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

the flow at the study site is confined with the baseflow channel; locally, this produces relatively
high mean velocities and high values of shear stress. The two points that lie far above the curve
in Figure 25 correspond to the riffle located in the span between cross sections 5, 6 and 7. At a
discharge of 125 m3/s, the flow through these sections is only about 1 m deep; however, because
the velocity is very high (up to 2.5 m/s), the energy slope through these sections is also relatively
high, i.e. roughly 50% higher than the reach average. With a slight increase in discharge at these
sections, flow begins to overtop the bar surface, causing an abrupt increase in width and
roughness, and a corresponding drop in velocity and shear stress. At a discharge of 175 m3/s the
flow through these sections still has an average depth of only about 1 m; however the area of the
channel bed that is inundated at this flow is considerably higher, thus the velocity and friction
slope decrease and rapidly converge on the reach-average values.
The smooth curve running through the data in Figure 25 defines a reach-average relation
for the boundary shear stress as a function of discharge,
ti=2.93Q1.40 (rz=0.51)
where r is in N/m2 and Q is in m3/s. The exponent in this equation (0.40) is somewhat lower than
values derived from field studies in other reaches of the Colorado River, but not anomalous in a
hydraulic or geomorphic sense (Pitlick et al., 1999; Pitlick and Cress, 2000). This equation can
be used with information on grain size to assess the validity of previous estimates of the
threshold for initial motion, based on (1), the relation for dimensionless shear stress, r*. Recall
that the relation for r* represents a force balance between the fluid stress, z, acting on the bed
versus the resistance provided by the unit weight of the grains, which scales with their diameter,
D. The stress given in the above equation represents the total fluid force averaged over the entire
channel reach; thus a reach-average estimate of r* can be obtained by balancing this force
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