Plastic transport and retention around river bifurcations

The transport of plastic in rivers is affected by a wide variety of factors, such as river discharge, wind drag, and tides. These dynamic processes include the travelling time or distance, retention time or location, and remobilization rate of plastic items, which can be quantified by using GPS-based trackers. However, these properties are still unknown in some specific locations, including river bifurcations where the changes in river discharge, flow velocity, and river morphology are significant. Here, we demonstrate the behaviour of plastic transport in a river bifurcation area not influenced by tides during flood season in the Red River system of Vietnam. While all trackers retained somewhere in the river after hours or days, we found that after 10.5 kilometers downstream of the bifurcation, 9 out of 10 trackers followed the main channel retained in the same approximately 6-kilometer-long river segment. Meanwhile, 50% of the 6 trackers that left the main channel to enter the tributary also retained in the same 2.5-kilometer-long river segment 9 kilometers downstream of the bifurcation. These findings are linked to the concept of rivers as plastic reservoirs, as none of the trackers that stranded on the riverbanks for several days was remobilized. Furthermore, the retention of trackers in the same area after leaving the bifurcation clearly indicates shared driving factors on plastic transport, which are likely the river discharge, wind direction or velocity, and river morphology. Our results underscore the need for future research on delineating exact accumulation zones of the plastics in riverbanks considering the effects of wind, river discharge, and river morphology.