Flood type drives river-scale macroplastic deposition

Plastic pollution is considered a global environmental challenge, prompting international regulation efforts such as the UN plastic treaty, a global initiative aimed at addressing the escalating plastic pollution crisis. Rivers with high connectivity to urban areas are particularly exposed to macroplastic pollution. Floods amplify macroplastic abundance in rivers by mobilizing previously deposited, and introducing new macroplastics. Recent observations suggest most of the flood-driven macroplastic transport is either exported downstream or stored within rivers. Yet, a comprehensive understanding of the fate of these mobilised macroplastics remains unresolved.

We assessed flood impact on macroplastic deposition along river floodplains, using data from fifteen events — five floods and ten non-flood conditions — across two Dutch rivers (the IJssel and Meuse). We quantified riverbank macroplastic concentrations under both non-flood and flood conditions, and floodplain macroplastic concentrations under flood conditions. Non-flood conditions were defined as events with return periods below bankfull discharge, while floods exceeded this threshold (1.5-year return period). We estimated riverbank macroplastic concentrations following five floods of varying magnitudes, with return periods ranging from 2 to 100 years. We attributed macroplastic concentrations to the main drivers of macroplastic deposition, using a parsimonious modelling approach. We considered ten factors, including river and floodplain characteristics, and proximity to potential macroplastic sources. Similar to sediment and large wood deposition, the longitudinal distribution of macroplastic along floodplains is likely influenced by the balance between supply and deposition factors, which determines floodplain capacity in storing macroplastics.

We found that higher flood return periods increased macroplastic deposition, with the two largest floods depositing two to three times more macroplastic than non-flood conditions. In addition, deposition mechanisms varied by flood type. Obstruction-based deposition dominated during an extreme summer flood (summer 2021 in the Meuse), when macroplastics mainly accumulated in inundated trees. Low-energy deposition prevailed during a long winter flood (winter 2024 in the IJssel), with high macroplastic concentrations found in wide floodplain sections where flow velocities decreased.

The identified typologies in floodplain macroplastic deposition patterns suggest varying spatial distribution patterns within the floodplains. These can help to develop interventions to reduce the floodplain macroplastic legacy. Due to its parsimonious nature, our modeling approach can be applied to other rivers and flood events. We anticipate that this may contribute to better understanding of the impact of floods on plastic pollution.