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

downstream-diminishing effects of reservoir regulation, and the added input of sediment from
unregulated tributaries. Thus, while the hydrologic and geomorphic effects of reservoir operations
are very apparent in these and other data (c.f. Pitlick et al., 1999), the results presented in Table 6
suggest that these effects tend to diminish downstream.
Table 6. Summary statistics and results of Mann-Whitney tests for differences in annual discharge
and sediment load. The test statistic, z, is the standard normal variate andp is the significance
probability associated with particular values of z.
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USGS Period of Annual Discharge Annual Sediment Load
Station No. Record Q z p QS z p
-------------------------------------------------------------------------------------------------------------------
9072500 1934-1958 68 0.49
1959-1997 62 1.45 0.073 0.29
9095500 1934-1958 114 1.81
1959-1997 109 0.93 0.176 1.35
9180500 1934-1958 203 6.46
1959-1997
-------------------------------------- 197 0.67 0.252
--------------------------------------- 4.77
-----------.
2.84 0.002
2.07 0.019
2.15 0.016
-------------------
Although the above results show that the quantity of suspended sediment carried by the Colorado
River has in general decreased, there does not appear to be any long-term trend in the grain size of
the suspended sediment load (Fig. 15). A time-series plot of sand-fraction data from the Cisco
gauge shows that, while there is a very wide range in the percentage of sand carried in suspension
(0-70%), the amount of sand versus silt and clay have remained essentially the same since the early
1950s (Fig. 15). This result does not imply that the river is carrying the same amount of sand now
as it was in the past, rather that the proportions of sand, silt and clay have stayed roughly the same
over time. Overall, the suspended sediment load of the Colorado River has decreased because the
construction and operation of reservoirs has reduced the magnitude and duration of peak flows.
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1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Figure 15. Percentage of sand in suspended sediment samples, Colorado River near Cisco, UT.
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