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528 TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 1 2 - MEAN BED STAGE (n = 3690) . MINIMUM BED STAGE (n :: 763) 0 I -, w Cl <( .2 .... <Jl -3 . ,." : l~j , . g ! j :: ! ~ 1 !I~ -4 rr#1"';f1A ...~" ~: I..j..~..,~ , .i~''tj . ; ~ ~ -. !i: lQ >l ~ 0 'II 0 ~ 0 a) ;t ~ ~ .. 2! 2! 2! 2! 2! 2! 2! 2! TIME (ealendar years) 4 I 3 w 2 Cl <( 1 Cl >- 0 <( ~ -1 w -' .2 CD <( u .3 .... -4 <( -. MEAN BED STAGE w -5 (n...3815) Cl ~ ......... MIN. BED STAGE -6 (n.1005) <Jl .7 0 ~ 0 ~ 0 ~ 0 ~ 0 b) N N <'> <'> . . ~ ~ .. ~ ~ ~ ~ ~ ~ ~ ~ 2! TIME (caJendar years) Figure 7. (a) Mean and minimum bed stage as a function of time at the Grand Canyon cableway. Values of mean bed stage are from the 3690 USGS discharge measurements made from 1922 through 1962; values of minimum bed stage are from the 766 USGS discharge measurements analyzed by Burlduvn [1986]. Stage is that measured at the lower gage (relative to the 5ame datum as at the upper gage). The linear regression used in the F-testtrend analysis of mean bed stage is shown. (b) Mean and minimum bed stage as a function of time at the upper Lees Ferry cableway. Values of mean bed stage are from the 3815 USGS discharge measurements made from 1921 through 1962; values of minimum bed stage are from the 1005 USGS discharge measure- ments analyzed by Burlduvn [1986]. Stage is that measured at cahleway gage (relative to a different datum than at the recording gage). Bold vertical lines indicate where the time series of mean bed stage was subdivided into the four segments for the F -test trend analyses, see text for justification of this subdivision; linear regressions used in the F-testtrend analysis of the data in each segment are shown. (c) Width as a function of stage at the Grand Canyon and upper Lees Ferry cableways. Note the channel narrowing caused by the December 19, 1944, rockslide at the Lees Ferry cableway. For comparison, data from the Grand Canyon cableway are also subdivided into two portions to illustrate the stability of the channel at that site. Stage at the Grand Canyon cableway is that measured at the lower gage; stage at the Lees Ferry cableway is that measured at the cableway gage. The 1922-1940 Grand Canyon data are from the 2406 USGS discharge measurements made from November 12, 1922, through December 30, 1940; 1941-1962 Grand Canyon data are from the 1284 USGS discharge measurements made from January 5,1941, through December 31,1962. The 1921-1944 Lees Ferry data are from the 2592 USGS discharge measurements made at the upper cableway from August 3, 1921, through December 7,1944; 1945-1962 Lees Ferry data are the 1223 USGS discharge measurements made at the upper cableway from January 10, 1945, through December 30,1962. (d) Discharge of water as a function of stage at the Grand Canyon and Lees Ferry gages. Data from the Grand Canyon gage are subdivided as in Figure 7c to illustrate the stability of the stage-discharge relationship from 1922 through 1962. Stage at the Grand Canyon gage is that measured at the lower gage. Data from the Lees Ferry gage are subdivided into the three periods with approximately stable stage-discharge relationships: August 3, 1921, through April 8, 1923, April 10, 1923, through July 31, 1929, and from August 2, 1929, through February 7, 1959. Unlike in Figures 7b and 7c, stage at the Lees Ferry gage is that measured at the recording gage. At Lees Ferry the August 3, 1921, through April 8, 1923, data are from the 237 discharge measurements made at the upper cableway during this period; April 10, 1923, through July 31,1929, data are from the 464 discharge measure- ments made at the upper cableway and the 41 discharge measurements made at the lower cableway during this period; August 2, 1929, through February 7. 1959, data are from the 2896 discharge measurements made at the upper cableway and the 286 discharge measurements made at the lower c~lhleway during this period.