We studied molluscan fauna at two sites in the Włocławek Dam Reservoir, differing with respect to the hydrological conditions: a rheolimnic site with a comparatively strong water flow (Soczewka) and a limnetic site, dominated by wave action (Dobiegniewo). Within each site, we sampled molluscs from three substratum types (sand, a thin layer of mud on sand and mud). Samples were collected monthly, from April to October. The taxonomic composition differed between the sites, with Sphaerium corneum (L. 1758), Pisidium sp. C. Pfeiffer, 1821, Bithynia tentaculata (L. 1758) and Viviparus viviparus (L. 1758) being more abundant in Soczewka, while Potamopyrgus antipodarum (Gray, 1843), Marstoniopsis scholtzi (Schmidt, 1856) and Dreissena polymorpha (Pallas 1771) occurred exclusively in Dobiegniewo. S. corneum was a strong dominant in most of the samples, and variation in its abundance was the only cause of the observed differences among the various substrata. Strong differences in the total density and biomass were also observed between the two sites, the molluscs from Soczewka being much more abundant. This phenomenon was particularly apparent on the muddy substratum, which was the most suitable in Soczewka and the poorest in Dobiegniewo. Our study revealed that moderate water flow in Soczewka affected molluscs by improving conditions for them, especially on the muddy substratum.