9670

• UCREFRP Catalog Number: 9670
• Author: Pitlick, J.
• Title: Channel Monitoring To Evaluate Geomorphic Changes On The Main Stem Of The Colorado River.
• USFW Year: n.d.
• USFW - Doc Type: Boulder, CO.
• Copyright Material: NO

against the reach-average median grain size, DSO. The average D50 of the bed surface sediment in
the study reach is 0.069 m. Normalizing the individual values of z by the average D50 of 0.069 m
(and appropriate constants) gives the relation shown below (Fig. 26). This relation is identical to
the one shown above, except in this case the dimensionless shear stress is used as the dependent
variable; the coefficient in the best-fit relation changes accordingly but the exponent is the same
(0.40).

0.040

0.035

0.030
w

M 0.025

0.020

a

c 0.015

0.010

0.005
5

0.000
0 100 200 300 400 500
Discharge (m31s)
Figure 26. Relation between dimensionless shear stress and discharge, RM 176,
assuming constant grain size.
The threshold dimensionless shear stress, *,, for initial motion in this reach was
estimated to be 0.025, based on the relation of Mueller et al. (2005) with a reach-average slope
of 0.002. The relation shown above indicates that this threshold is reached at a discharge of 286
m3/s. This value is within 3% of the previous recommended discharge for initial motion, Q, =
278 m3/s (Pitlick et al., 1999; Pitlick and Cress, 2000).
The smooth curve defining the reach-average dimensionless shear stress was developed
for a single grain size, which simplifies the analysis, but does not account for the fact that the

•
•
? _ 0.40
? • Rz _ 0.511

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