Sediment dynamics in the lower Mekong River: Transition from tidal river to estuary

Abstract

A better understanding of flow and sediment dynamics in the lowermost portions of large‐tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean. Flow velocity, salinity, and suspended‐sediment concentration were measured for 25 h at three cross sections in the tidal Song Hau distributary of the Mekong River, Vietnam. Two campaigns took place during comparatively high‐seasonal and low‐seasonal discharge, and estuarine conditions varied dramatically between them. The system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow. The changing freshwater input, sediment sources, and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow. The Dinh An channel of the Song Hau distributary exported sediment to the coast at a rate of about 1 t s−1 during high flow and imported sediment in a spatially varying manner at approximately 0.3 t s−1 during low flow. Scaling these values results in a yearly Mekong sediment discharge estimate about 65% smaller than a generally accepted estimate of 110 Mt yr−1, although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate. Fluvial advection of sediment was primarily responsible for the high‐flow sediment export. Exchange‐flow and tidal processes, including local resuspension, were principally responsible for the low‐flow import. The resulting bed‐sediment grain size was coarser and more variable during high flow and finer during low, and the residual flow patterns support the maintenance of mid‐channel islands.

Salt wedge dynamics lead to enhanced sediment trapping within side embayments in high‐energy estuaries

“…These coves can form by channel meander and braid cutoffs and by drowning of fluvial or glacial valleys during rising sea level. They are common along the coastlines of passive margins where low‐gradient rivers can have tidal reaches of 50–1000 km including the Amazon River [ Kosuth et al ., ], Mekong River [ Nowacki et al ., ], Fitzroy River [ Bostock et al ., ], and Columbia River [ Jay et al ., ]. Tie channels connect coves to the main river channel and may persist naturally due to tidal flushing or be actively maintained by activities such as dredging or wood removal.…”

Sedimentation and Survival of the Mekong Delta: A Case Study of Decreased Sediment Supply and Accelerating Rates of Relative Sea Level Rise

“…Sea level rise issues are often compounded by sediment supply issues that could result in deeper effective channels within the Mekong River distributaries, and this would affect the extent of saline water intrusion into the distributary channels. At present, the 2012-2015 Song Hau studies suggest that saline water can intrude up to 40 km from the river mouth at low river discharge (Nowacki et al, 2015;McLachlan et al, in press). Numerical modeling studies in other Himalayan large rivers, such as the Changjiang (Qiu and Zhu, 2013), that face similar rates of relative sea level rise suggest that the intensity of saltwater intrusion and stratification both increase as sea level rises, with distinct partitioning of the effect between individual distributary channels.…”

“…T r a n D e C h a n n e l Nowacki et al, 2015;McLachlan et al, in press). At high discharge, much of the channel floor is composed of exposed relict units, while at low discharge, the zone downstream of the estuarine flood limit is partly mantled by soft muds trapped by estuarine circulation.…”

“…0.25-1 m thick that likely are a result of stratification-induced estuarine trapping (Nowacki et al, 2015;McLachlan et al, in press). These fines are removed during the subsequent high-discharge period to the adjacent subaqueous delta.…”

“…Kummu et al (2010) and Kondolf et al (2014) estimated 15%-18% of the annual suspended sediment load is already being trapped behind dams in the catchment. Nowacki et al (2015) utilized measurements of the Song Hau distributary in the 2012-2013 water year and flow partitioning with the other distributaries (from A. Nguyen et al, 2008) to suggest total suspended loads may be as low as 40 MT yr -1 .…”