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Measurements with a compass of the orientation of the a-axis of a-axis upstream-imbricated clasts was used to determine flow paths on the bar. A-axis flow-parallel clast orientation is very common in f1uvially transported gravels, but it is by no means a unique indicator of flow direction because a-axis f1ow.transverse orientation and b-axis upstream-imbrication is also common (Hein, 1984; Yagishita, 1992). The results of the measurements at Mathers hole are plotted on Figure 3.3. The data indicate that sediments that are transported from the reach upstream of the bar have a trajectory down the bar axis. However, at about XS 10, flow begins to diverge towards the right side of the bar. By about XS 9 significant flow diversion into the left chute channel occurs. Clast orientation follows the trend of the left chute channel to the downstream end of the bar where flows diverge over a slipface. On the right side of the bar the orientation of the clasts parallels the trend of the right chute channel. 3.1.3. Hydraulic and Incipient Motion Analysis Hydraulic conditions within the study reach were evaluated for discharges ranging form 300 to 32,300 ets. The upper end of the range corresponds to the estimated peak discharge during May, 1984. The analysis was performed using the one-dimensional HEC-2 water-surface profile computer model (U.S. Army Corps of Engineers, 1990). Results were then used to estimate hydraulic parameters and incipient motion conditions for the range of discharges at specific locations within the reach. Hydraulic Model Development and Calibration The hydrauJic model was developed from fourteen cross sections and supplemental topography surveyed on August 26-28, 1993 (Figure 3.1). The model was verified using water-surface elevations, 1993 high-water marks and other historic high-water marks measured during the cross-section surveys. Some of the historic high-water marks are believed to correspond to the peak of the 1984 flood. The total discharge in the river at the time of the survey was between 300 and 350 cfs, based on stream gaging conducted on the aftemoon of August 30, 1993 at the study site (Q = 344 ets) and on August 31, 1993 at Cleopatra's Couch (Q- 300 ets). The measured values were compared with the sum of the recorded discharges on the Yampa River at Maybell and the UttIe Snake River at Uly, which is believed to provide a reasonable approximation of Yampa River flows in the study area (George Smith, USFWS, personal communication). Calibration of the model with the measured water-surface profile and high- water marks was achieved by adjusting the downstream control, Manning's wnw channel roughness values, expansion and contraction losses and effective flow areas within physically reasonable values for the range of flows considered. 3.9 Resource Consultants & Engineers, Inc.