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Impingement abundance monitoring was conducted at 15 Ohio River power plants as part of the Ohio River Ecological Research Program. Impingement rates were compared with environmental, power plant design, and operational factors that varied within and among the power plants, including water temperature, river flow and stage, change in flow and stage during the sampling events, volume of cooling water pumped, design pumping capacity, approach velocity, location of the intakes along the river, intake type, and intake configuration. The study demonstrated similarities in species composition, size distributions, and seasonal patterns over nearly 1,400 river kilometers of the Ohio River, results that were consistent with studies conducted nearly 30 years earlier. Dramatic annual differences in impingement rates during the 2‐year study indicated that impingement is largely a function of the recruitment levels of juvenile gizzard shad Dorosoma cepedianum, threadfin shad D. petenense, and freshwater drum Aplodinotus grunniens, measures that fluctuate widely in the Ohio River based on long‐term monitoring data and unpredictably based on current knowledge of the impingement process. The study also showed that most physical variables had little or no effect on impingement rates. Water temperature was identified through multiple regression analyses as the most important physical variable, with impingement tending to increase during the winter. Actual pumping rate during sampling events—the only factor evaluated that is under the direct control of the participating power plants—was one of the least important factors affecting impingement rates.

The Ohio River fish community was sampled three times (June, August, and October) in 2005 by electrofishing near 17 power plants encompassing nearly the entire length of the Ohio River (river kilometer 82–1,527). Six 500‐m zones were electrofished at each plant. Using a generalized additive model, we examined this data set of 306 electrofishing samples to determine how the abundance of 31 taxa varied over the length of the river. We also looked for trends in several measures of community health (catch rate, species richness, index of biotic integrity, and modified index of well‐being). Of the 31 taxa examined, all but three showed longitudinal differences over the course of the river. Based on the patterns for the 31 taxa and 5 community measures, we assigned each taxon or measure to one of seven classes. The largest class included 10 taxa that declined steadily in abundance from upriver to downriver. All community measures followed a similar pattern except that each measure increased near the downstream‐most plant. Conversely, only two taxa (Striped Bass Morone saxatilis and River Carpsucker Carpiodes carpio) showed the opposite trend (i.e., increasing in a downstream direction). Some taxa were restricted to the upper or lower end of the river or to certain reaches along the river. Species that were restricted to the upper river were primarily stream fishes (certain minnows Notropis spp. and several darters Etheostoma spp.). Conversely, fish species that were restricted to the lower river were primarily large‐river fishes as well as species that were typical of the Mississippi River fauna. Site‐specific differences in water temperature, Secchi depth, specific conductance, and dissolved oxygen were generally unimportant in explaining the observed patterns. Substrate type (percent cobble/boulder and percent silt) occasionally was important. The observed longitudinal trends appeared to be primarily in response to natural temperature and habitat differences between the upper and lower sections of the Ohio River. Received March 16, 2012; accepted March 10, 2013

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