We conducted an intensive fish survey in the tailwater reach of a large Ozark river 30 years after its impoundment and compared the recent fish assemblage with those prior to impoundment and shortly (4 years) after impoundment. Our primary objective was to assess whether relatively short-term monitoring following dam construction can adequately quantify the long-term effects of impoundment on downstream riverine fishes. The preimpoundment survey (1962-1963) described a fish assemblage composed of warmwater fish species, predominantly Cyprinidae, Ictaluridae, Centrarchidae, and Percidae. Yoke darter Etheostoma juliae (34%), central stoneroller Campostoma anomalum (24%), and Ozark madtom Noturus albater (7%) were the most abundant species. The postimpoundment surveys of 1965-1966 and 1968 documented immediate changes in the fish assemblage. No Ozark madtoms and only four yoke darters were collected shortly after impoundment. Central stonerollers accounted for 45-50% of the fish collected, and both short-term postimpoundment surveys collected five species of darters (Percidae) that accounted for 41-42% of the fish collected. Thirty years after impoundment, we found that the tailwater fish assemblage was composed almost entirely of coldwater species. Ozark sculpin Cottus hypselurus and four species of introduced trout (Salmonidae) accounted for 98% of the fish assemblage by number during the 1995-1997 surveys. The rank abundance of species was negatively correlated between our survey and the preimpoundment survey but not between our survey and the short-term postimpoundment surveys. Many species that we collected (54%) are habitat generalists, and we did not collect 77% of the fluvial-specialist species that were present in historical collections. All postimpoundment surveys documented dramatically reduced species richness and diversity. We conclude that short-term monitoring following impoundment is inadequate to determine the impact of dams on lotic fish assemblages and suggest long-term postimpoundment monitoring to determine when a fish assemblage has stabilized.