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.. 60 P. B. HOLDEN produce warm summer temperatures; deep intakes generally produce cold summer temperatures. Summer temperatures have generally been lower than natural in most instances where native fishes have been adversely affected. Winter temperatures may be elevated below deep-release dams. ; Most obligate riverine fishes are adapted to a certain daily, seasonal, and yearly temperature regime, as are most fishes in gen- eral (Allen, 1969; Hubbs, 1972). Therefore, changes in the preferred thermal regime will affect them, and the greater the change, the greater the impact or effect. Species that are relatively specialized in terms of temperature requirements may disappear altogether because they are unable to tolerate the "new" regime (Pfitzer, 1963; Spence and Hynes, 1971; Trautman and Gartman, 1974; Edwards, 1978). They are often found further downstream, where preferred temperatures are still available. Other species that are preadapted to the regulated temperatures often become more abundant. In many situations, the altered thermal regime is sufficient for adults, but temperatures are not adequate for reproduction (Pfitzer, 1963; Hubbs, 1972; Zakharyan, 1972). Therefore, the tailwater population is either replenished from downstream recruitment or, in the case of species with short life spans and little movement capabilities, the species disappear immediately below the dam. The regulation of rivers also tends to alter the turbidity and general water chemistry in the tailwaters. Turbidity is generally lowered and, therefore, could affect fishes that require turbidity. No information in the literature indicates that this has actually been the cause of a species or population decline. Smith (1976) indicated that increased turbidity below dams in the Trinity River, California, caused siltation of salmonid spawning areas. Water chemistry changes can also be great. Generally, rivers below dams reflect the Water chemistry from the intake level of the upstream reservoir. Therefore, the types of chemical changes vary between dams, and seasonally at anyone dam, as the reservoir stratifies and overturns. Again, there is little information to show that these kinds of chemical changes have influenced fishes. Of great concern in most large dam situations is gas super- saturation in tailwaters. The Waters become supersaturated when spilled over high dams, trapping air and plunging it to depths where high pressures enhance solubility. This has been a major problem in the Pacific Northwest, where gas-bubble disease has been one of the major mortality factors on young salmon and trout (Ebel, 1969; Collins, 1976; Raymond, 1976; Robinson, 1978). It has been solved in some cases by the use of deflectors in the spillways (Collins, 1976). .