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Chapter III Affected Environment and Environmental Consequences 24 The pre-dam aquatic ecosystem supported an array of native and non-native fish species. At the time of dam closure in 1963, eight species of native and eight species of non-native fish were present. By 1968, non-native fish species outnumbered native species, with trout dominating the now cold-water system immediately below the dam, and native species declining or becoming extirpated. The reasons for extirpations or declines of the native fishes are undoubtedly complex, but principal known factors are competition and predation by non-native fish, habitat changes including temperature changes, and a fragmented ecosystem brought about by construction and operation of Glen Canyon Dam. Prior to the dam, the principal source of energy to the river was the large quantities of woody and organic material and nutrients washed from the surrounding landscape. There was little primary productivity in the river because of reduced light penetration from consistent turbidity. In the post-dam river, the biological foundation (primary producers) of the aquatic system below Glen Canyon Dam is Cladophora glomera/a, a filamentous green alga. Clear river conditions created by the dam make possible the abundant growth of Cladophora, Together, Cladophora, diatoms, and associated invertebrates (Gammarus and insects) provide an important food source for other organisms in the aquatic food chain. CONSTRUCTION Work activities directly associated with installation of the temperature controls at Glen Canyon Dam would not adversely impact resources in Lake Powell or downstream in the Colorado River, The existing concrete caps on the trashracks towers would be saw-cut underwater. A relatively small amount of concrete (about 715 cubic yards) would be left in the bottom of the reservoir. The operating components of the system would be made mostly of structural steel, assembled off-site, hauled to the dam, and lowered into place from the top of the dam by a mobile crane. Some assembly and connection of the trashrack sections and trashrack cover plates would be completed underwater by divers. No coffer damming would be necessary to dewater the trashrack tower work locations. The main staging area, equipment storage, assembly, and construction yard would probably be located off-site at Federal facilities near the dam and the City of Page. There would be no ground disturbing activities at the construction yard that would put sediment into nearby water bodies. There would be protective measures included in the construction specifications to prevent possible water pollution from toxic materials, such as solvents, fuels, paints, hazardous materials, or other contaminants. WATER QUALITY AND AQUATIC RESOURCES The pre-dam aquatic ecosystem in Glen and Grand Canyons supported an array of native and non-native fishes. At the time of dam closure in 1963, eight species of native and eight species of non-native fish were present. By 1968, non-native fish species became more abundant than native species, with trout dominating the now cold-water system immediately below the dam, and native species declining or becoming extirpated. The reasons for extirpations or declines are undoubtedly complex, but principal known factors include