Surveys were conducted in late July and early August 1999 to assess community characteristics, population demography of dominant species, status of endangered species, and characteristics of nonindigenous populations of freshwater bivalves in the lower Ohio River (LOR). Data will be used to analyze ecological effects of construction and operation of a new lock and dam at River Mile (RM) 964.4. Primary focus has been on a prominent mussel bed just downstream of the project at Olmsted, IL. Studies have shown that this mussel bed extends approximately from elevation 84.7 m (278 ft) nearshore to 69.2 m (267 ft) 2 2 farshore (elevations referred to mean seal level). Moderate- (20-50 individuals sq m) or high- (> 50 individuals sq m) density assemblages of mussels tend to occur at more central elevations on the bed from approximately RM 966.4 to 968.3. The downstream and farshore limits of the mussel bed tend to be less distinct than the nearshore and upstream limits. The community downstream of the project is extremely dominated by Fusconaia ebena. A mussel bed being monitored upstream of the project, at RM 957 near Post Creek, is clearly dominated by this species, but much less heavily than at Olmsted. Also, the abundance of F. ebena is higher farshore than nearshore at the Post Creek site. Communities at both Olmsted and Post Creek have been resilient to long-term occurrence (since approximately 1960) of the Asiatic clam, Corbicula fluminea, and more recent occurrence of the zebra mussel, Dreissena polymorpha. Both mussel beds are characterized by extreme dominance of just two year classes, 1981 and 1990, of the dominant species. Demographically complete sampling of F. ebena (Lea) at Olmsted was conducted from 1983 through 1999. Dominance of the 1981 and 1990 cohorts allowed length-to-age relationships to be estimated directly from length-frequency histograms. Two linear relationships adequately described growth rates from ages 2 through 17 years.