The Shapes and Sizes of Macroplastics and Other Litter in Rivers

Limited data exist on the physical and geometric properties of river macrolitter. To resolve this, we reveal the physical-structural relationships of river macrolitter using two of the most comprehensive data sets generated to date. First, we dissect the properties of river litter using a detailed data set of 14,052 riverbank items, for which their dimensions (longest L₁, intermediate L₂, shortest L₃) and physical characteristics (mass, volume, density) are determined. These properties are mapped onto a data set of 239,290 River-OSPAR items collected from 22 river and riverbank sites across four continents, using multivariate statistics and database density estimates. 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 crisp 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 <0.4. In practice, these are flat objects with two larger dimensions and a third that is at least 1 order of magnitude smaller. This large, physically based data set enables the prioritization 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 cleanup technologies.