Two decades after the construction of the first major dam, the Mekong basin and its six riparian countries have seen rapid economic growth and development of the river system. Hydropower dams, aggregate mines, flood-control dykes, and groundwater-irrigated agriculture have all provided short-term economic benefits throughout the basin. However, it is becoming evident that anthropic changes are significantly affecting the natural functioning of the river and its floodplains. We now ask if these changes are risking major adverse impacts for the 70 million people living in the Mekong Basin. Many livelihoods in the basin depend on ecosystem services that will be strongly impacted by alterations of the sediment transport processes that drive river and delta morpho-dynamics, which underpin a sustainable future for the Mekong basin and Delta. Drawing upon ongoing and recently published research, we provide an overview of key drivers of change (hydropower development, sand mining, dyking and water infrastructures, climate change, and accelerated subsidence from pumping) for the Mekong's sediment budget, and their likely individual and cumulative impacts on the river system. Our results quantify the degree to which the Mekong delta, which receives the impacts from the entire connected river basin, is increasingly vulnerable in the face of declining sediment loads, rising seas and subsiding land. Without concerted action, it is likely that nearly half of the Delta's land surface will be below sea level by 2100, with the remaining areas impacted by salinization and frequent flooding. The threat to the Delta can be understood only in the context of processes in the entire river basin. The Mekong River case can serve to raise awareness of how the connected functions of river systems in general depend on undisturbed sediment transport, thereby informing planning for other large river basins currently embarking on rapid economic development.

Vertical gradational structures develop as sand infiltrates into static gravel beds. Understanding the vertical distribution of interstitial sand deposits will improve predictions of ecological suitability and hyporheic hydrodynamics. A series of flume experiments was performed to investigate fine infiltration processes. Four sand distributions were introduced into flows over gravel beds. After each experiment, bed cores were extracted and analysed in vertical layers to examine the gradational trends with depth. Vertical trends of fine content were highly sensitive to the relative grain-size distributions of the gravel bed and the introduced sand. For experiments with d 15gravel /d 85sand ratios 15AE4 and larger unimpeded static percolation was observed, where sand filled the voids relatively uniformly from the bottom of the gravel layer to the top. Experiments with ratios 10AE6 and smaller bridged. Sand clogged a thin layer of gravel pores near the bed surface, precluding subsequent infiltration. Interstitial sand deposits fined with depth of penetration for all experiments which was the result of three distinct but overlapping processes. (i) Granular sorting: As particles fell through the substrate, smaller material preferentially passed through the voids deeper into the gravel. (ii) Bed-load sorting: Size segregation occurs in the wake of the leading bed form as smaller particles saltate further and settle first. (iii) Hydraulic sorting: Smaller sand was transported preferentially as suspended load filling the deep voids of the furthest flume positions downstream. Finally, when the experiments that formed a bridge layer were replicated with higher bed shear stresses, less interstitial sand deposition was observed. Higher shear stresses transported coarse particles downstream more efficiently causing bridge layers to form earlier and allowing less time for suspended load to settle into the deeper substrate pores before the pathways were closed.

No abstract

No abstract