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-, 2.0 .--.. E 1.8 ~ w 1.6 0:: t5 1.4 ~ 1.2 J: Cf) Z 1.0 ~ (9 0.8 ~ w 0.6 -I Z 0 0.4 Cf) Z w 0.2 :2 0 0.0 5 1000 DISCHARGE (cfs) -T--~=L_! I i l 't *c = 0.03 rr----r -'I i i . I -\ r 2 3 5 10000 FIGURE 12. Variation in dimensionless grain shear stress (T.') at cross section 5.1 for two values of dimensionless critical shear stress (T.) at discharges ranging from 500 to 10,000 cfs in the right branch channel, RM 16.5, Yampa River. The median size of the bed material is 74 mm (2.9 in.). showed that the velocities in the pool up- stream of the bar begin to exceed 4 ft/sec at a discharge of about 5,000 cfs. At higher discharges, the velocities in the pool are capable of transporting significant quan- tities of sand and gravel into the branch channels. ~issection of the bar during re- cessional flows maintains the continuity of coarse sediment transport within the can- yon reach. Coarse sediments (gravels and cobbles) deposited on the midchannel bar because of backwater conditions during one high discharge event are entrained during both the falling limb of the same event and on the rising limb of the next event. Coarse sediment transport by the Yampa River within the canyon reach is, therefore, an episodic phenomenon. Backwater conditions during high flows cause deposition of a heterogeneous mass of very poorly sorted (silts to cobbles) sed- iment that apparently is unsuitable as a spawning substrate. The unsuitability of the primary bar sediments for spawning is further enhanced by recessional flow de- position of a mud drape. Observation in- dicates that, at discharges that fully sub- merge the midchannel bar, sand-sized sediments are the coarsest particles being I~ 126 transported over the bar and through the backwater zone downstream of the bar. The sedimentological evidence (Figure 4) and the results of the hydraulic mod- eling (Figures 7 and 8) that show reduced hydraulic gradient with increasing dis- charge, indicate that previous conclusions regarding the range of discharges (19,500- 27,500 cfs) required for flushing of fines at RM 16.5 were in error. At the high dis- charges previously thought to be the flush- ing flows, the predominant process at the RM 16.5 bar is deposition, as was observed and measured following the 1983 peak dis- charge by O'Brien (1984). However, these very high-magnitude, low-frequency (> 20 yr) discharges obviously are significant with respect to formation and maintenance of the midchannel bar, especially because they are the discharges required to entrain the coarse sediments delivered to the Yam- pa River by debris flows from the small tributary drainages downstream of Oeer- lodge Park. Flushing of fines to prepare the clean cobble substrate required for spawning and egg adhesion is the result of a complex set of both erosional and depositional pro- cesses that appear to occur during reces- Rivers. Volume 4, Number 2 April 1993