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<br />. <br /> <br />. <br /> <br />extremely high. During the summer the young HBC that survive in the mainstream tend <br />to occupy low-velocity, vegetated shoreline habitats; however, low winter survivorship <br />virtually eliminates the young-of-the-year HBC in the mainstream. Therefore, few if any <br />HBC spawned during the previous year are present in the mainstream in the following <br />spring. <br /> <br />Limited spawning of HBC occurs among other sub-populations in the Colorado River. <br />Valdez (1995) documented limited spawning success at 30-Mile Spring in upper Marble <br />Canyon, and few young HBC have been documented at Kanab Creek; however, these <br />sightings are rare compared with the reproductive success of those HBC that spawn in <br />the LCR. Catches in Kanab Creek do not indicate spawning in that tributary. Juvenile <br />chub have also been collected in the mouth of Bright Angel Creek, and in Shinumo <br />Creek (Valdez and RyeI1997). <br /> <br />Dietary analyses reveal HBC to be opportunistic feeders, selectively feeding on aquatic <br />and terrestrial invertebrates (Valdez 1995, Valdez and RyeI1997). HBC diet changes <br />over the course of the year in response to food availability and turbidity-related <br />decreases in benthic standing biomass over distance downstream from Glen Canyon <br />Dam (Stevens et a11997, Valdez and RyeI1997). Non-native Gammarus lacustris <br />occasionally comprise a large proportion of HBC diet, especially after high mainstream <br />flow events (Valdez 1995, Valdez and Rye11997) and Gammarus selectively feeds on <br />epiphytes (i.e., diatoms) associated with Cladophora glomerata, the dominant alga in <br />the upper reaches where clearwater conditions often prevail (Shannon et al. 1994). <br /> <br />Impacts of BHBF <br /> <br />High flows, such as the 1996 BHBF, had little detectable effect on the movement <br />patterns or distribution of adult HBC, and the 1996 BHBF did not appear to serve as a <br />spawning cue (Valdez 1997). The increased drift associated with that BHBF resulted in <br />an increase in Gammarus in HBC gut contents, an effect which is not surprising given <br />the opportunistic foraging behavior of this species (e.g., Tyus and Minckley 1988). <br />Given that this species evolved under the highly variable flow regimes that <br />characterized the pre-dam Colorado River, it is unlikely that short-duration BHBF flows <br />of 45,000 cfs could be demonstrated to affect subadult or mature HBC. <br /> <br />High mainstream flows may either benefit or harm younger HBC. High flows can <br />provide beneficial scouring of the benthos and rejuvenation of return current channels <br />and other mainstream backwater habitats. Based on results of the 1996 BHBF test, <br />scouring flows probably need to be higher to be of extended benefit. Acknowledging <br />that 45,0000 cfs flows of cold water can directly harm young HBC, most likely between <br />May through July, as larval and young HBC emerge from the LCR and occupy those <br />mainstream near-shore and backwater habitats, it's important to keep in mind that <br />under current mainstream temperatures, low winter survivorship virtually eliminates <br />those young-of-year humpback chub that do find their way into mainstream habitats. <br /> <br />1998 GCD Beach/Habitat Building Flow 14 <br /> <br />Biological Assessment <br />