Many rivers in the world carry loads of debris to the sea. Typically in urbanizing areas with limited waste collection systems, the amounts of plastic amongst the organic material can be substantial. This has an impact on e.g. water quality, clogging of the (urban) water infrastructure, and deterioration of habitats. Flood risk is increased when debris is accumulating at bridges, weirs and trash racks. Understanding the behaviour of plastics in rivers and their impact on river conveyance is important for assessing both water quality issues and flood risks. In this study samples of waste in the Citarum river basin near Bandung, Indonesia where taken using single and double trawls. The analysis showed that plastic content of those samples can amount to more than 40% of the total debris weight. Additional laboratory measurement were performed on a debris flow through a trash rack in a straight flume, to assess the effects of plastic content on debris behaviour and consequent increased energy loss in flows through a trash rack. Though limited by scale effects and simplifications in the experiment, it was concluded that the plastic content in the debris increased the rate of clogging as well as the flow resistance for a certain amount of debris. The rapidly increased resistance is considered a problem in areas with intermittent, heavy rainfall where plastics that accumulated on river banks are mobilised and flushed into the water system. The carpet formed by plastic-containing debris appears to be more coherent and to contribute more to flow resistance and a higher risk of flooding. Future research will include a wider variation of flow geometries and debris composition.