The river Rhine transports around 4,000 tonnes of macrolitter towards the North Sea each year.

Improper waste disposal has led to a rising accumulation of litter in aquatic environments, but long-term surveys of riverine macrolitter particles are rare. Here we report an analysis of continuous in-river monitoring over 16 months, using a custom-made physical-interception litter trap in the Rhine River, one of Europe's largest rivers. In collaboration with citizen scientists, we collected 20,339 floating macrolitter items larger than 1 cm that we classified according to international standards. We find that private consumers are the largest source of macrolitter at 56.4%. Plastics comprised 69.7% of macrolitter by particle number but only 14.8% by weight, underscoring the relevance of non-plastic materials. Based on our data, we estimate the annual macrolitter transport towards the North Sea to amount to 27 to 42.2 million items, or 3,010.5 to 4,707.5 t in the Rhine River, which significantly exceeds previous estimates. Macrolitter abundance fluctuated by a factor of 41 between biweekly samples and was higher at times of rising discharge. As such, we suggest that long-term monitoring is essential, and river discharge forecasts could serve as a predictor to guide clean-up efforts.