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1. SPAWNING BAR The Spawning Bar study site, RM 16.5 to 17.0, was first used in an instream flow analysis by Prewitt in 1982. Rose, in 1983, re-described the area with eight new transects and used the IFG4 program for hydraulic simulation. Wahle, in 1984, made four modifications to the data decks developed by Rose (Table 1), recalibrated the decks and reran the modified data decks through the HABTAT program. The data deck used in the present analysis retained three of the modifications made by Wahle: the corrected stage-of-zero flow (SZF) values, the compatible substrate code, and the water surface elevations. The fourth modification, the addition of 4.06 feet to the bed elevations in transects one and two, was not justified or documented by Wahle, and therefore was not retained. Applications of PHABSIM require a determination of relative channel stability because the hydraulic simulation programs assume erosion and deposition of sediments remains constant over time such that a balance of channel forming processes exist. The geomorphic stability of this area was evaluated through the use of aerial photography (Wahle and Johnson 1984) to delineate geomorphic zones based upon geology, channel slope, pattern, shape, topography, vegetation, and land use. The analyses of photographs, 1.5 miles downstream of the study site, indicated very minor changes in mid-channel bars between 1961 and 1982 and therefore, it was concluded that the study site was geomorphically stable. While flow regime essentially dictates the size of the channel, the amount of sediment and the ability of the system to transport it, jointly determines general channel shape. In the case of the Yampa River, the sediment load supplied is balanced by its ability to transport that load, and thus it is considered in a state of dynamic equilibrium. Andrews (1980) and O'Brien (1987) concurred in the finding that the Yampa River is in long term equilibrium. Calibration The Spawning Bar study site data were divided into three data decks to accommodate an emergent island (Figure 1). Transects 1 through 5 were divided into ri9ht and left channels, with thirty-five percent of the flow running through the right channel (looking downstream), and sixty-five percent flowing through the left. Transects 6 through 8 were undivided and modeled as a single deck. A test was performed on transects 6 through 8 in an attempt to determine the best physical habitat simulation approach (IFG4, WSEI4S/IFG4 combination, or one flow IFG4) for calibration and analysis of this historic site. It was assumed that the program which calibrated the best (i.e., maintained velocity- adjustment-factors (VAF's) closest to the range of 0.9 to 1.1) would also have the most accurate predictive capability and should therefore be used for the analysis. Five data decks were developed for the test. Two decks (A and B) retained three sets of velocity and water surface elevation (WSE) measurements. Deck A 3