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

formulated by calculating instantaneous suspended sediment loads, Q, from point measurements of
discharge, Q, and suspended sediment concentration, C,, using the following equation:
Q,=cCSQ (1)
where c = 0.0864 if Ct is expressed in mg/1 and Q is expressed in m3/s. Values of Q, are thus
given here in metric tons per day or metric tons per year. A relation between Q and Q, was then fit
by eye to these individual values (Fig. 5a), with the line chosen empirically to minimize the
difference between observed and predicted sediment loads (Fig. 5b). In this case, the fit was
straightforward because the data follow a simple log-linear trend.
a) b)
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T
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c 10,
9
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c 103
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C 0
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C 00
0
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10' o
101 102 103 10' 101 t03 104 105
Discharge (m3/s) Observed Sediment Load (tons/day)
1951-65 0
0p
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0
0 -
90
0
03 0
00.
0 0
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OZ 0 0
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egn. 3a
n,
Figure 5. Suspended sediment data for the Colorado River at Glenwood Springs (USGS station
9072500); (a) discharge and suspended sediment load, (b) comparison between observed sediment
loads, and loads predicted from equation 3a. Data from Iorns, et al. (1964).
Individual values of Q and Qs were plotted for each of the other gauging stations to produce a
series of sediment-load relations (Figs. 6-8). These data were further subdivided according to
whether the samples were obtained before or after the peak in the annual hydrograph. Typically, the
Colorado River carries much higher suspended sediment loads on the rising limb of the hydrograph
than it does on the falling limb. This trend is caused by a rise and fall in suspended sediment
concentration that precedes the rise and fall in water discharge. Another smaller subset of the data
includes samples taken in late summer and early fall that also have high sediment concentrations.
These late-season samples reflect brief but large increases in concentration caused by runoff from
localized thunderstorms or regional monsoon storms. Based on the number of such observations at
individual stations it appears that these events become more common towards the southwest
(downstream) part of the study area.
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