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Last modified
1/26/2010 12:50:40 PM
Creation date
10/11/2006 11:46:22 PM
Metadata
Fields
Template:
Water Supply Protection
File Number
8220.101.10.A
Description
Colorado River-Water Projects-Glen Canyon Dam/Lake Powell-Glen Canyon Adaptive Management-AMWG
State
AZ
Basin
Colorado Mainstem
Water Division
5
Date
3/3/2004
Author
USGS
Title
AMWG Meeting Attachments-March 3-4 2004-Variations in Sand Storage Measured at Monumented Cross Sections in the Colorado River Between Canyon Dam and Lava Falls Rapid-Northern Arizona 1992-99
Water Supply Pro - Doc Type
Report/Study
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<br />and others, 1978; Kieffer, 1985). The downstream ends <br />of these pools are controlled by bedrock outcrops or <br />altemating debris bars that are the outwash from the <br />upstream debris fans (Howard and Dolan, 1981; Webb <br />and others, 1989: Melis and Webb. 1993: Melis and <br />others, 1994). Because the debris fans raise the bed <br />elevation (Howard and Dolan, ] 981). large pools also <br />fonn upstream from rapids. The length of the pools is <br />detemlined by the spacings between debris fans. which <br />are controlled by bedrock geology (Howard and Dolan, <br />] 98]). These pools are recirculation zones and may <br />contain one or more eddies and low-velocity areas, thus <br />providing an environment for sediment deposition <br />(Schmidt and Graf, ] 990). <br />Prior to the closure of Glen Canyon Dam in ] 963. <br />the Colorado River in Grand Canyon had highly <br />variable discharge and suspended-sediment <br />concentration annually. Between May and July, melting <br />of the snow pack in the headwaters produced mean <br />annual peak discharge of 2,420 m3!s. Except for brief <br />but occasionally substantial summer tributary tlash <br />tloods, tlow in the Colorado River typically fell to less <br />than 200 m3!s after the snowmelt tlood subsided. Post- <br />dam tlow regulation has removed seasonal-tlow <br />variations by reducing the frequency of discharges that <br />are greater than powerplant capacity (940 m3/s) and <br />increasing the overall range and rate of change of <br />discharge during each day. <br />The average sediment load for the Colorado River <br />at Lees Ferry before the dam was 60 million metric <br />tons per year (Webb, Wegner, and others. 1999). The <br />suspended-sediment load of the post-dam Colorado <br />River has been reduced by 95 percent (Webb, Wegner. <br />and others, ] 999) and is primarily supplied by the <br />major tributaries, the Paria and Little Colorado Rivers. <br />and numerous smaller. ungaged tributaries. The mean <br />annual supply of fine sediment was 2.74 million metric <br />tons for 1947-76 from the Paria River and 8.4 million <br />metric tons for ]947-72 from the Little Colorado River <br />(Webb. Wegner, and others, 1999). The smaller <br />tributaries create and maintain the debris fans and <br />rapids on the Colorado River and serve as important <br />sources of fine-grained sediment (Powell. 1895; <br />Hamblin and Rigby, 1968: Dolan and others. 1974; <br />Howard and Dolan. 198]; Keiffer, 1985). Estimating <br />the sediment yield from these smaller tributaries is <br />complicated because sand is contributed by a <br />combination of tluvial and hillslope processes (Webb <br />and others, 2000). Using a sediment-yield model for <br />debris tlow. Webb and others (2000) estimate that <br /> <br />approximately 2.0 million metric tons of total sediment <br />enters the river from the ungaged tributaries between <br />Glen Canyon Dam and Diamond Creek (river mile -15 <br />to 226). <br /> <br />Controlled Releases and Gaged Tributary Inputs <br /> <br />Changes in area measured at cross sections within <br />the study area are affected by controlled tlows from <br />Glen Canyon Dam and sediment input from tributaries. <br />Under the MLFF preferred alternative specified in the <br />EIS, peak releases are limited to 708 m'/s, minimum <br />releases are limited to 227 m3/s during the day and <br />]42 m3!s at night, and the maximum daily tluctuation <br />or range in tlow is limited to 227 m3/s. The preferred <br />alternative also provides for shott duration, high-tlow <br />releases to accomplish specific management objectives <br />and to simulate some of the dynamics of a natural river <br />(U.S. Department of the Interior, 1995). In March- <br />April 1996. a controlled high-tlow release experiment <br />(1996 controlled tlood) was conducted. A tlow of <br />] ,275 m3/s was released from Glen Canyon Dam for <br />a period of 7 days to detennine the effectiveness with <br />which discharges greatly in excess of the normal <br />powerplant-restricted maximum could renew the <br />riparian environment along the Colorado River. Several <br />papers on the controlled tlood are compiled in Webb, <br />Schmidt, and others (1999). <br />For this study period, the box plots shown in <br />figure 2 graphically summalize the distribution of <br />maximum. minimum. and range (daily maximum - <br />daily minimum) of daily discharge values for water <br />years 1992--:99 at the USGS streamtlow-gaging station <br />Colorado RIver at Lees Ferry (09380000; fig. I I, <br />approximately 16 miles downstream from Glen <br />Canyon Dam. The distribution of maximum and <br />minimum daily discharge values is lower during water <br />years 1992-95 than during water years 1996-99. <br />Beginning in water year 1996, there is an increase in <br />the annual median daily maximum and minimum <br />discharge. The annual median daily discharge <br />decreased in water year] 999. but was still larger than <br />medians of the earlier period 11992-95). The maximum <br />daily discharge at this station occurred during the 1996 <br />controlled flood, and the minimum daily discharge <br />occurred in water year 1992. Table I shows annual <br />summary statistics for discharge at Lees Ferry. <br />Suspended-sedimem discharge from the Glen Canyon <br />reach, measured at the Lees Ferry gaginu station, is <br />negligible and therefore is not included in table] or <br />other analyses in this report. <br /> <br />02""4 <br /> <br />Introduction 5 <br />
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