Recent growth of the construction industry has fuelled the demand for sand, with considerable volumes being extracted from the world’s large rivers. Sediment transport from upstream naturally replenishes sediment stored in river beds, but the absence of sand flux data from large rivers inhibits assessment of the sustainability of ongoing sand mining. Here, we demonstrate that bedload (0.18 ± 0.07 Mt yr −1) is a small (1%) fraction of the total annual sediment load of the lower Mekong River. Even when considering suspended sand (6 ± 2 Mt yr −1), the total sand flux entering the Mekong delta (6.18 ± 2.01 Mt yr −1) is far less than current sand extraction rates (50 Mt yr −1). We show that at these current rates, river bed levels can be lowered sufficiently to induce river bank instability, potentially damaging housing and infrastructure and threatening lives. Our research suggests that on the Mekong and other large rivers subject to excessive sand mining, it is imperative to establish regulatory frameworks that limit extraction rates to levels that permit the establishment of a sustainable balance between the natural supply/storage of sand and the rate at which sand is removed.