The freshwater mussel (Unionidae: Bivalvia) communities of the French Creek watershed are nationally recognized for their importance to biodiversity. The goal of this research was to gather more information on the distribution and densities of these species throughout the river. This study utilized two-phase sampling with timed searches to characterize mussel species richness in a large number of sites and to calculate catch-per-unit-effort values. The results of the timed search were used to select a subset of sites for quantitative mussel surveys to estimate density and abundance. Starting in New York, the main stem of French Creek was divided into 5.6-km lengths, and one site was randomly chosen within each of those lengths, favoring habitat (large riffle/runs) for rare species and high species diversity. Snorkelers collected as many unionid individuals as possible, with a target search rate of 0.5 m² min^-1. Thirty-two main-stem sites were surveyed with timed searches. Mean species richness was 11.8 (SE = 0.94), and mean CPUE was 59.5 mussels per person-hour (SE = 9.32). Quantitative sampling was performed at ten sites using a double sampling design. Mean density estimates ranged from 0 to 27.98 m^-2. Abundance estimates range from 0 to 69,848 live mussels per site. For the main stem, we calculated regression models to estimate densities and abundances at qualitatively sampled sites based on the CPUE at quantitatively sampled sites. Extrapolation yields approximately 22 million mussels in the 2.04 million m² of large riffle-run habitat in the main stem of French Creek.