Watershed land cover predicts the abundance of macroplastic and other anthropogenic litter in streams

Macroplastic and other types of macroscopic anthropogenic litter (AL; trash, particles > 5 mm) are pervasive across ecosystems, persistent in the environment, increasing in abundance, and can degrade into microplastics (particles < 5 mm). Rivers retain and transform AL prior to export downstream, but improved predictions of AL distribution and movement within rivers are needed. In particular, equations linking watershed land use with the abundance and assemblage of AL in rivers are lacking. We measured macroscopic AL abundance in three North American watersheds spanning a gradient of land‐cover types, with four sites per watershed, distributed from small streams to larger rivers. We quantified relationships between watershed land cover and AL abundance, measured as number and mass of particles per stream area. Across sites, AL abundance was 0.014–1.051 items/m 2 and 0.011–87.482 g/m 2 . Watershed impervious surface area and urban cover were significant predictors of AL by count (pseudo R 2 = 0.418), likely due to greater numbers of AL sources and enhanced connectivity between terrestrial landscapes and urban streams. However, relationships between land cover and AL mass were weaker (pseudo R 2 = 0.19–0.29), suggesting strong reach‐scale influences on AL mass. Habitat type influenced AL assemblages: floating and overhanging zones showed higher proportions of lightweight materials, while benthic zones contained more dense materials. Our results show watershed urbanization and habitat‐specific retention processes structure AL abundance and assemblage in rivers, improving predictions of AL distribution at larger spatial scales and informing AL prevention and mitigation strategies.