To estimate residency and population sizes, endemic fishes were marked and released during October 1998 at confluence of Colorado River and Havasu Creek (Grand Canyon National Park, Coconino County, Arizona). Ripe Catostomus (Pantosteus) discobolus females and newly hatched fry confirmed late season (i.e., early October) spawning by this species. Reproduction by other catostomids was inferred through physical examination of adult C. latipinnis and by capture of a possible Xyrauchen texanus larva. The latter is an endangered species not considered a constituent member of the endemic Grand Canyon fish community; its potential presence in Grand Canyon will influence adaptive management of endemic and introduced fishes. Results of ANCOVAs (with fishing effort as covariate) indicated numbers of aggregating C. latipinnis were significantly associated with flows in Havasu Creek but not those in the mainstem Colorado River. The ecology of big-river endemic fishes in Grand Canyon is clearly linked to tributary outflows, and environmental cues which elicit spawning of native fishes are discussed.