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WSP12656
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Last modified
1/26/2010 4:17:10 PM
Creation date
8/6/2007 1:52:23 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
7/1/2004
Author
Schmidt - Topping - Grams - Goeking
Title
The Degraded Reach - Rate and Pattern of Bed and Bank Adjustment of the Colorado River in the 25 km Immediately Downstream from Glen Canyon Dam - 07-01-04
Water Supply Pro - Doc Type
Report/Study
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<br />30 <br /> <br /> <br />002480 <br /> <br />measure of channel width (Figure 16). Within these narrow elevation ranges, width is not <br />correlated with stage (Figure 17). The channel narrowed in each year that successive <br />measurements were made at similar water surface elevations. In most cases, the channel <br />partially re-widened that same year or the next year. Since dam closure, the deposit is <br />rarely inundated and is stabilized by riparian vegetation. <br />A detail ofthe left bank of the Lower Cableway cross-section illustrates the <br />formation of the sand bar that narrowed the channel (Figure 18). Inundation discharges <br />were determined for each elevation from the stage-discharge relation for the Lower <br />Cableway. The narrowing began with a deposit that first appeared in June J 935 and <br />aggraded the deposit from the 465 m3/s stage to the 1892 m3/s stage. The time series of <br />channel width indicates this deposition occurred between June 15 and June 22, 1935 <br />(Figure 16a). The next major episode of deposition occurred the next year between May <br />8 and May 11, 1936 (Figure 16b). That deposit aggraded the bar to at least the 1477 m3/s <br />stage (Figure 18). By May 27, 1948, the bar had eroded approximately to the June 14, <br />1935, level. Two days later, the bar aggraded to the 2147 m3/s stage, the highest <br />elevation deposit measured at that location on the cross-section. The 1949 to 1957 <br />measurements show the deposit consistently at an elevation somewhat below the <br />elevation of the 1948 deposit, indicating some erosion but an otherwise stable deposit. <br />The January 24, 2000 measurement by GCMRC indicates that the onshore portion <br />of the deposit aggraded to the 3298 m3/s stage and that a further offshore part of the <br />deposit aggraded to the 1159 m3/s stage. In our aerial photograph mapping, the higher- <br />elevation onshore deposit was mapped as high-tamarisk terrace (htt) and the lower <br />elevation deposit was mapped as an undifferentiated post-dam flood deposit (fs/hf). <br />Thus, the htt deposit must have formed during the 1957 and 1958 floods, because those <br />are the only floods that could have aggraded the deposit to that elevation. The lower fS/hf <br />deposit would have been inundated in those years and in 1962, 1965, 1983, and 1984. <br />Because the 1984 aerial photographs indicated recent deposition on this surface, the <br />deposit most likely aggraded to its current elevation during the 1983 and 1984 floods. <br />
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