Preliminary results from an active combined sewage overflow event in the City of Ottawa, discharging to the Ottawa River, suggest microplastic concentrations are approximately 400 times greater during these active discharge events. The city of Ottawa Ontario and Gatineau Quebec have over 50 combined sewage overflow locations that direct sewage overflows into the Gatineau and Ottawa Rivers. One overflow location in downtown Ottawa has an indication signal of active overflow events. Baseline microplastic concentrations were measured at this location in addition to concentrations during an active overflow event. The baseline concentrations were 0.41 microplastics per cubic meter, with concentrations increasing to 167 microplastics per cubic metre during the overflow event. This demonstrates that sewage overflows are a significant conduit of microplastics to river environments and presents the necessity for more detailed investigations and analysis on this pathway of microplastics to freshwater systems.