8274

• UCREFRP Catalog Number: 8274
• Author: Pitlick, J. and R. Cress.
• Title: Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
• USFW Year: 2000.
• USFW - Doc Type: Recovery Implementation Program Project 48-C,
• Copyright Material: NO

The model runs provide, among other things, estimates of the flow depth It and energy slope Sf at
each cross section. This information is used with (4) and (5) to calculate the dimensionless shear
stress at each cross section, and to formulate an empirical relation between Q and z, as shown in
Figure 23. A power-law fit of the average values of f for this site gives a curvilinear relation with
an exponent of 0.73. The value of the exponent is slightly higher than the uniform-flow value
(0.60) because, here, the Sf increases from 0.0018 to 0.0021 over the range of flows considered.
0.08
0.06
0.04
0.02
0.00
Discharge (m3/S)
3.0
2.5
2.0
ti/2
c
1.5
1.0
0.5
0.0
Figure 23. Relation between discharge and dimensionless shear stress, RKM 345.
From the above relation, we estimate that the threshold for initial motion in this reach is exceeded at
a discharge of -210 m3/s (recall that initial motion is the flow corresponding to f = 0.030, which is
the point where a few framework particles are just beginning to move). The bankfull flow (or the
point where most all of the framework particles are moving) is reached at a discharge of about 390
mN. These discharges are somewhat low in comparison to our reach-average estimates (see
below) because the grain size measured at this site (48 mm) is small in comparison to the reach-
average value for this segment of the Colorado River (57 mm; Table 1).
Similar measurements and calculations were done at two additional sites in the lower part of the
study area. One site is located at RKM 150, about 4 km downstream of the Cisco gauge; the other
site is located at RKM 106, about 4 km upstream of the Moab bridge. Water surface elevations
were measured at site RKM 150 on three different occasions, and at RKM 106 on four different
occasions. These measurements allow us to verify model results over a broad range of discharges,
rather than relying on a single set of observations as was done at the site discussed above. We
obtained excellent agreement between the observed and calculated water surface elevations at the
upper site (RKM 150) by choosing n values of 0.027-0.029 (Fig. 24a). Similar results were
obtained at the lower site (RKM 106) choosing n values of 0.025-0.026 (Fig. 24b).
34
0 100 200 300 400 500 600 700