Significant Reduction in Tonle Sap Lake's Critical Flood Pulse Caused by Human-Induced Riverbed Lowering

The Tonle Sap Lake (TSL), a vital component of the Mekong River, is renowned as one of the world’s most productive lake-wetland systems. The lake’s high productivity is intimately related to an annual flood pulse that is driven by Mekong River flood waters forcing a unique flow reversal along the Tonle Sap River into the lake. During the dry season the floodwaters are returned to the Mekong River, sustaining vital freshwater fluxes to the downstream delta, inhabited by 23 million people. Recent observations have revealed notable changes in the timing and duration of the reverse flow into the TSL, resulting in associated reductions in lake inundation extents. Previous work has identified changes in flow regimes as a possible cause of the observed decline of the reverse flow. In contrast, here we show how riverbed lowering along the mainstem of the Mekong River – driven by accelerating channel bed sand mining and trapping of sediments through upstream hydropower damming – of 3.06 m (σ= 2.03 m), has resulted in a reduction of the water flux into the TSL by up to 47% from 1998 to 2018. We additionally show that projected future (to the year 2038) riverbed lowering, resulting from ongoing sandmining, of up to 5.92 m (σ) = 2.84 m), would result in a further decline of water flux into the TSL of ~ 69% relative to the bathymetry condition in 1998. These ongoing reductions are reducing the maximum extent of seasonally flooded areas by ~ 40% around the lake, presenting a critical threat to its biological productivity and the entire functioning of the TSL flood pulse system. Additionally, these changes in the reverse flow would increase, by around 26 billion m³, the flow that would be transmitted downstream into the Mekong delta during the monsoon season, potentially contributing to increased flood risk downstream as well as reducing dry season ‘return’ water fluxes to the delta by 59%, presenting risks of accelerated saltwater intrusion and reduced agricultural productivity within the delta. Taken together our modelling results show the importance of sediment and river bed levels to the sustainability of the TSL flood pulse and that its future function will be significantly diminished if current levels of sediment extraction from the Mekong system continue.