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~. . increased hydraulic gradient during recessional flows with the down- stream hydraulie control (e.g., sharp bend, or a channel contraction) being spatially fixed by erosion-resistant material. The high discharges must be able to entrain and transport the bar- forming gravels and cobbles to the backwater-affected locations. 2. On a mesoscale level, the backwa- tered and confined flows must cause a bar to be formed that provides for both the resting-staging and depo- sition-fertilization phases of spawn- ing during the recessional flows, when the local hydraulic energy in- creases. 3. Finally, on a microscale level, the in- teractions between the hydraulic en- ergy of the flows, the bar sediments, and sediment delivery from upstream must produce a clast-supported, open- framework conglomerate that forms a clean spawning substrate in which the constituent cobbles are at a con- dition of incipient motion. Existing data and observations allow only limited testing of the process-response model. Similar topographic, sedimento- logie, and fishery data were collected at the Alternate spawning bar at RM 18.5 (Figure 2). Hydraulic analysis and field observa- tions confirmed the presence of the macro-, meso-, and microscale components of spawning habitat formation. Hydraulic data for a range of discharges between 500 and 5,000 cfs at a riffle (cross section 214) and a tertiary bar (cross section 569) in the left chute channel are presented in Table 1. At a discharge of about 1,200 cfs, the hydraulic analysis indicated that the bed material at cross section 214 (D50 = 77 mm [3.0 in.]) should not be mobile (T*' < 1) and this was confirmed by field observation. No Colorado squawfish were captured in the associated pool. In contrast, at the same discharge, the hydraulic model indicated that the bed material at cross section 569 (D50 = 88 mm [3.5 in.]) should be mobile (7*' > 1) and this was confirmed by field observation. Average velocity in the pool upstream of the spawning bar at this dis- charge was about 2 ft/sec, and therefore, sediment supply to the tertiary bar in the chute channel would have been low. A tu- I~ 128 Rivers . Volume 4, Number 2 April 1993 TABLE 1 Hydraulic and incipient motion data at Alternate spawning bar, RM 18.5, Yampa River. Discharge (cfs) Dimensionless grain shear stress (T *' )a T*c = 0.03 T*c = 0.047 Velocity (ft/sec) Cross section no. 2l4b; median grain size (D50) = 77 mm (3.0 in.) 5.1 1.6 4.1 0.9 3.9 0.8 3.5 0.5 3.9 0.5 500 800 1,207 2,000 5,000 1.1 0.6 0.5 0.3 0.3 Cross section no. 569c; median grain size (D50) = 88 mm (3.5 in.) 500 6.4 1.9 1.2 800 7.6 2.4 1.5 1,207 7.9 2.6 1.6 2,000 8.7 2.9 1.9 5,000 6.9 1.3 0.9 a When T*' > 1 the bed material is mobilized; T*c is the dimension:ess critical shear stress. b Cross section 214 is the downstream riffle. c Cross section 569 is a tertiary bar. berculated adult Colorado squaw fish was captured in the pool associated with the tertiary bar at cross section 569. The anal- yses conducted at the Alternate spawning bar at RM 18.5 appear to validate the pro- cess-response model at this known Colo- rado squawfish spawning site. During the recessional stage of the 1992 runoff season, adult Colorado squawfish were captured at two other locations; one in the lower Yampa Canyon (Mathers Hole, RM 17), and the other on the Green River in Gray Canyon (approximate RM 155). Both sites appear to fit the requirements of the process-response model. A dissected backwater-induced bar is present at Math- ers Hole and at a discharge of about 600 cfs, 15 adult Colorado squaw fish were cap- tured in a pool at the downstream end of a chute channel. Of these squawfish, 14 were ripe and tuberculated males and 1 was a ripe female. No hydraulic analysis of this site has been conducted to date, but the process-response model indicates that this bar could be a spawning bar at an, as yet, unknown range of discharges. At the Gray Canyon site on the Green River at