Plastic pollution in riverbeds fundamentally affects natural sand transport

Abstract Rivers link terrestrial and ocean environments, distributing fresh water, nutrients, and sediment to diverse ecosystems. Over the past 50 years, rivers have become increasingly significant vectors for plastic pollution. Lowland riverbeds exhibit coherent features including ripple and dune bedforms, which transport sediment downstream via well-understood processes, yet the impact of plastic on sediment transport behaviours is largely unknown. Here we use a flume tank to show that when plastic particles are introduced to sandy riverbeds, even at relatively low concentrations, novel bedform morphologies and altered processes emerge, including irregular bedform stoss erosion and dune “washout”, causing topographic bedform amplitudes to decline. We detail i) new mechanisms of plastic incorporation and transport in riverbed dunes, and ii) how sedimentary processes are fundamentally influenced. Hence, plastic is not a passive component of river systems; it directly affects bed topography, whilst increasing the proportion of sand suspended in the water, which has the potential to impact river ecosystems and the wider landscape. The resulting plastic distribution in the sediment is heterogeneous, highlighting the challenge of representatively monitoring plastic concentrations. Our insights establish a new branch of process sedimentology: plastic and sand interactions, set to be increasingly relevant amongst emerging challenges of the Anthropocene.