River plastic during floods: Amplified mobilization, limited river-scale dispersion

Abstract Plastic pollution in the world's rivers and ocean is increasingly threatening ecosystem health and human livelihood. In contrast to what is commonly assumed, most mismanaged plastic waste that enters the environment is not exported into the ocean. Rivers are therefore not only conduits, but also reservoirs of plastic pollution. Plastic mobilization, transport and retention dynamics are influenced by hydrological processes, and river catchment features (e.g. land-use, vegetation, and river morphology). Increased river discharge has been associated with elevated plastic transport rates, although the exact relation between the two can vary over time and space. The precise role of an extreme discharge event on plastic transport is however still unknown. Here, we show that fluvial floods drive plastic transport and accumulation in river systems. We collected unique observational evidence during the July 2021 flood along the complete Dutch part of the Meuse. Plastic transport multiplied with a factor of 141 upstream of the Dutch Meuse, compared to non-flood conditions, making the Meuse one of the most polluted rivers measured to date. Over one-third of the annual plastic transport was estimated to occur within the six-day period of extreme discharge. Towards the river mouth, plastic transport during the flood period decreased by 90\%, suggesting that the dispersal of plastic mobilized during the flood is limited due to the entrapment on riverbanks, in vegetation, and on the floodplains. Plastic accumulation on the riverbanks decreased significantly along the river, corroborating the river's function as plastic reservoir. Using new observational evidence, we demonstrate the crucial role of floods as driver of plastic transport and accumulation in river systems. Floods amplify the mobilization of plastics, but the effects are local, and the river-scale dispersion is limited. We anticipate that our findings serve as a starting point for improving global estimates of river plastic transport, retention, and export into the sea. Moreover, our results provide essential insights for future large-scale and long-term quantitative assessments of river plastic pollution. Reliable observations and a fundamental understanding of plastic transport are key to designing effective prevention and reduction strategies.