The sizes and shapes of plastics in rivers

Limited data exists on physical and geometric properties of river litter. To resolve this, we reveal the physical-structural relationships of river litter, using two of the most comprehensive datasets generated to date. First, we dissect the properties of river litter using a detailed dataset of over 14,000 riverbank items, for which their dimensions (longest L₁, intermediate L₂, shortest L₃) and physical characteristics (mass, volume, density) are determined. These properties were then mapped onto a dataset of nearly 240,000 River-OSPAR items collected from 22 river and riverbank sites across four continents. We then identify the most persistent River-OSPAR litter categories, together with kernel density estimations of their principal dimensions and geometries. Results show that only 25 River-OSPAR categories account for 80% of all river and riverbank litter, with soft plastic pieces/films and candy, snack, and crisps packaging being the most abundant. Flat, 2D shaped macrolitter are the most persistent litter items, with 48% of the top 25 items sharing similar geometric properties: L₁ between 1 - 10 cm, and a flatness ratio (L₃ / L₂) of less than 0.4. In practice, these are flat objects with two larger dimensions and a third that is at least one order of magnitude smaller. This large, physically based dataset enables prioritisation of which shapes, sizes, and densities should be targeted by future plastic transport models, informing what plastics may be missing in current monitoring protocols, and the design of river clean-up technologies.