Flooding has been shown to benefit native fishes in unregulated desert streams by disproportionately removing or displacing nonnative fishes. We examined how the presence of an upstream impoundment, Lake Patagonia, affected the fish assemblage in Lower Sonoita Creek, Arizona. Following flooding, catch and relative abundance of most nonnative species found in the reservoir increased in Lower Sonoita Creek. Non-native species not found in the lake decreased in the creek following flooding. Reservoirs may serve as sources of nonnative fishes to downstream areas inhabited by native fishes. Preventing nonnative fishes from escaping reservoirs during flooding would aid in conservation efforts of native fishes downstream.