Annual winter impingement of large numbers of freshwater drum (Aplodinotus grunniens) on screens of a power station intake on the Mississippi River led to an investigation of the cause of impingement. Impingement occurred most abundantly in winter and early spring and primarily involved juvenile fish. Pronounced drift of moribund and dead fish was found in the main channel above and below the power station in late winter. Laboratory studies indicated that juvenile freshwater drum became disoriented, incapacitated, and suffered increased mortality as water temperature dropped to 1 °C and below. In winter and early spring, temperatures were 0 °C in the main and side channels of the river, but pockets of water above 1 °C existed in some backwaters. Dissolved oxygen concentrations declined through the winter, becoming very low in some backwaters. An aggregation of fishes including freshwater drum was observed in the warmer backwaters. Variations in river flow and dissolved oxygen depletion in some backwaters were postulated to cause periodic disruption of the thermal refuges and an associated appearance of incapacitated and dead juvenile freshwater drum in the drift. If man-induced changes to the river eliminate backwater winter refuges, the ichthyofauna of the river could ultimately be altered.

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Isolation of causative factors has proved challenging in characterizing the physical, chemical, and biological effects of shoreline hardening on the nearshore environment because of logistical challenges in identifying comparable sites. Extensive shoreline hardening and interspersion with unaltered shores in the large, shallow lakes in central Wisconsin provide an opportunity to surmount this. We compared the effects of shoreline protection on wave climate, bottom topography and substrate, water quality, and plant and animal assemblages at five adjacent pairs of natural and armored (riprapped) shorelines. Armored shorelines were characterized by coarser, more compacted substrates with lower organic content; cooler temperatures with higher dissolved oxygen; and greater water clarity. Differences in physical and chemical properties likely influenced plant growth forms and fish feeding guild differences between riprapped and natural sites. For example, floating-leaved plants were more abundant and fish were nearly twice as abundant and were represented by larger individuals at natural versus armored shorelines. Substrate characteristics may account for the differences in water quality and plant and animal associations observed in this study. As shoreline property owners continue to install riprap as protection against erosion, we expect the nearshore environment and associated biological communities to increasingly reflect this practice.

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