Population densities of the Asiatic clam, Corbicula fluminea, in the thermally influenced areas of the New River at the Glen Lyn power plant, Glen Lyn, Va., increased from zero before 1975 to approximately 11 522/m² by February 1978. Since their establishment, clams in the thermal effluent have served as a source of propagules for clam invasion into other areas of the New River during the warmer seasons.

Copper was more toxic (LC₅₀ = 0.04 ppm) to the clam than zinc (LC₅₀ = 6.04 ppm) or copper-zinc simultaneously (LC₅₀ = 0.05 ppm) in 96-h static bioassays. Ninety-six-h LC₅₀ determinations in artificial stream bioassays were at least one order of magnitude higher when compared with the static test values (the LC₅₀ values for copper and copper-zinc combined were 0.49 and 3.06 ppm, respectively). The LC₅₀ values appeared also to be the effective concentration at which at least 50 percent of the clams demonstrated a gaping response and cessation of filtering activity. Elemental bioaccumulation of copper and zinc was more prevalent in the visceral tissue than in concentrations measured from clam valves or from the water in both laboratory bioassays and field determinations. When copper bioaccumulation in the viscera increased to approximately 275 to 400 ppm, gaping and a lack of filtering activity were evident in 96-h tests. Behavioral mechanisms—cessation of filtering activity and the closure of valves during stress—were considered to be likely factors in producing the higher LC₅₀ values, when compared with other invertebrate responses.