WSP12655 - Laserfiche WebLink

Home Browse Search

002373 CI Figure 24. Graph en 9.0 2000 (") showing mean bed u) :I: elevation and the 0:::2 ~ discharge of water ~::l - 1500 ::0 w!;;( 8.0 G) measured at the Lower - discharge at time JT1 :20 of measurement - 1000 Marble Canyon gage z>- Z cableway between -.0:: (") Z<l: 7.0 - c: 1983 and 2001. Arrow 00:: - 500 ttl indicates January -I- 0 1-- ~~ s::: 1993 flood in Little w<( 6.0 ~ 0 m Colorado River, ...Jz -t located immediately m w<( - -500 ::0 downstream from the Ow en w> 5.0 "tI cableway. alo m Zal - -1 000 ::0 ~<( en :2 m 4.0 -1500 (") C"') 00 C"') 00 0 co 00 0) 0) Z 0) 0) 0) 0) 0 ...... ...... ...... ...... - - - - ...... ...... .... .... "2: C "2: C (lJ (lJ (lJ (lJ ..., ..., ..., ..., ,_~_l prior 2 years. Degradation caused by the latter flood was probably related to erosion of boulders in Bright Angel Rapid that caused a long-term shift in the stage-discharge relation. There was a small amount of degradation during the rising limb ofthe 1996 Controlled Flood, because the stage-discharge relation shifted by -0.35 m. Degradation at a slow rate occurred between 1969 and 1971 and between 1974 and 1983, because Bright Angel Rapid was presumably eroded to some degree. Similar patterns of aggradation and degradation were measured at the Lower Marble Canyon gage between 1983 and 2000 (Fig. 24). Here, aggradation in the measure- ment reach occurred during the few days when floods from the Little Colorado River, located approximately 1 km downstream (Fig. 15C), entered the Colorado River. These tributary floods temporarily increased the water-surface elevation at the gage, decreased the velocity, and induced bed aggradation. Ponding of the flow was measured on January 22 and 23, 1993, during a flood from the Little Colorado River, when the stage-discharge relation shifted +0.64 m. Although not measured at the time, a flood from the Little Colorado River that occurred on January 12, 1993, probably shifted the stage-discharge relation by approxi- mately 1 m, because this flood exceeded the January 23 peak by about 40%. These ponding events caused bed aggradation of about 0.3 m under the cableway, and the bed did not de- grade to its pre-1993 elevation again until August 1997. 5.3.3 Changes in Main Channel Pools Offshore from Eddies between 1992 and 2000 Annual measurements of 16 main channel pools between 1992 and 2000 by NAU further confIrm that there has been no system-wide accumulation of fIne sediment during the era of environmental management. In fact, these data indicate that there was probably a pro- gressive loss of bed sediment. Many of these pools scoured during the 1996 Controlled Flood, remained scoured during the experi- mental releases of 1997, and fIlled when dam releases decreased in 1998 (Fig. 25). The average trend in bed elevation between 1992 and 2000 is negative (p=9.8x10-4). This loss of sediment included gravel as well as sand, because pipe-dredge samples from these pools included both. The data from each of the 16 pools were also normalized and averaged into one time series, because each pool is only a few channel 5.0 Changes in the Topography of the Main-Channel Bed 35