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A. en o c:: - 0 Ol - > c:: '';:; 0 CO :.= ~E ..c::- ,-,en ",en Olen ~ - .!: u-) "'-: '" 0> '" ::l E<( "0 c:: c:: 0 '" - en '" ;5 B. - '" ;5_ B ~ 2.2 g; 9 .~ .~ a;;: 1.1 ~..s '-'N "'0 e ~ 0.0 .':: -.. ~~ '" ::l E ~-1.1 ~ 0 '" en c. - '" ;5_ o '" 2.2 _ c:: Ol 0 > - 'g .2 a;;: 1.1 ~E '-'M "'0 010 .~ :.. 0.0 ~~ '" ::l E' -g 5 -1.1 '" en Influence of Glen Canyon Dam Operations on Downstream Sand Resources 25 6.6 _. _ _ Mass-balance uncertainty envelope 5.5 -- Water discharge 4.4, U. 3.3 -.. . ~ 1,;,11 ... . _.-. III ... .. ..,.. 2.2 t 1.1 Accumulation 0.0 -1.1 ~~:~~~ en 40,000 u 35,000 ~K~~~ ~ lK~~~ ~ 1 0,000 ~ 5,00~ .f '" Ol Ol -' Ol ;5 - '" Ol ~ '" ..c:: '-' .~ o I' _-., Erosion,. r '.. ,.....- I ------. I "'- I.......... \,.._1_-...1 ---....... --. -2.2 '. -3.3 '-,..! .....--.........__....:r'.. -4.4 ..-.....1--........- .5.5 ~ ~gg g g~~ ~ ~g8 8 ggg g ggg .- .....NN N N~........... .....NN N NNN N NNN g'~.g ~ g Ei.g t; g g.g t; g~.g ~ g~.g t; <X zu..~ <t zu....::2 <C Zu..:2; <t zu..:2: <( zu...~ 3.3 35,000 30,000 15,000 ~ 10,000 ~ 15,000 ~ 10,000 ~ 5,000 >- o :: Ol LL '" Ol Ol -' - '" Ol ~ '" ..c:: '-' .~ o ... -. Mass-balance uncertainty envelope .- Water discharge 11M' .a:.ftr4Ml'a;Nl ----, .,t , ' I _.. _ __.. -hcumui'ation-...... _.. ._-. Erosion r- '. \ Linle Colorado River flood -2.2 J\l\~ 1111l\e~.1\)\l\o\o~ .1\)\llJall.lllll'1;\at. 11l1l, 1;\a~ 1\)\l\\l\~ 11l1l, 3.3 35,000 30,000 15,000 en 10,000 ~ 15,000 ~ 1 0,000 ~ 5,000 ~ o O:i LL '" Ol Ol -' - '" Ol ~ '" ..c:: '-' .~ o - -. Mass-balance uncertainty envelope - Water discharge '!URU Mil/Mrm ml~~~M\i~ ,______ t ...... - Accumulatio!l .. -. ... Erosion "- -".----- "'. \ . Paria River flood -2.2 J\l~ 101l, se~.l\)\l' \'lo~ .11l1l' Jail. 1IlMN\at.l01l~1;\a~ 11l1l~ J\l\~ 11lM Figure 7. Mass balance of sand between Lees Ferry and Grand Canyon gages from August 1999 through July 2004 (A) and separately for sediment years (July-June) 2003 (8) and 2004 (C). Mass balance is cJmputed by subtracting measured, mainstem suspended-sand export (10% uncertainty) from estimated and measured sand inputs from the Paria River (20% uncertainty) and Little Colorado Riv'3r (30% uncertainty). as well as from estimated inputs from numerous lesser tributaries (50% uncertainty). The measurements illustrate the rapid export of tributary inputs by high dam releases and the continued overall loss of sand from Grand Canyon under the modified low fluctuating flow (MLFF) alternative, even during the drought-hydrology, minimum-volume release years of 2003 and 2004 (modified and updated from Rubin and others, 2002). others (1976). Most significantly, the sand mass balance remained negative during water years 2000 through 2004, despite 5 consecutive years in which minimal release volumes (8.23 million acre-feet (10,148 million m3)) from Lake Powell occurred during prolonged drought in the upper Colorado River Basin. These measurements and calculations of sand transport also show that tributary inputs are typically transported downstream and out of the canyon within a few months under typical Record of Decision opera- tions (Rubin and others, 2002). . Repeat topographic mapping of sandbars (Hazel and others, 1999) showed that the 1996 BHBF did increase the surface area of high-elevation sandbars, but more than half of the sand depos- ited at higher elevations was taken from the lower portions of the sandbars (Schmidt, 1999) rather than being derived from tributary sand supplies accumulated on the channel bed, as originally hypothesized in the 1995 EIS. . Repeated surveys of channel cross-sections (Flynn and Hornewer, 2003) revealed erosion at 55 of the 57 locations between 1991 and 1999, even though daily operations were constrained during the time series of repeat measurements. . Schmidt and others (2004) conducted geomorphic mapping from air photos and land surveys for the predam and postdam periods. They estimated the loss of sand to be about 25% of the area typically exposed at base flow in predam photographs, but estimates range from 0% to 55% depending on study reach and method of analysis. Their studies further suggested that loss of the sandbar area continued at a relatively steady rate between 1983 and 2002, despite constraints on daily operations imposed after 1991. Importance of Continuous Long-term Sediment-transport Data Because of a lack of continuous data on sediment inputs and export that would have allowed for a sedi- ment budget based on measured data, the EIS study team used stable sand-transport rating curves. Stable rating curves assume that for any given flow there is a single value for the corresponding sand concentration and, therefore, a predictable sand-transport rate related to flows released from Glen Canyon Dam. The recent