9670 - Laserfiche WebLink

Home Browse Search

inundated periodically over the period of time covered by the photographs, however, the plants would not have become established if the sediment forming the bed surface had been mobilized to any extent in any of these years. Vegetation growth on these low-lying surfaces is ubiquitous, and provides clear evidence that bed load transport within this reach of the Colorado River is very limited at flows much less than half the bankfull discharge. SUMMARY and CONCLUSIONS This study coincided with a period of intense drought with very low runoff produced throughout the upper Colorado River basin. Streamflow records maintained by the USGS suggest that water years 2002-2004 were perhaps the lowest in the upper Colorado River basin in the last 100 years. Nonetheless, coordinated reservoir operations were implemented for 7 days in 1998 and 10 days in 1999, resulting in increases in discharge of 40-56 m3/s (1500-2000 ft3/s), in the 15-mile reach. The bypass flows were successful in boosting background flow levels by 10- 15%, which was sufficient to mobilize the bed material in a number of places in the 15- and 18- mile reaches. The limited availability of water in subsequent years prevented further tests of the geomorphic effects of bypass flows. Flows that did occur in 2000-2004 were generally well below average, thus thresholds for mobilizing the bed material were exceeded very infrequently. Over the 7-year period of the study, the discharge required to produce initial motion (-1/2 the bankfull discharge) was exceeded for a total of 78 days, which is only about 1/3 the frequency recommended in previous reports. The discharge required to completely mobilize the bed (bankfull discharge) did not occur at all. Geomorphic changes in the 15- and 18-mile reaches were monitored using periodic surveys of main-channel cross sections and backwaters, and comparative analysis of aerial photographs taken in 1993 and 2000. These measurements indicate that, overall, the large-scale 57