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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, I 527 10 9 FlOOD ",0", WATER-SURFACE STAGE PEAK ___ MEAN BED STAGE · I .--...--- MINIMUM BED STAGE ~ ....0-.- 1948-1a58 MEAN BED STAGE ~ ~ . A x... n CN.S.& MEAN BED STAGE) : . .~.~(:{ ,.X"~. :"+,,nIMINIMUMBEDSTAGE) 5 O-O-{}..Q.-Q..o- q MIN. BED ~ 2.2'); .^-.Q'Q-2'i2-{}.-o--o " ELEVATION Y ~ 3 t 2 i. -l '.I..I..I.. 1 !.! '. o ., -2 .,x",x",x"X")("M...X" . oX x..~___~..X-'M.-M"K ~ Xi ." Ii()( ~ ~ .)Ii -3 -+- -+- +..+ . .-....:.+.:...:..".0.......+...+.000.+..0..+ +-. ...+-........+._+.. .+' "+-.+--.......,+. .. I w '" -= '" ~ -' lC -= o .... -= w '" ~ '" .. .. .. 75 50 , 25 o ~ ~ ~ ~ _ Jun ~ ~g Ssp ~ b ~ 200 w " 180 z w a: 160 a: ::> 8 140 c 120 ~ " 100 ! 80 -= 9 60 c i!l 40 ~ 20 a: 0 z 0 ~ SAND-OEPLET1QN EVENTS ---0... DECAEASING-CQNCENTRATION INCONCLUSIVE EVENTS ----.- SAND-ENHANCEMENT EVENTS _.. -.6,... INCAEASING-CONCENTRATlON INCONCLUSIVE EVENTS ... -x-.. - n (total number 01 the 4 rypes of sand-8l4lPIY events) X-"X- ._~...x..-x"- J8I1 Fob ... Ap' May b) ;,.. .Jo4=..M..M...X'"..x...~..X:..)(..)(_.~ '->;:'''X''':lo'.. . Jun Jul Aug Sep Oct Noy D8C 50 , o Figure 6, (oontinued) Lees Ferry gage annual technical file, 1945) and caused ap- proximately 10 m of channel narrowing (Figure 7c). Because of these externally foreed changes in geometry, the bed-stage data from the upper cableway were analyred in four segments: (1) from August 3,1921, through April 8, 1923, (2) from April 10, 1923, through July 31, 1929, (3) from August 2, 1929, through December 7, 1944, and (4) from January 10, 1945, through February 7, 1959. To ensure that only natural long- term trends were detected in this analysis, the period after completion of the cofferdam at the Glen Canyon Dam site on February 11, 1959 [Maron, 1989], was not included. Though the effect of the oofferdam was probably smaU at the upper cableway during 1959-1962, by trapping some of the upstream supply of sediment, the oofferdam may have slightly enhanced the soour at the upper cableway (Figure 6b) [see Pembenon, 1976]. From August 2, 1929, through February 7, 1959, and as observed at the Orand Canyon cableway, the trend in bed stage at the upper Lees Ferry cableway was slightly, but significantly, negative (Figure 7b). Trends in mean bed stage from August 3, 1921, through April 8, 1923, and from April 10, 1923, through July 31, 1929, are not significant at either the 5% or 10% level, however. From August 2. 1929. through December 7, 19.\.\. the bed at the upper cableway sooured at a rate of 2.5 cm/yr; this trend is significant at the 1.6 X 10-' level. From January 10, 1945, through February 7,1959, the bed sooured at a rate of3.0 cm/yr; this trend is significant at the 3.9 X 10-4 level. Thus, given the approximate 120-m width of the channel at the upper cableway, about 3.0 m' more sediment was eroded from this cross section than was supplied to it each year during 1929- 1944, and about 3.6 m2 more sediment was eroded from this cross section than was supplied to it each year during 1945- 1958. From August 2, 1929, through February 7, 1959, the I5-min discharge of water at tbe Lees Ferry gage increased by only 10%, and the 15-min water-surface stage increased by only 3% (L. E. Vierra and D. J. Topping, unpublished data, 1999). Thus long.term changes in either discharge or stage can be ruled out as the cause of the 1929-1958 erosion. Further- more. because the August 2, 1929, to February 7, 1959, stage. discharge relationship was effectively oonstant (Figure 7d), the 1929-1958 decrease in the amount of sediment at the upper cableway suggests that Glen Canyon, like Grand Canyon, was supply-limited with respect to fine sediment. However, the previous analyses (shown in Figures 4, 5, and 6) suggest that the degree of sediment supply limitation in Glen Canyon was much less than in Grand Canyon.