Plastic pollution found on riverbanks and floodplains originates either from terrestrial pathways (direct littering or dumping) or from riverine transport. Similar to some sediments, macroplastics are more easily deposited on the riverbanks along areas with stagnant water, low flow velocities, low channel slopes and high riparian vegetation densities (Bruge et al., 2018). During floods, riverbank plastic can be transported and deposited in floodplains as well, where it will most likely be buried by natural sediments (Lechthaler et al., 2020; Roebroek et al., 2021b). However, the retention mechanisms and time scales of plastic on riverbanks and in floodplains remain largely unresolved, especially as sedimentation rates and river characteristics play an important, yet unknown role (Lechthaler et al., 2021). Roebroek et al. (2021a) found retention times of shorter than six months, whereas Tramoy et al. (2020b), who investigated the use-by dates on medical packaging, showed that 37% of the litter was discarded before 1984 (Tramoy et al., 2020b). Both the spatiotemporal distribution and the retention times of macroplastics on riverbanks and in floodplains must be further explored in the future, for which we recommend upscaling and expanding methods. The latter may include using expiration dates or the language on packaging, sampling plastics at deeper layers in the riparian zone, and quantifying retention within specific features such as vegetation.

Vegetation along the riverbanks, in estuaries and floating at the river surface can function as both a carrier and a reservoir of macroplastics. Variations in river water level can result in the deposition of macroplastics in grass, bushes and trees (Williams and Simmons, 1996). The retention time of plastics trapped in riparian vegetation greatly depends on (seasonally varying) flow dynamics, and the characteristics of the vegetation, such as the height, plant density and spatial configuration, architecture, and geometry. These factors determine to what extent the water interacts with the vegetation, and in turn influences the likelihood of entrapment. A recent study observed that arboreal and shrubby vegetation entrap more macroplastic compared to herbaceous, reed and bush types (Cesarini and Scalici, 2022). The presence of branches, thicker foliage, as well as higher height and density compared to the other vegetation types might play an important role in macroplastic trapping and retention. Moreover, floating aquatic vegetation has been found to trap and carry considerable amounts of macroplastics downstream. In the Saigon river, drifting patches of water hyacinths were found to carry as much as 78% of the floating plastic (Schreyers et al., 2021). However, the effects of bidirectional flows due to the influence of tidal regimes, and the stranding of large hyacinth patches on the riverbanks further complicates this conception of floating vegetation as a transport carrier of plastic into the sea.

Dense macroplastic items (>1,000 kg/m3) can sink to the river bed in the absence of strong turbulence or currents. These negatively buoyant plastic types include items made of polyvinyl chloride, polyethylene terephthalate, polypropylene and polystyrene. During the transport process, initially buoyant macroplastic items, such as bottles, can fill with water or have biofilm grown on their surface, and thus lose their buoyancy and sink (Gabbott et al., 2020; Lechthaler et al., 2020; Al-Zawaidah et al., 2021). These items can be deposited in areas of the river with low flow velocities (Liro et al., 2020) and high natural sedimentation rates. The macroplastics can either be buried by natural sediments, which increases their remobilization threshold, be transported as bed-load transport, or get resuspended into the water column due to stronger flow velocities or turbulence (van Emmerik and Schwarz, 2020). Possible methods to measure deposited macroplastics include the analysis of dredged sediment (Constant et al., 2021), or analyzing samples from sediment traps (Enders et al., 2019; Saarni et al., 2021). There are only a few measurement methods to quantify macroplastics in riverbed sediments or as bed-load, yet, those are essential to identify macroplastic retention zones and understand their spatiotemporal distribution.

Only a small fraction of the world's rivers remain free flowing (Grill et al., 2019). Many river systems have been anthropogenically altered to guarantee shipping, water supply for consumption and irrigation, or generate power. Examples include dams, water inlets, groins, ports, sluices and canals. This type of infrastructure can act as accumulation zones for macroplastics. Larger plastic concentrations have been found upstream of dams, both in surface waters and sediment (Zhang et al., 2015; Castro-Jiménez et al., 2019; Watkins et al., 2019). Macroplastics become trapped around hydroelectric power plants, and are deposited on riverbanks and in sediments around groins (Skalska et al., 2020; Tramoy et al., 2020a). In urban water systems, plastics are trapped by rack structures, pumping stations or around bridges (Tasseron et al., 2020). Consequently, the water system becomes (partially) obstructed, which leads to higher upstream water levels (Honingh et al., 2020). Targeted infrastructure, such as floating litter booms or litter traps, is placed to intentionally accumulate plastics. Besides capturing plastic for removal, these traps also provide a rich source of data. Several assessments to estimate plastic mass transport, identify plastic mass flows, or characterize plastic waste, analyzed waste captured at such infrastructure in the Netherlands (Vriend et al., 2020), France (Gasperi et al., 2014) and Malaysia (Malik et al., 2020). Further work at the river system scale is needed to shed additional light on the role of infrastructure on plastic accumulation, and the plastic mass balance on catchment level.

Exorheic lakes, i.e., lakes that ultimately drain into the ocean, are found in river systems all over the world. Macroplastics have been found afloat in lake surface waters (Faure et al., 2015), buried in bottom lake sediments (Egessa et al., 2020), and deposited on lake shorelines/beaches (Faure et al., 2015; Egessa et al., 2020). These plastics either come from local activities (e.g., littering, fishing gear, direct wastewater drainage from a nearby urban area or direct surface runoff) or have been conveyed by rivers that discharge into the lake. Once a river flows into a lake, the plastics are carried by the lake surface currents and can concentrate in small temporary gyres (Faure et al., 2015). Wind-induced surface currents, especially during storms, also transport and deposit substantial amounts of plastic on the shorelines of lakes (Zbyszewski et al., 2014). The full retention capacity of lakes remains unresolved, and as many river systems worldwide feature lakes, quantifying how much upstream riverine plastics make it through a lake to the downstream river, is crucial for accurate estimates on riverine plastic transport and emissions.

