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FINAL REPORT, November 2003 High-flow Requirements for the Duchesne River morphology, bank height at the two surveyed sites was assumed to be representative of bank height through the full modeled reach. Model Calibration HEC-RAS models were constructed for three discharges whose water surface levels were observed at the 24-hour Camp study site: 2,450 fels, 1,050 ft3/S, and 226 fe/so One calibration model was constructed for the Above Pipeline site, 770 ft3/s. At Wissiup Return, models were constructed for two flows: 1,840 ft3/s and 1,040 ft3/s. Model calibration at all sites involved varying values of Manning's n at each cross-section station to attain a satisfactory fit between predicted and observed water-surface elevations at each cross section for each observed discharge. All modeled water-surface elevations were brought to within 3 cm of observed values using values of Manning's n appropriate for this stream type (Figure 12). In most rivers, n values typically decrease with discharge. Where the variation in n between the higher discharges at a given cross section was modest, the n value for the largest observed discharge was taken to be the appropriate n value for higher discharges. At 24-hr Camp, n values for 1,050 ft3 Is and 2,450 ft3/s were very different at some cross sections. At these cross sections (stations 470,668, and 709), the n value for higher discharges was estimated with the aid of a logarithmic curve fit to the n values determined for each of the three calibration discharges. Cross sections were assigned their estimated values of Manning's n for all modeling runs extrapolating stage at higher discharges (Table 8). Values of n in the chute modeled at 24-hour Camp was calibrated by adjusting n and discharge into the chute to match water-surface elevations and discharge observed in the chute during May 2001. Discharge into the chute at modeled discharge levels greater than those observed was determined by adjusting discharge into the chute until the total energy head at the most upstream chute cross section matched the total energy head in the main channel at the chute's upstream junction. As no stage-discharge measurements were made for stage-discharge calibration in the chute channel at Wissiup Return, cross sections in this chute were arbitrarily assigned moderate values of Manning's n between 0.035 and 0.04. When running the model for higher discharges, it was necessary in increase Manning's n in some cross sections to prevent the prediction of supercritical flow at certain discharges. Discharge into the chute for high-flow extrapolations was estimated by adjusting discharge into the chute until the total energy head at the most upstream chute cross section 27