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.. 68 P. B. HOLDEN It appears possible that the major cause of the loss of reproduction is reduced flows and resulting ramifications. The detrimental effect of low flows on reproduction of Colorado squawfish was recently noted in a portion of the Green River in Utah. Young-of-the-year squawfish had been found in the Green River between Jensen, Utah, and Desolation Canyon, a distance of about 160 km, in the early 1970s (Holden and Stalnaker, 1975). Studies in 1977, a drought year with low natural flows, located no young-of-the-year squawfish in that area, but numerous juveniles of the 1975 and 1976 year classes were found (Holden, 1977). A study in 1978, a normal water year, found young-of-the-year near Jensen, but the remaining area was not sampled (Holden and Crist, 1979). These data indicate that in this portion of the Green River, espe- cially at Jensen, squawfish reproductive success is correlated with flow and that during a low-flow year (1977), reproduction was not successful, whereas it was successful during more normal- or high-flow years. Glen Canyon Darn has altered the Colorado River in Grand Canyon from a warm, turbid river with tremendous seasonal flow variations to a cold, clear river that fluctuates 2-3 m daily due to power demands. The native fishes below Glen Canyon Dam have largely been replaced by stocked rainbow trout. Much of the change in the fish fauna can be attributed to the cold summer tailwater temperatures (Holden and Stalnaker, 1975), although the tremendous daily fluc- tuations must also be partly responsible. Spawning of most of the native suckers and minnows is presently restricted to tributaries. Juveniles of these species are seldom found in the mainstem. It appears probable that the ephemeral nature of the habitat, caused by the rapid, daily fluctuations, is the major factor preventing juvenile fishes from utilizing the Colorado mainstem. The importance of delayed impacts of regulation on native Colorado basin fishes is difficult to assess. The major area where such factors may be important is the Colorado River below the inflow of the Gunnison River at Grand Junction, Colorado, over 160 km below the Curecanti Darns. As mentioned, Colorado squawfish no longer reproduce in this area, although adults are present. Several factors other than decreased flows are present that may be affecting the squawfish. Exotic species, especially green sunfish (Lepomis cyaneZlus) and largemouth bass (Micropterus saZmoides), are relatively common in this area (Kidd, 1977), much more so than in other areas of the upper Colorado Basin (Holden and Stalnaker, 1975). The primary reason for their abundance is the presence of flooded gravel pits that are joined to the river near Grand Junction, Colorado. These permanent backwaters afford the centrarchids with spawning and