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• and because it was thought to represent the typical hydraulic environment of Gray's Canyon. In addition, Rattlesnake Rapids restricted upstream movement at a relatively low flow and was not passable at high flow. This study site had a cobble/boulder substrate, high cliffs on the right bank and a small floodplain on the left. Included within the section were riffles and pool hydraulic features, characterized by six cross sections. Stratum (segment) "F" was represented by river mile 143 between Jensen and Ouray, Utah. Features at this site included sand substrate and no vegetated alluvial islands; the reach was suspected to have a rather shifting bed. Six transects were used to describe the hydraulic and habitat conditions. Access was through the Ouray National Wildlife Refuge. PHABSIM Data Collection, General Field measurements were conducted at all stations based upon estab- lished techniques (Bovee and Milhous 1978: Trihey and Wegner 1980). Velocities were obtained with a Marsh-McBirney direct-readout current meter. All differential leveling was performed with either Nikon or Geotech automatic levels. Depths were determined either with a calibrated topset wading rod or using a sounding weight and a USGS A-55 calibrated reel mounted on a 14 or 16 foot aluminum boat. • Measurements of the channel geometry velocity distribution and water. surface elevations were used as input to either the IFG-2 (Dooly 1975) or IFG-4 (Main 1978) hydraulic simulation computer programs. In short, the IFG-2 program used predictable backwater relationships to compute stage (water surface elevation) at flows other than the measured flow. From the stage and known stream geometry, depths, velocities and wetted substrate were determined for each of the measured points and/or verticals. The IFG-4 program also predicted depths, velocities and substrates but required a minimum of three measurements of stage and velocities to develop flow vs. stage and flow vs. velocity regressions. All field measurements were taken so that they could serve as input to either the IFG-2 or IFG-4 programs. Colorado River Depth, velocity, substrate and overbank condition data were col- lected at the Moab and Potash reaches during the weeks of July 1, July 14, and October 20. These measurements were successful in determining velocities and depths at all preselected cross-sections and verticals. The Black Rocks reach was sampled during the week of July 1 at a flow of 15,000 cubic feet per second (cfs) and was found unworkable because of excessive velocities. Water surface elevations and some moderate veloci- ties were determined there, but collection of a complete data set was not possible. 5