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Urban rivers are hypothesized to be major transporters of plastic pollution into lakes and oceans, with storm events playing a pivotal role. However, few studies investigate microplastic and macroplastic contamination and transport across a river basin, and how it varies with flow. Here, we sampled microplastic (less than 5 mm) and macroplastic (greater than 5 mm) from four sites along an urban river in Ontario, Canada, during baseflow and stormflow. To contextualize their fate and transport through river reaches, we sampled macroplastic stored in the riparian zone, overhanging vegetation, floating in surface water and riverbed and sampled microplastic from the surface water, water column and sediment. At baseflow, most macroplastic was found in the riparian zone (ranging from 0.1 to 4.7 pieces per m²). During stormflow, concentrations (micro and macro) rise and fall with discharge. Moreover, the composition of microplastics in the water column shifts from fibre- to rubber-dominated during higher flows. The mobilization of denser (e.g. rubber) particles during flow is consistent with greater water velocities during storms. Finally, using our data and flow patterns from 2022 to 2023, we estimate that approximately 522 billion microplastic particles and 20 754 macroplastic items, equalling approximately 36 000 and 160 kg by mass, respectively, are transported to Lake Ontario annually. This article is part of the Theo Murphy meeting issue ‘Sedimentology of plastics: state of the art and future directions’.