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Last modified
1/26/2010 4:17:09 PM
Creation date
8/6/2007 1:47:16 PM
Metadata
Fields
Template:
Water Supply Protection
File Number
8220.101.10.B
Description
Colorado River - Water Projects - Glen Canyon Dam-Lake Powell - Glen Canyon TWG
State
CO
Basin
Colorado Mainstem
Water Division
5
Date
10/1/2004
Author
Schmidt - Topping - Grams - Hazel
Title
System-Wide Changes in the Distribution of Fine Sediment in the Colorado River Corridor Between Glen Canyon Dam and Bright Angel Creek - Arizona - Final Report - 10-01-04
Water Supply Pro - Doc Type
Report/Study
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<br />002359 <br /> <br />5.2.1 Rate and Magnitude of Degradation <br /> <br />~".......l <br /> <br />Data that demonstrate the rate and <br />magnitude of bed degradation are resurvey of <br />22 cross-sections established by Reclamation <br />in 1956 between Glen Canyon Dam and the <br />riffle at the Pari a River confluence, just <br />downstream from Lees Ferry (Fig. 18, Table <br />6). Reclamation also described the size and <br />depth of bed sediments by boring, or using a <br />jet probe, at 16 locations. Borings and probes <br />extended into the underlying gravel, entirely <br />through the overlying sand. A subset of these <br />cross-sections was resurveyed in 1959, 1965, <br />1975, 1983, 1990, and 2000 (Grams et aI., <br />2004). Supplemental data about bed degrada- <br />tion was determined by analysis of discharge <br />measurements and the cableways of the Lees <br />Ferry gage. <br />Degradation of riffles and pools began <br />soon after dam construction began, and there <br />was significant erosion within 7.5 km from <br />the dam by 1959 (Fig. 19). The thalweg of <br />the riffle 1.0 km downstream from the dam <br />was eroded 3 m and pools were eroded 1.25 <br />to 2.5 m. Further downstream, there was no <br />change in bed elevation. Pemberton (1976) <br />suggested that the cofferdam's storage capac- <br />ity of3.58 x 107 m3 was sufficiently large to <br />reduce the concentration of suspended sedi- <br />ment of the 1957 and 1958 spring snowmelt <br />floods, whose peak discharges were 3,567 and <br />3,001 m3/s, respectively, thereby reducing the <br />magnitude of fill that would have occurred <br />during flood recession. <br />The 1965 channel cleaning flow <br />achieved its intended purpose and caused the <br />greatest amount of bed degradation ever <br />measured in the study area. Resurvey of <br />cross-sections in September 1965 indicated <br />that cross-sections in the upstream part of <br />Glen Canyon were further eroded and that <br />cross-sections in the downstream part of Glen <br />Canyon were eroded for the first time. Riffles <br />as far downstream as 18.4 km from the dam <br />were eroded, and the pool at the upper cable- <br />way of the Lees Ferry gage was eroded by <br /> <br />about 5 m (Fig. 19A). Approximately 5.0 x <br />106 tons were eroded from Glen Canyon during <br />the 3 mths of these high flows (Topping et aI., <br />2003). <br />The rate of bed degradation decreased <br />greatly thereafter, and there was some aggrada- <br />tion in the 1990s. The thalweg of pools typi- <br />cally eroded an additional 0.5 to 2.0 m be- <br />tween fall 1983 and fall 1990 and aggraded by <br />less than 1.0 m in the same pools between <br />1990 and 2000. In some cases, such as at <br />cross-sections 10 and 11A, the channel wid- <br />ened although the thalweg did not degrade <br />further. <br />Degradation of pools and riffles differed <br />in magnitude and longitudinal pattern. Pools <br />degraded 5 times more than riffles, and the <br />magnitude of degradation was just as large in <br />the downstream end of Glen Canyon as in the <br />upstream end. In contrast, the magnitude of <br />degradation of riffles decreased downstream, <br />and the bed at the riffle immediately down- <br />stream from the Lees Ferry gage did not <br />change at all. <br />There are two implications of the decreas- <br />ing downstream magnitude of bed degradation <br />at riffles. The first is that the magnitude of <br />shifts in stage-discharge relations decreased <br />downstream. Water surface elevation at 150 <br />m3/s is now about 2.3 m lower than it was in <br />1956 near the dam, but there has been no <br />change in water surface elevation at a cross- <br />section 20.2 km downstream from the dam <br />(Fig. 20). The second implication is that the <br />gradient of the Colorado River in Glen Canyon <br />decreased by about 25%. In 1956, the gradient <br />was 0.00034 and 0.00037 at 79 and 2067 m3/s, <br />respectively, and today the gradient is between <br />0.00025 and 0.00028. <br />The cumulative volume of sand and <br />gravel eroded from Glen Canyon was about <br />10.7 x 106 m3. In 1956, the Dso of the bed was <br />approximately 0.2 mm and the Dso of the <br />underlying gravel was about 20 mm (Fig. 21). <br />The entire sand layer of the pre-dam bed was <br />removed, but half of the total degraded bed <br />sediment was gravel (Fig. 22). <br /> <br />5.0 Changes in the Topography of the Main-Channel Bed 31 <br />
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