Plastic pollution in aquatic ecosystems is a growing threat to ecosystem health and human livelihood. Recent studies show that the majority of environmental plastics accumulate within river systems for years, decades and potentially even longer. Long‐term and system‐scale observations are key to improve the understanding of transport and retention dynamics, to identify sources and sinks, and to assess potential risks. The goal of this study was to quantify and explain the variation in floating plastic transport in the Rhine‐Meuse delta, using a novel 1‐year observational data set. We found a strong positive correlations between floating plastic transport and discharge. During peak discharge events, plastic transport was found up to six times higher than under normal conditions. Plastic transport varied up to a factor four along the Rhine and Meuse rivers, which is hypothesized to be related to the complex river network, locations of urban areas, and tidal dynamics. Altogether, our findings demonstrate the important role of hydrology as driving force of plastic transport dynamics. Our study emphasizes the need for exploring other factors that may explain the spatiotemporal variation in floating plastic transport. The world's most polluted rivers are connected to the ocean through complex deltas. Providing reliable observations and data‐driven insights in the transport and dynamics are key to optimize plastic pollution prevention and reduction strategies. With our paper we aim to contribute to both advancing the fundamental understanding of plastic transport dynamics, and the establishment of long‐term and harmonized data collection at the river basin scale.

Plastic debris and other anthropogenic litter has negative impacts on ecosystem health and human livelihood (van Emmerik & Schwarz, 2020). Despite several global initiatives to tackle this emerging environmental challenge, plastic production and leakage into the environment is expected to further grow in the coming decades (Borrelle et al., 2020). Rivers have been assumed to be the main conveyors of land-based plastic waste into the ocean (Meijer et al., 2021;Schmidt et al., 2017). However, recent work has suggested that plastic pollution can be retained within river systems for years to decades, and potentially even longer . Plastics accumulate on riverbanks, in vegetation, around hydraulic structures, and within estuaries, where they are exposed to environmental weathering leading to degradation and fragmentation (Delorme et al., 2021). The secondary micro-and nanoplastics that arise from this may lead to additional environmental risks, and may eventually be exported into the ocean (Koelmans et al., 2022). Understanding transport and retention dynamics

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.