Declines of native big-river fishes of the Colorado River Basin, southwestern United States, have been attributed in part to emplacement of hypolimnial-release dams. Lowered spring-summer tailwater temperatures inhibit spawning and embryonic development, depress swimming performance and growth, and reduce survival of early life stages. We examined effects of three temperatures (10 C, 14 C, 20 C) on aspects of growth, development, and physiology of larval and early juvenile life stages of Xyrauchen texanus (Razorback Sucker), Catostomus latipinnis (Flannelmouth Sucker), Gila cypha (Humpback Chub), and Ptychocheilus lucius (Colorado Squawfish) in the laboratory. Lengths, weights, and specific growth rates of all species were significantly lower at 10 C and 14 C than at 20 C, and time to transformation from larval to juvenile life stage was delayed at colder temperatures. Transfer of fishes from 20 C to 10 C (simulating transport from warm tributaries to cold tailwaters) caused loss of equilibrium in youngest life stages. All these effects increase exposure of larvae to existing sources of mortality. Conservation of the big-river species in hypolimnial-release tailwaters will require thermal modification of dam discharges to lessen detrimental effects of cold water temperatures. Because such action may also benefit nonindigenous biota, recovery of the native fauna may ultimately depend upon implementation of comprehensive (and likely expensive) control measures against nonnative species.