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

A second effect of having more sand on the bed is to reduce the value of the critical dimensionless
shear stress, e, Recent flume experiments by Wilcock (1998) indicate that as the percentage of
sand on the bed increases the f, of coarse particles decreases, and their overall mobility increases.
Wilcock's data show that with 25% sand in the bed the fc is about 0.025, and with 30% sand, the
f. is about 0.020. To account for the potential effect of an increase in sand, we adjusted the value
of f,, downward to 0.025 for the reaches between Westwater Canyon and Big Bend (strata 6-3),
and downward further to 0.020 for the reach near Moab (strata 2). These values were chosen on
the basis of an increase in the proportion of sand in the subsurface sediment (see Fig. 18b), and
because they produce results consistent with our observations in the 15-mile and 18-mile reaches,
where flows equal to about half the bankfull discharge begin moving the framework gravel (Pitlick
et al., 1999). This adjustment has an important added effect of potentially lowering the threshold
for significant motion (or widespread bed mobilization), since that value is keyed to f.. Data from
the lower reaches of the study area (below Westwater Canyon) indicate that the cross-sectional area
of the channel increases markedly here, resulting in much higher bankfull discharges. These high
flows undoubtedly rework most of the bed material, but it seems possible that transport stages
corresponding to 1.5f c may occur at flows less than bankfull. Additional field work is needed to
test this hypothesis and to verify the geomorphic effects of specific flows in these reaches.
Equations (4), (5) and (7) were used with the measured values of w, h and S, and appropriate values
of f, and n, to estimate discharges corresponding to initial motion (Q) and bankfull flow (Qb).
The n values are based on results from step-backwater calculations at 10 different sites (the three
discussed above, plus the seven others discussed by Pitlick et al., 1999). Table 7 lists our estimates
of Q, and Qb for each reach (individual values for each cross section are listed in Table A-5). To
facilitate comparisons between strata, the results in Table 7 are grouped with respect to the two
major tributaries in the area, the Gunnison River and the Dolores River. Grouping the results as
such shows that the estimated discharges are similar within strata bounded by these tributaries. For
the reaches above the Gunnison River (strata 9-11), we estimate that discharges of 211 to 278 m3/s
will begin moving framework gravels, while discharges of 580 to 623 m3/s reach the bankfull level
(Table 7). For the reaches between the Gunnison River and the Dolores River (strata 6-8), we
estimate that discharges of 497 to 548 m3/s are required for initial motion, and discharges of 979 to
1320 m3/s are required to reach the bankfull level. For the reaches below the Dolores River (strata
2-5), we estimate that discharges of 561 to 659 m3/s will initiate motion, and discharges of 1500 to
2000 m3/s will reach the bankfull level (the median bankfull discharge of 2929 m3/s in strata 4 is
not particularly meaningful because flows of this magnitude have not occurred in historic time).
We do not list a discharge for initial motion in the lowermost reach (strata 1) because the bed
material in this reach is predominantly sand that moves essentially all year long.
To get a sense for the range in these estimates, Figure 26 and 27 show box plots of the distribution
of individual values of Q, and Qb within each reach (for those not familiar with box plots, the line
across the middle of the box indicates the median; the ends of the box represent upper and lower
quartiles; the whiskers represent the range in data, excluding outliers, which are indicated by open
circles). These plots indicate that the estimated values of Q, and Qb can vary by a factor of 3 to 4
within any given reach. The large range in Q, and Qb is not unexpected and can be explained by the
fact that local variations in channel width and depth occasionally result in anomalous values of Q,
and Qb. The true variation in Q, and Qb within individual subreaches is likely to be much less,
otherwise the basic geomorphic requirement of sediment continuity would not be satisfied, i.e. the
river would be transporting large amounts of sediment at some locations and not at others. If this
was occurring on a widespread basis, we would expect to see systematic aggradation and
degradation, when in fact the channel appears relatively stable overall.
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