Wastewater discharge to rivers is a controversial practice that compromises water quality, aquatic habitats and human health worldwide. Here we show how untreated wastewater laced with microplastics and raw sewage is routinely discharged into UK river flows that are too low to disperse the microplastics downstream. These ‘dry weather’ spills lead to acute microplastic contamination of river bed habitats. Many aquatic fauna feed in the benthic zone, the quality of which affects the entire riverine ecosystem. All microplastic types accumulate to high concentrations on the channel bed until flushed downstream by floods. These findings pose fundamental questions about the sustainable management of urban wastewater. Treating the wastewater would shut down the major source of microplastic fragments and microbeads in such rivers and prevent their transport to the ocean. Riverine microplastic transport is primarily partitioned between: (1) continuous transport at low concentrations of synthetic fibres from treated wastewater effluent; and (2) episodic flood-driven transport of the full microplastic assemblage entrained from contaminated channel beds. Focusing only on the buoyant non-flood microplastic load can produce highly unrepresentative assessments of riverine microplastic contamination. Climate warming and urban population growth will intensify the microplastic burden on many river ecosystems as summer baseflows decline and wastewater fluxes increase.