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

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Discharge (m3/ s )
Figure 8. Relations between discharge and suspended sediment load, Colorado River near
Cisco, UT (USGS station 9180500). Symbols are the same as in Figure 6. Data from Iorns, et
al. (1964) and annual USGS Reports of Water Resources in Utah (published annually).
The scatter exhibited by the data in Figures 6-8 is relatively large, but in our experience, not out of
the ordinary. In most cases the data follow trends defined by a curve of the general form
Qs = k (Q - Qo)a (2)
where QS is the estimated suspended sediment load, and k, a and Qo are coefficients; k and a
determine, respectively, the position and slope of the sediment rating curve, while Q. determines the
shape; the larger the value of Qo, the more concave the rating curve. Specific values of k, a and Q.
are listed for each gauging station in Table 2. These values were chosen by trial and error to
minimize the difference between observed and predicted suspended sediment loads, as shown in
Figures 9-11. For the Glenwood Springs gauge and the Cameo gauge, the selection of specific
parameter values was straightforward, resulting in good correspondence between observed and
predicted suspended sediment loads over the entire range of discharges (Fig. 9). For the State Line
and Cisco gauges the selection of parameter values proved to be more difficult, partly because the
data from these gauges follow more irregular trends, and because in the case of the Cisco gauge, the
data do not appear to be stationary over the period of record. The equations for these two stations
(Table 2) were formulated so that the curves gave positive values of QS over most of the range of Q;
this meant that the curve did not necessarily pass through the lowest values. As a result, the
correspondence between observed and predicted suspended sediment loads at the State Line and
Cisco gauges is poor at low discharges, but moderately good at high discharges (Figs. 10 and 11).
Fortunately low flows carry relatively little of the total annual sediment load, so the potential error
associated with estimates in this range of discharges is not of great concern.
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