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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 553 1997 TEST -A..OW DEPOSITS -+-- RM 8 (Jackass)3 m from hwm __ AM 22122-ffi'1e bar) 14.3 m from hwm - .. - RM 44 Upper Eminence Break return channel)5.5 m from hwm __ - AM 44 Upper Eminence Breakl!i.5 from hwm _____ AM 44 Upper Eminence Break 11 m from hwm ~ ~~ ~ ~~~e~ i:~~~~l) 12 m hwm SUSPENDED SEDIMENT ____ AM 63 ~BeIOW Crash Canyon) 14.5 m from hwm DURING THE 1997 TEST FlOW =.g- : ~~: ~~:~)~.~~ =~~.1 m from hwm --Etr-lOWER MARBLE CYN (RM 61) -0-- RM 68 Iltlner) 11.2 m from hwm -& GRAND CANYON (RM 87) -a- AM 81 (Grapevine)2.S-3.5 m from hwm 1.0 (Top) z :;: ........ j:Cii 08 ....0 . "a. ~~ W;:: 0.6 "0 C...J WLL 0.4 ~~ ;t~ ~t- 0.2 o z z o :D -;:: <D" <DC "N 0.6 ;;lg) ~:::! 0.4 .."i: ,..m 00 ::;:c 0.2 Jl Z '" ~ 1.0 " ........ j:~ 0.8 ....0. 0 "w ~o 0.6 ' ,,;:: \ 09 \ I ~ ::: \ '\-- "- \1 ~~ b "- (Beglmlng 0.0(8000)' of~ 0.0 b) 1> 10 20 30 40 50 80 70 dischsJge) SILT & CLAY CONTENT (% OF SAND. SILT, AND CLAY) . o I I I ;~Z ~ i b a) 0.0( Bass) 0.10 0.08 SAND D", (mm) (End of 10 high . dlschorgo) 0.8 0.20 (Beginning of high 0.0 discharge) (End of 1 0 ~ . dlscNrge) 0.8 z o Jl -;:: <0" <DC "N 0.6 -tm mO ~:::! 0.4 "TI~ ,..m 00 ::;:c 0.2 :D Z '" Figure 9, (a) Median grain size of sand as a function of normalized height within the deposits of the 1997 test flow and measured median grain size of sand in suspension as a function of normalized time during the 2 days of steady high discharge during the 1997 test flow. See Figure 1 for the locations of the eight bars that were sampled. The phrases in italics refer to tbe lateral distance of the sample site (in meters) from the test-flow high-water mark. (b) Silt and clay content as a function of normalized height within the deposits of the 1997 test flow and measured silt and clay content in suspension as a function of normalized time during the 2 days of steady high discharge during the 1997 test flow. 5.2. Results: 1983 During the 1983 utUe Colorado River flood, suspended- sand concentrations increased at all three of the downstream gages on the Colorado River (Figure lOb). At the Grand Can- yon gage (41 km downstream from the mouth of the ullle Colorado River (LCR), suspended-sand concentrations were 0.002% the day before the beginning of the Little Colorado River Rood. The peak discharge of the Little Colorado River flood passed the LCR near Cameron gage (located 73 km upstream from the confluence with the Colorado River) on September 30 (Figure lOa). Within 2 days, suspended-sand concentrations had increased to 0.11 % at the Grand Canyon gage, a faClor of 55 increase in concentration (Figure lOb). At the Grand Canyon gage this increase in concentration was associated with a fining of the suspended sand, with the median size decreasing from 0.20-0.25 mm to about 0.13 mm (Figure IOc). Because the Little Colorado River flood had only" small impact on the discharge of water at the Grand CanYL HI gage (< 12% during the flood peak), this increase in suspended-sand concentration was most likely due to the enhancement of the upstream supply of sand in the Colorado River during the Little Colorado River flood. Moreover, if the increase in con- centration were due to an increase in the discharge of water in the Colorado River, the grain size of sand in suspension should have coarsened and not fined. Because their approach does not allow for fining of the sand on the bed of the Colorado River during tributary floods, Randle and Pemberton [1987] greatly underestimate the concentrations of suspended sand at the Grand Canyon gage during this period (Figure lOb). The short-term response of the suspended sand in the Col- orado River to the Little Colorado River nood was not limited to only the upper portion of Grand Canyon. At the Nalional Canyon gage (172 km downstream from the mouth of the Little Colorado River), suspended-sand concentrations in. creased from about 0.004 to 0.03% (a factor of 7.5 increase) in response to the Lillie Colorado River nood (Figure lOb). Like