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1 ~ Introduction ,~ Three fishes native to the Upper Colorado River Basin are currently listed .~ as endangered: Colorado squawfish Ptychocheilus lucius, humpback chub Gila cypha, and bonytail G, elegans. A fourth fish, razorback sucker Xyrauchen texanus, is extremely rare and a candidate for listing as an endangered species; current management efforts are directed toward preventing its further decline. The U.S. Fish and Wildlife Service, in cooperation with other federal, state and private organizations, recently developed a Recovery Implementation Program for the endangered fish s ecies P in the U er Colorado PP River Basin USFWS 1987 ( ) Amon g other things, the recovery program outlines a number of options for the management and recovery of these species. Effective management of water to benefit the endangered species is an important component of this recovery program. The magnitude of seasonal river discharge is believed to affect the reproductive success of Colorado squawfish. Jones and Tyus (1985) found a significant negative relation between the number of age-0 Colorado squawfish in fall in the Green River and the river discharge of mid to late summer--fewer young of the year (YOY) occurred after high summer flows. Age-0 Colorado squawfish abundance was lowest in 1983 and 19$4, years of unusually high runoff throughout the basin. Haynes et al. (1985) reported, however, that during those u same years relatively large numbers of Colorado squawfish larvae occurred in the Yampa River, an important spawning area for Colorado squawfish in the upper Green River Basin. These data suggested that high spring runoff increased Colorado squawfish reproductive success in the Yampa River, but continued high flows during the summer in the Green River downstream reduced the survival of the resulting young that had entered the Green from the Yampa as larvae. The present study examined the relation between maximum-annual discharge and the relative 1 t