The livelihoods of millions of people living in the world's deltas are deeply interconnected with the sediment dynamics of these deltas. In particular a sustainable supply of fluvial sediments from upstream is critical for ensuring the fertility of delta soils and for promoting sediment deposition that can offset rising sea levels. Yet, in many large river catchments this supply of sediment is being threatened by the planned construction of large dams. In this study, we apply the INCA hydrological and sediment model to the Mekong River catchment in South East Asia. The aim is to assess the impact of several large dams (both existing and planned) on the suspended sediment fluxes of the river. We force the INCA model with a climate model to assess the interplay of changing climate and sediment trapping caused by dam construction. The results show that historical sediment flux declines are mostly caused by dams built in PR China and that sediment trapping will increase in the future due to the construction of new dams in PDR Lao and Cambodia. If all dams that are currently planned for the next two decades are built, they will induce a decline of suspended sediment flux of 50% (47-53% 90% confidence interval (90%CI)) compared to current levels (99 Mt/y at the delta apex), with potentially damaging consequences for delta livelihoods and ecosystems.

25The Mekong delta is recognised as one of the world's most vulnerable mega-deltas, being 26 subject to a range of environmental pressures including sea level rise, increasing population, 27and changes in flows and nutrients from its upland catchment. With changing climate and 28 socioeconomics there is a need to assess how the Mekong catchment will be affected in 29terms of the delivery of water and nutrients into the delta system. Here we apply the 30Integrated Catchment model (INCA) to the whole Mekong River Basin to simulate flow and 31water quality, including nitrate, ammonia, total phosphorus and soluble reactive phosphorus. 32The impacts of climate change on all these variables have been assessed across 24 river 33 reaches ranging from the Himalayas down to the delta in Vietnam. We used the UK Met 34Office PRECIS regionally coupled climate model to downscale precipitation and temperature 35to the Mekong catchment. This was accomplished using the Global Circulation Model GFDL-36CM to provide the boundary conditions under two carbon control strategies, namely 37representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. The RCP 4.5 38 scenario represents the carbon strategy required to meet the Paris Accord, which aims to 39 limit peak global temperatures to below a 2 o C rise while seeking to pursue options that limit 40 temperature rise to 1.5 o C. The RCP 8.5 scenario is associated with a larger 3-4 o C rise. In 41 addition, we also constructed a range of socio-economic scenarios to investigate the 42 potential impacts of changing population, atmospheric pollution, economic growth and land 43 use change up to the 2050s. Results of INCA simulations indicate increases in mean flows 44 of up to 24%, with flood flows in the monsoon period increasing by up to 27%, but with 45 increasing periods of drought up to 2050. A shift in the timing of the monsoon is also 46 simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in 47 nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of 48 these nutrients also increase by 5%, which reflects the changing flow, land use change and 49 population changes. 50

Abstract. The world's large rivers are facing reduced sediment loads due to anthropogenic activities such as hydropower development and sediment extraction. Globally, estimates of sand extraction from large river systems are lacking, in part due to the pervasive and distributed nature of extraction processes. For the Mekong River, the widely assumed estimate of basin-wide sand extraction is 50 Mt per year. This figure is based on 2013 estimates and is likely to be outdated. Here, we demonstrate the ability of high-resolution satellite imagery to map, monitor, and estimate volumes of sand extraction on the Lower Mekong River in Cambodia. We use monthly composite images from PlanetScope imagery (5 m resolution) to estimate sand extraction volumes over the period 2016–2020 through tracking sand barges. We show that rates of extraction have increased on a yearly basis from 24 Mt (17 to 32 Mt) in 2016 to 59 Mt (41 to 75 Mt) in 2020 at a rate of ∼8 Mt yr−1 (6 to 10 Mt yr−1), where values in parentheses relate to lower and upper error bounds, respectively. Our revised estimates for 2020 (59 Mt) are nearly 2 times greater than previous best estimates for sand extraction for Cambodia (32 Mt) and greater than current best estimates for the entire Mekong Basin (50 Mt). We show that over the 5-year period, only 2 months have seen positive (supply exceeds extraction) sand budgets under mean scenarios (5 months under the scenarios with the greatest natural sand supply). We demonstrate that this net negative sand budget is driving major reach-wide bed incision with a median rate of −0.26 m a−1 over the period 2013 to 2019. The use of satellite imagery to monitor sand mining activities provides a low-cost means to generate up-to-date, robust estimates of sand extraction in the world's large rivers that are needed to underpin sustainable management plans of the global sand commons.

Abstract. The world's large rivers are facing reduced sediment loads due to anthropogenic activities such as hydropower development and sediment extraction. Globally estimates of sand extraction from large river systems is lacking, in part due to the pervasive and distributed nature of extraction processes. In the Mekong River, current basin wide estimates of sand extraction are 50 Mt, and based on estimates from 2013. Here, we demonstrate the ability of high-resolution satellite imagery to map, monitor and estimate volumes of sand extraction on the Lower Mekong River in Cambodia. We use monthly composite images from PlanetScope imagery (5 m resolution) to estimate sand extraction volumes over the period 2016–2020 and show that rates of extraction have increased year on year from 24 Mt (17 Mt to 32) in 2016, to 59 Mt (41 Mt to 75 Mt) in 2020 at a rate of ~8 Mt yr−1 (6 Mt yr−1 to 10 Mt yr−1); where values in parenthesis relate to lower and upper error bounds, respectively. Our revised estimates for 2020 (59 Mt) are nearly two times greater than previous best estimates for sand extraction for Cambodia (32 Mt) and greater than current best estimates for the entire Mekong Basin (50 Mt). We show that over the five year period, only two months have seen positive (supply exceeds extraction) sand budgets under mean and upper bound scenarios (five months under the lower bound estimates). We demonstrate that this net negative sand budget to the river is driving major bed incision with a median rate of −0.26 m a−1 over the period 2013 to 2019. The use of satellite imagery to monitor sand mining activities provide a low-cost means to generate up-to-date, robust estimates of sand extraction in the worlds large rivers that are needed to underpin sustainable management plans of the global sand commons.