Freshwater plastic pollution is critically understudied in Southeast Asia (SEA). Recent modelling studies indicate that SEA rivers contribute vast quantities of plastic to the world's oceans, however, these fail to capture the complexity of individual systems. We determine the volume of mismanaged plastic waste (MPW) entering Tonle Sap Basin (TSB)-the largest freshwater lake-river system in SEA, between 2000 and 2030. Using economic, population and waste data at provincial and national levels, coupled with high resolution population and flood datasets, we estimate that ca. 221,700 tons of plastic entered between 2000 and 2020, and 282,300 ± 8700 tons will enter between 2021 and 2030. We demonstrate that policy interventions can reduce MPW up to 76% between 2021 and 2030. The most-stringent scenario would prevent 99% of annual MPW losses by 2030, despite substantially higher waste volumes and population. If successfully implemented, Cambodia will prevent significant losses in natural capital, material value and degradation in TSB worth at least US$4.8 billion, with additional benefits for the Mekong River and South China Sea.
