Microplastic concentrations were investigated in water and sediment in the Ottawa River watershed in Quebec and Ontario, Canada. Microplastic concentrations were measured temporally during precipitation events in an urban creek in the City of Ottawa and on the main channel of the Ottawa River within the Ottawa/Gatineau urban area. The temporal events sampled included heavy rainfall, snowfall and snow melt with microplastic concentrations measured from these events compared to concentrations during non-precipitation events. The results showed sewage overflows contribute to large short-term inputs of microplastics to the Ottawa River, however, spring snowmelt presented the highest increase of microplastic concentration for both the urban creek and main channel of the river. Additionally, the research examined the spatial distribution of microplastics in river water throughout the Ottawa River at 105 sample points on the main channel and tributaries. An ANCOVA analysis demonstrated only two significant spatial factors related to microplastic concentration, with distance downstream from the river source and an increase of microplastics at boat launch locations. However, these were both only weak relationships. The research incorporated two citizen science projects to investigate microplastic concentration in water and sediment in the Ottawa River watershed, while evaluating the potential of citizen science as a complementary sampling tool for microplastic research. With robust project design and implementation, citizen science is an excellent complementary tool for examining microplastic concentration in freshwater environments as it can reduce research costs, while increasing spatial scope of microplastic projects. Additionally, increasing citizen science capacity in microplastic research and monitoring is a useful tool to engage volunteers and involve them in environmental education while contributing to advancing the understanding of microplastic pollution.