Fisheries and Oceans Canada, in partnership with Environment and Climate Change Canada and the Ausable Bayfield Conservation Authority, has established the Unionid Monitoring and Biodiversity Observation (UMBO) network to track species-specific mussel abundance and population trajectories across seven watersheds in southern Ontario: Ausable River, Bayfield River, Grand River, Maitland River, Saugeen River, Sydenham River, and Thames River. The dataset includes results from 51 monitoring sites: 8 sites in the Ausable River, 1 site in the Bayfield River, 7 sites in the Grand River, 6 sites in the Maitland River, 4 sites in the Saugeen River, 13 sites in the Sydenham River, and 12 sites in the Thames River. Each river was sampled on a 5-10 year cycle, with Ausable River and Sydenham River sites having been sampled 3 times, and sites in all other rivers sampled twice.