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<br />. <br /> <br />. <br /> <br />The Government Bridge study site was modeled with a single IFG4 data deck by <br />combining 3 sets of stage-discharge measurements and one set of velocity <br />measurements. Results of the calibration are presented in Table 12. Only <br />sixty-seven percent of the velocity adjustment factors fell within the "good" <br />category. While the predictive water surface elevations were good (Table 11), <br />the velocity adjustment factors were poor. Overall, this data deck is rated <br />marginal to fair. <br /> <br />WUA Predictions <br /> <br />The calibrated data deck was run through HABTAT4 for adult Colorado squawfish <br />(Size 5, 400+mm). The resulting habitat versus discharge relationship (Table <br />13) suggests that adult Colorado squawfish habitat is maximized at 400 cfs <br />with a 90 percent habitat retention level at approximately 150 and SOO cfs. <br />Compared to the historic flow of record at the Government Bridge site <br />(Attachment 3), 400 cfs is exceeded 100, S7.7, 35.4 and 13.S percent of the <br />time in June, July, August and September respectively. As with the previous <br />study sites, the model's predicted optimal habitat versus discharge represents <br />a rare historic event in June and July. <br /> <br />Discussion <br /> <br />Bovee (19S2) suggested that in most cases the Instream Flow Incremental <br />Methodology (IFIM) should be used as a decision making tool to compare <br />alternative water management scenarios associated with a particular project <br />proposal. He further advised that IFIM should not be considered an ecosystem <br />model intended to generate a single solution but rather a predictive technique <br />best used to analyze relative impacts of different flow regimes. PHABSIM is <br />but one component of IFIM and predicts the availability of suitable physical <br />micro-habitats as a function of discharge. In this case, PHABSIM was applied <br />in an attempt to develop stream flow recommendations to optimize habitat <br />conditions for endangered fishes. Its relative usefulness, particularly on <br />large rivers, is problematic. The combination of hydraulic data collected <br />along stream cross-sectional transects, and SI curves assumes that suitable <br />habitat for fish species can be accurately described by three microhabitat <br />variables (depth, velocity and substrate), when macrohabitat variables, such <br />as channel stability, temperature and water quality, are assumed suitable <br />throughout a given range of flows. These assumptions, however, may not be <br />valid if other physical, chemical or biological variables are influencing <br />species distribution and abundance. Kaeding and Osmundson (I9S9), as well as <br />others, have pointed out that variables other than depth, velocity and <br />substrate may be of equal or greater influence on the actual microhabitat use <br />of the fish. In this analysis, the potential influence of other variables was <br />not included nor modeled. <br /> <br />Bovee (19S6) stated that the development of a valid utilization function <br />requires the unbiased measurement of microhabitat variables at specific fish <br />locations. Development of SI curves which accurately reflect the actual <br />microhabitat used by target fishes in large, turbid rivers is currently <br />constrained by state-of-the-art measuring techniques and sampling gear <br />limitations. Determining the exact location of fish in turbid rivers is <br />virtually impossible. Thus, microhabitat information collected from radio- <br />telemetered fishes employing customary USGS measuring techniques may not <br /> <br />15 <br />