EAS, working with Pacific Northwest National Laboratory, conducted snorkel surveys in the Hanford Reach to locate, identify, and measure freshwater mussel species inhabiting near-shore habitats. The study was intended to provide a more comprehensive characterization of freshwater mussel species composition and distribution in the Hanford Reach, estimate mussel densities, and determine the age structure of mussel populations.

The field team surveyed 52 transects (for a total 28,482 m2 of river bottom) at water depths of 1 to 2 m on the Benton County shoreline of the Reach. Transects were located where mussel beds were known or suspected to exist and areas near Hanford Site decommissioned reactor facilities. At each sampling point, mussel species composition and the number of individuals observed were recorded. Mussel density was estimated based on near-shore habitat characteristics, including substrate size, substrate embeddedness, relative abundance of aquatic vegetation, and large-scale geomorphic/hydrologic characteristics of the Hanford Reach.

The field team observed and measured 201 live native mussel specimens. Three species belonging to the Anodonta genus were common in habitats dominated by sand/silt substrates. The western floater and Oregon floater were found in virtually all substrate types sampled in approximately 60% of the Hanford Reach. They were frequently encountered in locations where the riverbed was at least partially embedded. The California floater was found in areas primarily characterized by high substrate embeddedness and low river water velocities. The western pearlshell mussel was almost completely absent from the 2004 survey. However, several shells found in two transects indicate a few small populations of the western pearlshell mussel may still exist at water depths greater those surveyed.