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silts and minor amounts of clay to large cobbles. The surface samples show a coars- ening trend from the primary bar to the riffles or tertiary bars. The average median size (050) increases from 31 mm (1.2 in.) for the primary bar surface to 58 mm (2.3 in.) for the secondary bar and 73 mm (2.9 in.) for the riffles or tertiary bars. The 16th percentile average values (016) also coarsen as fines are removed by the bar dissection process. The 016 values for the individual mapping units are 18 mm (0.7 in.), 28 mm (1.1 in.), and 44 mm (1.7 in.). The 84th per- centile (084) average values do not vary substantially and range from 96 mm (3.8 in.) to 106 mm (4.2 in.). This coarsening of sediment from primary to tertiary bars demonstrates that removal of fines (flush- ing) was the result of recessional-flow bar dissection and not high discharge flushing as previously described (O'Brien 1984; Tyus and Karp 1989). Comparison of the subsurface grada- tions for the primary and secondary map- ping units also demonstrates the coarsen- ing process. The average 050 of the primary bar is 12 mm (0.5 in.) and that of the sec- ondary bar is 37 mm (1.5 in.). Because the riffles or tertiary bars were subaqueous fea- tures, it was not possible to obtain subsur- face sediment samples. However, signifi- cant amounts of finer sediments were not found in the surficial sediments, and there- fore, it was assumed that the riffles or ter- tiary bars did in fact represent the cleaned cobble substrate that is required for suc- cessful spawning. The mud drape deposited during flow recession is composed primarily of silt. The drape was observed between the coarser surface sediments on both lower primary bar and secondary bar surfaces. Once it is sunbaked, the drape greatly inhibits re- moval of the surrounding sediments, and even when the drape is moist it appears to create cohesion of the surficial sediments. Pits dug into the sediments that comprise the various mapping units visually con- firmed the coarsening of the bar sediments. The primary bar is composed of a poly- modal, clast-supported, very poorly sorted matrix conglomerate. The secondary bar is composed of a clast-supported bimodal well-sorted matrix conglomerate. The ter- tiary bars are composed of clast-supported open-work conglomerate. HYDRAULIC AND INCIPIENT MOTION ANALYSIS Hydraulic conditions within the study reach were evaluated for discharges rang- ing from 500 to 20,000 cfs. 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 hy- draulic parameters and incipient motion conditions for the range of discharges at specific locations within the reach. Hydraulic Model Development and Calibration The hydraulic model was developed from eight cross sections surveyed on 12 and 13 July 1991 (Figure 3). The model was verified using the results of stream gaging (discharge, depth, and velocity measure- ments) conducted on the morning of 13 July 1991 and water-surface elevations measured during the cross-section surveys. The total discharge in the river at the time of the gaging was approximately 1,200 cfs. I~ 120 The amount of flow in each of the branch channels around the midchannel bar at the time of the stream gaging is shown in Fig- ure 3. The surveyed cross sections provided a core around which to build the hydraulic model. Based on field observations, how- ever, hydraulic controls occur at several points within the reach that were not ad- equately described by the surveyed cross sections. In addition, a chute channel had formed across the secondary bar surface (Figure 3). To account for the local varia- tions not described by the surveyed cross sections, additional cross sections were added to the model. The locations and shapes of the addi tional cross sections were estimated from surveyed data, features vis- ible on aerial photographs taken in 1990 by the USFWS, and detailed notes taken during the field reconnaissance. Elevations for the estimated cross sections were based on general information from the field notes and were adjusted to obtain water-surface Rivers . Volume 4, Number 2 April 1993