Estuaries are influenced by tidal dynamics and freshwater discharge, leading to diurnally changing water level, salt concentrations, flow velocity, and flow direction (Savenije, 2012). The scarce observational evidence available suggests that plastic transport and accumulation is impacted by these dynamics at different timescales. Within tidal cycles, plastic flux close to the river mouth has been found to be about the same during both ebb and flood tide, suggesting that the actual net transport from rivers into the sea is very limited (van Emmerik et al., 2020c). As long as the net discharge is low, the plastics accumulating in the estuary have a growing likelihood of beaching on the riverbanks (van Emmerik et al., 2020a), getting entrapped in riparian vegetation (Martin et al., 2020), deposited in the sediment (Acha et al., 2003), or to degrade and fragment into smaller particles (Lebreton et al., 2019). Increased freshwater discharge, especially during flood conditions, may cause a peak of plastic export into the near coastal zone. Other work has suggested that plastics may be retained on estuary shores for years or even longer (Tramoy et al., 2020b). The role of estuaries on plastic transfer between rivers and the ocean, and the time scales of retention, remain largely unresolved. Future work should therefore explore the factors that drive plastic accumulation, (re)mobilization, and the time scales of retention in estuaries.

Plastics in river systems are commonly fragments of soft and hard plastics or foam (Castro-Jiménez et al., 2019; Tramoy et al., 2019; van Emmerik et al., 2020a). While some plastic items remain intact in the environment for decades (Tramoy et al., 2020b), others break down into smaller and smaller particles, due to physical fragmentation or chemical degradation (Delorme et al., 2021). Physical fragmentation is caused by mechanical influences such as waves, and abrasion through sediment (Barnes et al., 2009) but also chewing and digestive fragmentation through organisms (Dawson et al., 2018; Mateos-Cárdenas et al., 2020). Chemical degradation processes include biodegradation, photodegradation, thermo-oxidative degradation, thermal degradation, and hydrolysis. Exposure to UV-light and air accelerate the degradation process (Andrady, 2011; Waldschläger et al., 2020). During fragmentation and degradation different processes interact, depending on environmental conditions and properties of the plastic items (Andrady, 2011; Song et al., 2017).

Once broken down into microplastic particles, they could either be deposited in the sediment, be transported in the water column, or ingested by biota (Leslie et al., 2017). The interaction of fragmentation and degradation, and transport of plastics through different river compartments remains largely unknown, and a quantification method for degradation rates has not been found yet (Ford et al., 2022). We do expect smaller particles to be remobilized more easily, travel further downstream, and eventually reach the ocean. Microplastics have been found to be flushed away after floods or storm events (Hurley et al., 2018; Treilles et al., 2022), and do not experience the same interaction with many of the entrapment factors as mentioned in section Where are River Plastics Retained? The plastic reservoir within rivers may therefore be emptied on the form of fragmented secondary microplastic particles.

We have seen that plastics accumulate within various compartments in river systems, leading to the exposure of biota to plastics, with all its potential detrimental consequences. The longer plastic accumulates in natural ecosystems, the higher the number of species that will encounter plastics and therefore the higher the chance it leaves a negative impact. Recent evidence shows that macroplastics can be retained in rivers for several decades (Tramoy et al., 2020a), increasing the likelihood of ingestion by biota. In the Amazon, as much as 80% of the fish species had ingested plastic particles (Andrade et al., 2019), which can be life-threatening, especially when (toxic) waterborne contaminants are adsorbed to the plastics (Bellasi et al., 2020). Given the pivotal role of freshwater biota in the food chain, future research must continue to explore the degree and duration of the exposure of biota to plastics in order to clarify the threats they face in different river compartments.

During high wind speeds, high rainfall intensities, or high river discharges, accumulated plastics are likely to be remobilized and removed from retention zones. Ample studies have demonstrated an increased riverine plastic transport during flood events (Moore et al., 2011; Jang et al., 2014; Mihai, 2018; van Emmerik et al., 2019c; Roebroek et al., 2021b). Flood events inundate the riverbanks, floodplains and tidal flats and thereby (re)mobilize the plastics that have been accumulating in these reservoirs during normal conditions. Additional plastics also enter the river system during such events. Estuaries that during normal conditions tend to have a zero net transport of plastics into the oceans, export plastics during and just after a flood event (Liro et al., 2020). Even plastics buried within riverbed sediments can get removed from this reservoir when the active sediment layer of the riverbed is destabilized by extremely high water flow velocities (Ockelford et al., 2020). However, extreme events do not solely (re)mobilize plastics, they can lead to plastic deposition as well. When the water flow velocities in the river channel are much higher than above the submerged floodplains, plastics can become transferred from the faster flow (channel) to the slower flow velocity area (floodplain) and settle there (Ciszewski and Grygar, 2016). Another phenomenon, often referred to as the “Christmas tree effect,” whereby plastic debris carried by the flood becomes entrapped in riparian trees and is left behind when the water level drops again, demonstrates the reservoir filling capacity of a flood event (Williams and Simmons, 1996). Future research must explore the relative importance between (re)mobilization and (re)deposition mechanisms during extreme events.