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4. Discussion The layout of the Quarry study site may have contributed to underestimating WUA at higher flows. At flows over 9000 cfs most of the stream channel is a deep run habitat with a few strong eddies. At these flows a few backwaters are created when the water backs up into draws on the north bank. These habitat features are located between transects and not reflected in the model. Although they may not have affected WUA substantially, these type of backwaters are thought to be valuable at high flows when fish may move out of the main river channel into low velocity areas (Wick 1983). The optimum depths and velocities for Colorado squawfish and razorback sucker life stages examined in this report occur around 3-5 ft and 0-1.68 ft/s. According to our field data, these velocities and depths occur around 5000 cfs. This is consistent with the predicted WUA's which maximize between 3000 and 6000 cfs. A broader range of habitats exist at flows under 5000 cfs. Narrow runs with higher velocities, large eddies with slow velocities and large backwaters occur at these lower flows. At flows of 1000 cfs and less the optimum depths of 3-5 ft are not available, and even though lower velocities are prevalent, there is a decrease in over all suitability. The majority of the site is a sand/silt substrate which correlates to a suitability of 1.0 on the substrate curves. The cobble, rubble, boulder substrates are found along the shoreline and are only submerged at higher flows. Since these substrates have a lower suitability maximum WUA is pr.edicted at lower flows. With the exception of backwater modelling the P~ABSIM model appears to have accurately predicted physical habitat at the Quarry site. Historic mean monthly flow records (Appendix A) were obtained from the U.S. Geological Survey (USGS) for the years 1955 to 1986. The historic discharges were measured at the USGS "Jensen gage," near Jensen, Utah, on the Green River. These records indicate that modeled flows that produce maximum WUA are generally typical for the months used in this analysis. An exception, is May and June when flows are usually much higher than the flow the model predicts as optimizing WUA for razorback suckers size 4 (May-June). C. Ouray Studv Site In the summer of 1986, the National Ecology Research Center, Fort Collins, Colorado was given the lead responsibility to evaluate and document the capability of PHABSIM to accurately characterize depth, velocity and area of backwater habitats on the Green River. The objective of the study was to quantify changes in backwater habitat for young-of-year Colorado squawfish due to changes in streamflow for management, impact assessment, and mitigation purposes. 34