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.. ECOLOGY OF RIVERINE FISHES IN REGULATED STREAM SYSTEMS 61 Gas supersaturation in the St. Johns River of New Brunswick occurred when air was injected into the water as it passed through the turbines of a dam during low flows (MacDonald and Hyatt, 1973). Effects on young Atlantic salmon were similar to those found in Pacific salmon. A major habitat alte"tation that occurs in regulated streams in arid regions is dewatering, or reduced flows. Dams in the Southwest are often built for irrigation storage, and much of the water may be diverted into canals. The impact of reduced flows on fishes has received much recent attention. The primary impact of reduced streamflows is a reduction in usable habitat, especially critical types of habitat such as spawning areas (Smith, 1976). A consider- able amount of work has been conducted in the last few years delineating the effects of reduced streamflows on species of the family Salmonidae (Banks et. aI, 1974; Hazel, 1976; Mullan et al., 1976; Nickelson, 1976; Bovee, 1978). Also, substantial effort has been expended in trying to formulate methodologies to adequately pre- dict the impact of reduced flows (Orsborn and Allman, 1976; Stalnaker and Arnette, 1976; Bovee and Cochnauer, 1977). Zakharyan (1972), studying sturgeon in the Kura River system of Russia, indicated that reduced flows below irrigation dams dewatered spawning areas, hence impacting reproduction. The low flows also negatively impacted downstream migrating juveniles. Another type of habitat alteration below hydroelectric dams that may impact riverine fishes is daily fluctuation due to power peaking. Daily fluctuations in water level as great as 2 mare found in some areas. These changes in water surface elevation cause tremendous changes in available habitat. There is little information in the literature regarding the effect of this phenomenon, perhaps because altered temperature or some other factor is also present. One major concern is that daily fluctuations preclude the establish- ment of warm, rich, slow-moving areas usually favored by young riverine fishes. Kroger (1973) noted that abrupt reductions in flow stranded fish in the Snake River below Jackson Lake (Wyoming). While cold tailwaters may be detrimental to native warm-water species, the artificial environment often provides conditions suit- able for trout fisheries. The primary reason is that trout are maintained by stocking rather than by natural reproduction. Mullan et al. (1976) summarized the problems with tailwater trout fisheries in the upper Colorado River Basin. Major problems revolved around temperatures that were too cold, flows that were less than optimum, and excessive water velocities in relation to available shelter. The cold temperatures had a secondary effect of reducing the availability of invertebrate food organisms.