Louise Schreyers , Rahel Hauk, Nicholas Wallerstein, Adriaan J Teuling, Remko Uijlenhoet, Martine van der Ploeg, Tim van Emmerik 

Plastic pollution is considered a global environmental challenge, prompting international regulation efforts such as the UN plastic treaty to end plastic pollution. River basins, with high population densities and poor waste management, are particularly exposed to plastic pollution. Floods amplify plastic presence in rivers by mobilizing previously deposited and introduce new plastics. Yet, the fate of these mobilized plastics remains unclear, with observations suggesting either downstream export or floodplain deposition. This study assesses flood impact on plastic deposition along river floodplains, using data from fifteen events — five floods and ten non-flood conditions — across two Dutch rivers. Non-flood conditions were defined as events with return periods below bankfull discharge, while floods exceeded this threshold (1.5-year return period). Higher flood return periods increased plastic deposition, with the two largest floods depositing two to three times more plastic than non-flood conditions. Deposition mechanisms varied by flood type. Obstruction-based deposition dominated during an extreme summer flood (summer 2021 in the Meuse), when plastics mainly accumulated in inundated vegetation. Low-energy deposition prevailed during a long winter flood (winter 2024 in the IJssel), with high plastic concentrations found in wide floodplain sections where flow velocities decreased. Floodplain characteristics, hydrological conditions and proximity to plastic sources drive the plastic depositional patterns on floodplains. Flood severity and plastic entry into the environment are both projected to increase. We therefore expect an even more prominent role of floods in the global distribution of plastic pollution.