Thousands of tonnes of litter flow into the North Sea through the Rhine River each year, harming water quality, according to new research. The litter contains toxic substances like heavy metals and microplastics. For example, tyres carry zinc and other metals dangerous to the ecosystem in high amounts. Dr Leandra Hamann, lead author and zoologist at the University of Bonn, said, “Even natural, less persistent materials, such as worked wood, paper, cardboard and food waste may contain toxic or harmful additives.” Rivers like the Rhine carry much litter into seas and oceans. The study found the rubbish in the Rhine is very similar to that in other European rivers including the Thames. Large amounts of litter also threaten drainage systems and increase flood risks. Citizen scientists and researchers from the University of Bonn collected litter from the Rhine between November 2022 and November 2023. They found the river carried up to 4,700 tonnes of "macrolitter" yearly. Macrolitter means pieces larger than 25mm. A floating litter trap in Cologne caught nearly 2,000 kg of rubbish. Of that, 15% was plastic and 28% came from food or drink packaging. Dr Hamann praised the citizen scientists who helped gather and log data using a custom app. The study was published in Communications Earth & Environment. Dr Hamann added that the Rhine is a major source of Europe’s macrolitter but many other rivers still need to be studied further.