Making waves: Unraveling microplastic deposition in rivers through the lens of sedimentary processes

River networks are the major pathways for microplastic (MP) transport from terrestrial environments to oceans. It is essential to understand where MPs reside and how they move along river networks because of their potential to negatively impact ecosystems. However, the ability to quantify the water-sediment exchange of MPs, locations of deposition, and the time scales over which burial occurs is limited. To fill this gap, previous work on processes that control MP deposition are briefly reviewed in this Perspective paper, with the aim of enhancing our understanding of the dynamic interplay between flow, sediment transport, and MP movement through river networks. Detailed studies on MP deposition onto surficial sediment show that MP transport can be explained by the shear stress theory, hyporheic exchange, and bioturbation. Nevertheless, these processes cannot fully explain the observed distribution of MPs in deeper river sediments. It is proposed that bedform movement, channel reworking, bar formation, and aggradation/degradation at the river network scale should be included when estimating MP deposition. It is argued that incorporating data on MP distribution in riverbeds with fluvial geomorphological and particle transport models will improve the current evaluation of MP transport in river networks and their burial residence time distribution.