flood tides) at Can Tho and My Thuan stations during the 2009–2016 period, monthly and annual flow and suspended sediment flux of the lower Mekong River were calculated. The present water discharge of the Mekong River to the sea can be estimated to be 400 km3 yr−1, +/− 100 km3 yr−1 depending on El Niño Southern Oscillation (ENSO), and the present sediment supply to the sea can be estimated to be 40 Mt yr−1, +/− 20 Mt yr−1 depending on ENSO. The ENSO influence (proxied by the Southern Oscillation Index—SOI) on Q (water discharge) and Qs (sediment flux) is at maximum at a time lag of 8–9 months. The 2010–2011 La Niña event increased the water supply by almost 30% and the sediment supply by 55% in 2011. The 2015–2016 El Niño event decreased the water supply by 20% and the sediment supply by 50% in 2015–2016. The average net water discharge was 12,550 m3 s−1 in neutral years. The Tien River at My Thuan accounted for ~52%, and the Hau River at Can Tho for ~48% of the total Mekong River discharge, with small variations (of the order 1%) amongst years. In terms of suspended sediment delivery to the sea, the Tien River accounted for a more variable portion of 55% in neutral years, 57.3% in 2015–2016 (El Niño) and 62% in 2011 (La Niña). 80% of water discharge occurred during the flood season (July–December) and 20% occurred in the low flow season (January–June), the proportion being 91% and 9% for the sediment supply. Previous estimates of annual sediment supplies to the sea (145–160 Mt yr−1) had been established before 2003. Possible origins of the recent reduction by ~75% of sediment supply are presented and discussed.
Highlights
Weathering and mechanical erosion, which constrain the fluxes of sediment and associated elements from the continent to the ocean, are attributed to a combination of natural processes related to geomorphology, tectonic activity, and climate influences as well as land cover [1,2,3,4,5,6]
In terms of suspended sediment delivery to the sea, the Tien River accounted for a more variable portion of 55% in neutral years, 57.3% in 2015–2016 (El Niño) and 62% in 2011 (La Niña). 80% of water discharge occurred during the flood season (July–December) and 20% occurred in the low flow season (January–June), the proportion being 91% and 9% for the sediment supply
Mean water discharges and suspended sediment fluxes of the Mekong River at Can Tho and My Thuan stations were calculated over the eight-year monitoring period (2009–2016) (Figure 2, Table 3)
Summary
Weathering and mechanical erosion, which constrain the fluxes of sediment and associated elements (e.g., trace elements, nutrients, organic matter) from the continent to the ocean, are attributed to a combination of natural processes related to geomorphology, tectonic activity, and climate influences as well as land cover [1,2,3,4,5,6]. The rivers draining the Himalaya Mountains into South-East Asia have been recognized as significantly contributing to suspended particulate matter (SPM) delivery to the global ocean [4,5,13,20]. In this key area, recent and rapid changes in population and economic growth have strongly affected the functioning of the river-systems leading to major human pressure on its sustainable development Important uncertainties persist, mainly due to the nonstationary nature of sediment fluxes and the use of dated and/or questionable data (short-term sampling, inappropriate sampling frequency and/or data collected before dam/reservoir construction, deforestation or climatic changes [9,17,18,19]).
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