Spatial and temporal patterns during reproduction and early life history of fishes were studied in a manmade cooling lake. Lake Columbia, impounded in 1974, near Portage, Wisconsin, has an area of 190 ha, a mean depth of 2.1 m, and a 15C temperature gradient derived from the thermal effluent of a 527-MW fossil-fueled generating station which began operating in 1975. The lake was initially colonized by fishes when filled with Wisconsin River water. Observations suggest a decline of species diversity of the fish community due to direct action of upper lethal temperatures, absence of colonization by warm-water, lake-dwelling species, and lack of recruitment for certain species. Spatial and temporal patterns of spawning of black crappie were altered by a rapid rise in water temperatures following plant start-up after a three-week shutdown. Elevated temperatures subsequently shortened the spawning season, induced resorption of ova, and caused loss of secondary sexual characteristics. After initially drifting with water current, juvenile stages of sunfish and gizzard shad responded to changes in the thermal gradient by horizontal and vertical shifts in abundance.