Plastic pollution in rivers threatens ecosystems, increases flood risk due to its accumulations at hydraulic structures and its final emissions into the ocean threaten aquatic life, especially and probably most in coastal urbanized areas. Previous work suggests that plastic pollution in these urban rivers is influenced by hydrometeorological and anthropogenic factors. However, the transport dynamics of the plastics in such rivers are non-linear and complex and remain largely unresolved. Here, we show that tidal dynamics can be the main driver of plastic transport closest to the river mouth. Outside the tidal zone, rainfall and river discharge were identified to be more important drivers. We monitored plastic transport in the Odaw river, Ghana during the dry season. The Odaw drains the densely populated city of Accra and discharges into the Gulf of Guinea. Data were collected between March and May 2021 (dry season), using visual counting at four bridges along the river, of which two were located within the tidal zone. We explored the correlations between river plastic transport, and rainfall, tidal dynamics, and river discharge. Finally, we estimated the total plastic mass transport by using item-to-mass conversion data from previously published literature. We observed a peak in plastic transport at the upstream bridge within the tidal zone after an increase in rainfall (7.3 times larger). We found a gradient of the hydrometeorological factors driving plastic transport. Closer to the river mouth, tidal dynamics were more strongly correlated with plastic transport than upstream. The daily mass transport was estimated to be between 1.4–3.8 × 102 kg/d, which is lower than previous model estimates. These results add to the evidence of inconsistent correlations between plastic transport and hydrometeorological variables. Long-term monitoring data is required to further investigate this. The results also support the hypothesis that tidal dynamics are a crucial factor in controlling the emissions of plastics from rivers into the ocean. The findings provide a baseline for the Odaw river during the dry season and allow for comparison with the wet season. The approach adopted here also serves as a blueprint for similar urban river systems, regionally and globally.

<p>Rivers have been highlighted to play a key role in the transport of plastics into the ocean and especially, urban water systems act as a major source of plastic leakage into the marine environment.  However, observed data on plastic pollution in these urban water systems are scarce.  Our study focuses on the Odaw river basin which is the main drainage systems in Accra, Ghana, and is heavily polluted with macroplastics.  This is due to the high anthropogenic activities which have increased the indiscriminate dumping of waste into the water system.  These macroplastics in the water system accumulates over long periods, and a part of it is emitted into the ocean during high discharges.  Due to limited data on the quantification of macroplastics in the Odaw river, the seasonality and hotspot accumulation zones of these macroplastics in the river are unknown.  Such information is crucial for policymakers to prioritize future plastic debris monitoring and mitigation strategies.  We aimed to quantify the macroplastic emissions from the Odaw river into the ocean.  Using visual plastic counting from four bridges together with a hydrodynamic model, estimations for total yearly plastic fluxes through the Odaw river were made.  This model used rainfall data from 2016-2021 at the input nodes to simulate the discharges in the water system.  We estimated a total plastic flux from the Odaw river into the ocean between <em>2.6 x 10<sup>1</sup></em> and <em>1.7 x 10<sup>3</sup></em> tons per year.  Low plastic fluxes were observed at the bridges during dry periods, but a 10 fold increase in plastic fluxes was observed during and after a rainfall event.  Except for two sampling days, negative plastic fluxes were observed at the two bridges closest to the ocean due to the effect of tides.  These observations made the estimation of the total plastic emissions into the ocean challenging because of the bidirectional flow dynamics of the macroplastics at these locations.  The findings of this study provide baseline data for macroplastic transport through the Odaw river into the ocean.  Future research could focus on investigating the accumulation zones of macroplastics at the bridges closest to the river mouths due to the role of tidal dynamics on the river plastic transport and export into the ocean.</p>