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". l04i-- "--- -- T-- --- ----- --,--- j--- ~-----+ --- - - \ ' _l------1~5Jl--- B6 -t-'Xs~8~-----~_XS~~,--- B4-i---:-~::---~- ~ ---- ~----.----- o 500 1(00 1500 HORIZONTAl DISTANCE (ft) = FlGUR E 7. Bed profile and energy grade line pro- files for discharges between 500 and 10,000 cfs in the right branch channel, RM 16.5, Yampa River. Tertiary bar or riffle locations are also shown. stress to mobilize the bed material; when 7*' > 1, particle mobilization is indicated. The effect of the riffles or tertiary bars on the local hydraulic conditions in the study reach can be seen in the velocity and dimensionless grain shear stress profiles for the right channel. At all discharges the riffles or tertiary bars are zones of locally higher velocity, and in general the profiles indicate that velocity increases with dis- charge at all locations (Figure 6). This is especially true in the pool upstream of the midchannel bar (upstream of cross section 5.1) where at low flow (500 cis) the velocity is < 1 it/sec and at high discharge (10,000 cis) it is >5 ft/sec. In contrast, depending on location, the dimensionless grain shear stress is greatest at lower discharges (Fig- ure 9). At cross section 0.8 the maximum dimensionless grain shear stress occurs at 500 cis, at cross sections 2.7 and 5.1 it occurs at 1,200 cis. Without exception, in both branch channels, the highest dimension- less grain shear stresses occur at the riffles ::r--J--~----r-~- i -_____r--- , I 100.. . !-i~...-'. I ~~~i~~~E~: :t----!J\.5:-=~~~~1~~5.=~~~I~ ~ ~~ I 84--<- ___ _ _J_ ----~--- o SCXl 1aD 1'5ID 200) 2SXl 3Co:) HORIZONTAl DISTANCE (ft) FIGURE 8. Bed profile and energy grade line pro- files for discharges between 500 and 10,000 cfs in the left branch channel, RM 16.5, Yampa River. Ter- tiary bar or riffle locations are also shown., ~ 3_0T, --------. ~ I ~ 2,51-~----- 0::: _ ____, t;) xso_a; ~ 2.0-j--- r ! (f) ~ '.5~__ ill 10 I :;i o ~ 0.5 UJ ::;; is XS2_7 ,(55.' I ---e-- lOOXlds = 0 -~------t~_;: r 2500 0.0 o soo ----I 1001 1500 2(0) 2SXl HrnlZONT AL DISTANCE (ft) FIGURE 9. Dimensionless grain shear stress (T.') profiles for three discharges in the right branch channel, RM 16.5, Yampa River. Tertiary bar or riffle locations are also shown. When T.' > 1 bed material is mobilized. or tertiary bars at discharges less than 10,000 cis. Comparison of the velocity pro- files (Figure 6) and dimensionless grain shear stress profiles (Figure 9) for the three riffles or tertiary bars in the right branch channel demonstrates that average veloc- ity alone does not adequately represent the force applied to the bed material by the range of flows that control the bar dynam- lCS. Spawning Habitat Identification Analysis of the variations in dimension- less grain shear stress (7*') over a range of discharges at each of the riffles or tertiary bars in the right branch channel provides an explanation for the association between the fish-capture sites and the riffles or ter- tiary bars. Considering first the down- stream riffle (Figure 3, cross section 0.8), where the median grain size (050) is 68 mm (2.7 in.), the dimensionless grain shear stress (7*') falls below 1.0 (incipient mo- tion) at discharges between 970 and 1,300 cis depending on the value of 7 *c being used (Figure 10). Dimensionless grain shear stress reaches a maximum at this riffle at a discharge of 800 cis when the hydraulic gradient is steepest and decreases as dis- charge increases and the riffle is drowned out by increasing backwater (Figure 7). At the time of the field sampling of the bed material in 1991, when the discharge in the river was about 1,200 cis, the bed ma- terial in this riffle was not mobile. No fish were captured in the associated down- stream eddy formed by the convergence of I M. D. Harvey et al. 123 II~