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~ 66 P. B. HOLDEN decline. The dams blocked movement of the riverine fish. Colorado squawfish and razorback suckers both reportedly migrated up trib- utaries to spawn (Minckley, 1973), but recent evidence suggests that this may not be the case (HOlden, 1977). Therefore, whether the dams actually impeded necessary migration is questioned. Short-term movements were definitely impeded. Habitat alteration was the major detrimental factor wrought by the dams. The large number of reservoirs changed hundreds of kilo- meters of river to still water (Minckley and Deacon, 1968). Most of this change occurred in areas inhabited by the large river fishes. Colorado squawfish and humpback chub seldom are found in reservoirs, except immediately after impoundment or sporadically in later years. Colorado squawfish are extinct in the lower Colorado Basin (Miller, 1961; Minckley and Deacon, 1968; Minckley, 1973). Adult bony tail chubs and razorback suckers are found in some of the lower basin reservoirs where there are no large, inflowing tributaries, but recruitment is lacking (Minckley, 1973). They are seldom found in reservoirs, such as Lake Powell, where they can escape into large, warm rivers. This suggests that these fish may survive in reservoirs where they are trapped, but will eventually disappear due to a loss of recruitment. This information also strongly indicates the need these species have for large rivers. Habitat alteration due to cold tailwaters in summer also has adversely affected the Colorado River fishes. Native species were effectively eliminated from 105 kID of river below Flaming Gorge Dam and were replaced with a trout fishery (Vanicek et al., 1970). Similarly, the Colorado River below Glen Canyon Dam in Grand Canyon remains too cold for most native fishes for over 400 km (Holden and Stalnaker, 1975). A relic reproducing population of humpback chubs persists in the Little Colorado River, the largest tributary in the Grand Canyon; and occasional individuals are found in the mainstream (C. Minckley, pers. comm., Museum of Northern Arizona, Flagstaff). The scarcity of adult rare fishes in these cold tailwaters suggests that summer temperatures are colder than they prefer. Below Flaming Gorge, they could move downstream, where large tributaries produced near-normal temperatures, whereas in the Grand Canyon area they could migrate only to Lake Mead or the Little Colorado River. Recent studies below Flaming Gorge Dam have shown that the tailwater temperatures changed 5 or 6 years after initiation of dam releases (Mullan et al., 1976). At first, it was thought that this was the result of stratification in the reservoir (Holden, 1973; Bureau of Reclamation, 1976; Mullan et al., 1976), but a recent analysis indicates that the temperature change is due mainly to an increase in flow (Holden and Crist, 1979). Releases from Flaming