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Trends in Suspended Sediment Loads The suspended sediment load of the Colorado River increases systematically downstream as sediment from erosive areas in western Colorado and eastern Utah enters the main stem. At Glenwood Springs, near the eastern edge of the Colorado Plateau, the annual suspended sediment load averages about 0.4 x 106 tons/yr (Table 5). Between there and the Cameo gauge, the annual load increases by a factor of four to about 1.5 x 106 tons/yr. The annual load at the State Line gauge is about 3.4 x 106 tons/yr, with most of this increase being due to the Gunnison River. At the Cisco gauge the load is about 5.4 x 106 tons/yr (Table 5). This increase is relatively large considering the Cisco and State Line gauges are only 55 km apart. However, the difference is not unreasonable given the characteristics of the Dolores River basin (sparse vegetation, widespread exposures of shale bedrock) and the influence of the late season storms. The annual suspended sediment load at Cisco, while high (--87 t/km2/yr), is typical of large rivers in semi-arid regions. Table 5. Summary statistics of average annual discharge and annual sediment load for 4 gauging stations on the main stem of the Colorado River. ------------------------------------------------------------------------------------------------------------------- USGS Drainage Period of Annual Discharge' Annual Sediment Load2 Station No. Area (km2) Record' Q SD C, QS SD C, 9072500 11,800 1934-1997 64 19 0.29 0.37 0.30 0.82 9095500 20,800 1934-1997 111 33 0.30 1.53 1.02 0.66 9163500 46,200 1952-1997 180 72 0.40 3.43 2.67 0.78 9180500 --------------- 62,400 --------------- 1934-1997 -------------------- 200 ----------- 76 ------- 0.38 ------------- 5.43 ------------- 3.67 ---------- 0.68 --------- 1. Periods of record for the Glenwood Springs gauge (9072500) and the Cisco gauge (9180500) were arbitrarily truncated at 1934 to coincide with the period of record for the Cameo gauge (9095500); the record for the State Line gauge (9163500) begins in 1952, thus no adjustment was made. 2. The mean (Q) and standard deviation (SD) are expressed in the same units as the data themselves, e.g., in m3/s or millions of tons/yr; the coefficient of variation (C, = SD/Q )is dimensionless. The large increase in sediment load between the Cameo and Cisco gauges reflects both an increase in average sediment concentration and an increase in discharge. Referring back to eqn. 1, note that Q, is a function of both CS and Q. The difference in Q between the Cameo and Cisco gauges is not large enough by itself to account for the difference in QS, thus the downstream increase in sediment load must be due to differences in suspended sediment concentration. To evaluate the importance of this difference, we analyzed suspended sediment data from the Cameo and Cisco gauges further by counting the number of observations in which suspended sediment concentrations fell within specified ranges (excluding late-season events which produce concentrations >10,000 mg/1). The results of this analysis (Fig. 13) show that, first, the frequency distributions of pre- and post-peak suspended sediment concentrations are roughly log-normal at both gauges. Second, it is apparent in comparing the data that sediment concentrations are generally higher at the Cisco gauge than they are at the Cameo gauge. This is particularly true of the post-peak samples, in which concentrations commonly exceed 500 mg/l at Cisco, but rarely reach these values at Cameo. In practical terms, these results indicate that summer-time sediment concentrations and turbidity are typically higher in downstream reaches than in upstream reaches. These differences are likely to have some effect on ecological processes because, in general, primary productivity decreases as turbidity increases. 22