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)
100.0
98.0
c
0
m 96.0
m
w
94.0
92.0
0
b)
101.0
99.0
s
97.0
c
O
R
95.0
W
93.0
91.0
RKM 150

. A ------ .----5 ----------- ----------A ------------ A -------- .-A..---.....-I
100 200 300 400 500
Distance (m)
RKM 106
---A ------------ A----------- A ----------- a----------- o........... A----------- I
-.--o-------0------o------0------o-----o-----+
0 100 200 300 400 500 600 700
Distance (m)
Figure 24. Bed and water surface elevations in reaches near (a) RKM 150 and (b) RKM 106.
In deriving the relations between Q and r' for these sites, we obtained some unexpected results. If
we formulate the dimensionless shear stress using the observed depth, slope and D50, we obtain
values of f that are low in comparison to what might be expected for the conditions. For example,
measurements at RKM 150 during a near-bankfull discharge of 1229 m3/s give a reach-average f
of 0.031 (Fig. 25a), which is only slightly above the assumed threshold for initial motion (0.030),
and well below the value expected for bankfull flow. Similarly, observations at RKM 106 during an
overbank discharge of 1456 m3/s give a reach-average f of 0.029 (Fig. 25b). These results imply
that bankfull flows in the lower reaches are barely capable of transporting coarse sediment,
assuming a threshold for motion of f = 0.030. That situation seems very unlikely, and it is clearly
not supported by field evidence. There are three possible explanations for this discrepancy: (i)
these two sites are not representative of the river as a whole; (ii) the measured grain sizes are not
representative of the reach; or (iii) the assumed value of 0.030 for the critical f is not appropriate
35