The conundrum of dams to freshwater mussels in small rivers.

Ohio Journal of Sciences (abstr.) 118: A28 The 90,580 registered dams in the United States are an average of 56 years old and provide drinking water, irrigation, hydropower, flood control, and recreation to our communities and economy. Each dam also creates areas of impoundment that together compose about 17% of stream reaches. The relationship between stream flow and freshwater mussel (Unionidae) assemblages was assessed in two river systems impacted by dams, the Cuyahoga River that famously led to the Clean Water Act, and the nearby Mahoning River, currently considered one of the most degraded systems in the nation. Timed searches throughout the length of both rivers were made to characterize variation in the assemblages of mussels above and below dams. Dams and their impoundments isolated populations, but also homogenized flow rates and reduced variation in habitat types for long stretches of river downstream even as they partially protected downstream areas from scouring floods. A moderate assemblage diversity (ca. 10 species) was found flowing reaches distant from dams. Headwater dams, which were built as reservoirs or for flood control, induced unexpected and a perhaps insidious decline in mussels as many adult mussels die likely from old age with little recruitment. As an outcome, species generalists like Lampsilis siliquoidea and Pyganodon grandis have become dominant even where water quality improved. Throughout the nation, single stream studies continue to document enigmatic mussel loss, where removal of point source problems fail to lead to improvements in the fauna. Flow rate variation may be an important culprit.