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<br />Executive Summary <br /> <br />downstream and warms. it would quickly reach temperatures preferred by native and <br />endangered (warmwater) fish. Warmwater releases may not be made in every year to <br />help control competitors to the humpback chub. Release periods might be as short as a <br />month to prevent thennal shock as the chub descend into the main channel, or as long as <br />the May through September season to promote better growth rates. <br /> <br />Seoping Issues - Input received at scoping meetings identified several areas of <br />particular interest to the public and scientific community. More detailed discussions are <br />included in Chapter III of this assessment. In summary, this report finds: <br /> <br />. Lake Powell Fishery - Temperatures in Lake Powell are already low enough to <br />cause occasional winter-kill of threadfin shad, an important forage fish for the <br />lake (game) fishery. It was recommended that Reclamation carefully evaluate the <br />potential impact on this resource of releasing warm water from the reservoir. <br />Computer modeling of the reservoir shows that temperatures in the surface layer <br />of Lake Powell would be cooled by up to I oC if warm water releases were <br />maintained through the entire "summer" season (140 days). Shorter duration <br />releases would have even smaller effects on lake temperatures. Little or no <br />impact is expected to threadfin shad or the lake fishery. However, the existing <br />lake monitoring program would be continued to confinn or refute these <br />predictions. <br /> <br />. Primary Productivity - Field and laboratory experiments conducted for <br />Reclamation by Northern Arizona University indicate that the existing <br />macroinvertebrates in the downstream environment can tolerate warmwater <br />releases of20oC for at least 30 days (the duration of the study). Release <br />temperatures for the proposed action would nonnally be limited to a maximum of <br />150C. Computer modeling of Lake Powell shows that surface withdrawals from <br />the reservoir would increase nutrient and detritus release levels below the dam by <br />up to 300 percent; thus. potentially improving the productivity of the river <br />ecosystem. Studies of warm water releases at Flaming Gorge Dam suggest that <br />some minor changes (increases) may occur in diversity and abundance. The <br />effects would be carefully monitored through the adaptive management process <br />and the Grand Canyon Monitoring and Research Center. <br /> <br />. Endangered Fish - Under present conditions (the no-action alternative), <br />endangered fish do not reproduce in the river, young fish thennal shock while <br />entering the main channel from warm tributaries, and those that survive are easy <br />prey due to their reduced swimming efficiency. The proposed temperature control <br />alternative would allow releases to be made to improve the reproductive success <br />of humpback chub by reducing thennal shock with late-summer, warmwater <br />releases. Additionally, summer-long warmwater releases may be used in an <br />attempt to further improve conditions for endangered species; however, non- <br />native fish may also benefit from sustained releases. <br /> <br />Of primary concern is competition between carp/catfish and humpback chub. <br />Summer-long, warmwater releases would likely cause carp and catfish <br />populations to increase in the mainstem (along with other non-native species). <br /> <br />11I <br />