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, ECOLOGY OF RIVERINE FISHES IN REGULATED STREAM SYSTEMS 63 number and size of inflowing streams. The channel of the Colorado River was lowered for 148 kID below Hoover Dam by the removal of over 115 million cubic meters of bed material from 1935 to 1951 (Gottschalk, 1964). Another factor that influences riverine fishes in regulated streams is predation or competition from introduced species (Kimsey, 1957; Long and Krcma, 1969). At times, exotic fish are introduced for their sporting potential, and, therefore, the impact is immediate. In situations where there is no mass injection of fish, that natural movement and establishment of piscivorous or competitive species may take several years. Buildup of toxic gases (HzS) (Ford, 1963) or the depletion of a required gas (OZ) (Koryak, 1976) are delayed impacts in tailwaters due to maturation in the upstream reservoir. These problems gener~ ally affect tailwaters for only a short distance, as aeration tends to normalize the situation. The synergistic effect of two or more altered factors in down- stream areas appears to be a potential problem worthy of future study. A rather slow change in downstream habitat could very grad- ually shift conditions so that an exotic or natural competitor or predator gradually becomes more favored and therefore displaces a riverine species. As the habitat becomes marginal for the obligate riverine species, it becomes more preferred for the exotic competi- tor. At that point, the obligate species becomes more detrimentally impacted by the exotic species than by the habitat change. SUMMARY Whether a factor is immediate or delayed, the ultimate impact of regulation on obligate riverine fishes depends on the degree of change and the tolerance level of the fish to that change. In general, the least tolerant (most highly adapted) species tend to sustain the greatest impact of regulation (Pfitzer, 1963; Lewis, 1974; Trautman and Gartman, 1974; Blake, 1977; Edwards, 1978). In warm-water areas in the eastern and southern U.S., where diverse fish faunas are common, tailwater species are usually repiaced by other native species where changes are not drastic (Pfitzer, 1963; Spence and Hynes, 1971). Drastic changes, such as a reduction in summer temperature from warm to cold, usually displace all native fishes, and exotic species such as planted rainbow trout usually predominate. In the more arid Southwest, with its relatively depauperate but highly specialized fish fauna, regulation tends to replace native fishes with exotic species (Minckley and Deacon, 1968).