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.