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Last modified
7/14/2009 5:01:47 PM
Creation date
5/20/2009 5:15:30 PM
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UCREFRP
UCREFRP Catalog Number
9331
Author
Allred, T. M. and J. C. Schmidt
Title
Channel narrowing by vertical accretion along the Green River near Green River, Utah
USFW Year
1999
USFW - Doc Type
Geological Society of American Bulletin
Copyright Material
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<br />CHANNEL NARROWlNG BY VERTICAL ACCRETION, GREEN RIVER <br /> <br /> 3000 <br /> Thalweg elevation ~ Flaming GDrge Dam completed <br />~ 2500 <br />::l 0 <br />!(( <br />0 <br />~ 2000 w <br /><C (!) <br />a: c: <br />I- -1 <t <br />CO :r: <br />a: () <br /><C ~Cil <br />z- 1500 0;;,- <br /><cS ~.s <br />UJ <( <br />6 0 <br />CO -2 Mean daily discharge z <br /><C <t <br /> W <br />Z 1000 :;E <br />0 <br />Ei <br />> <br />UJ -3 <br />..J <br />UJ 500 <br /> <br /> <br /> <br />-4 <br />1930 <br /> <br />1940 <br /> <br /> <br /> <br /> <br />1950 <br /> <br />1960 <br />YEAR <br /> <br />1970 <br /> <br />o <br /> <br />1980 <br /> <br />1990 <br /> <br />Figure 5. Elevation of the bed at its deepest point and mean daily discharge for the period of record at the present cableway. Although the range <br />in elevation of the thalweg has not changed, the average elevation of the thalweg has cyclically varied by about 0.3 m. <br /> <br />Stratigraphy of AUuvium near the <br />Present Cableway and Similarity to <br />Suspended Sediment Load <br /> <br />The fine-grained sediment that comprises the <br />inset alluvial deposit on the right bank at the pre- <br />sent cableway is horizontally bedded (Fig. 13), <br />similar to bank deposits of other Colorado Plateau <br />streams (Hereford, 1984, 1986, 1987a, 1987b; <br />Hereford el al., 1996). Similar deposits elsewhere <br />in the study reach are also horizontally bedded. <br />This pattern of bedding indicates that the entire <br />deposit exposed in the excavation formed by ver- <br />tical accretion. The beds that form this deposit do <br />not systematically fine in an upward direction; the <br />highest proportion of sand in beds occurs both <br />near the base of the excavation and near the top <br />(Table 5). This deposit is comprised of many dis- <br />crete layers, and the thickness of these layers is <br />greatest at the top, where two thick sandy layers <br />cap the deposit. Ripple drift cross-lamination is <br />variable and includes centroclinal cross-stratifi- <br />cation, which is indicative of deposition around <br /> <br />vegetation stems (Underwood and Lambert, <br />1974). The middle part of the exposure is com- <br />prised of thinner silty-sand beds, and two organic <br />rich layers that each had been the ground surface <br />at one time. Below these units are beds composed <br />primarily of sand. <br />The entire portion of the deposit exposed in the <br />excavation formed after 1958, based on tree ring <br />dating of multiple root crowns of buried saltcedar. <br />The lowest crowns are approximately 0.7 m be- <br />low the organic layers and these crowns dated to <br />1959 or 1960. Root crowns at the lower organic <br />layer date to approximately 1965. <br />This inset deposit is composed of sand and <br />finer sediment that can be suspended at discharges <br />less than the 2-yr recurrence flood. Typically, <br />more than 90% of the sand in this deposit is finer <br />than 0.125 mmand virtually all of the sand is finer <br />than 0.175 mm (Table 5). Allred (1997) measured <br />velocity and suspended sediment distribution at a <br />discharge of 515 m3/s between May 29 and 31, <br />1996, at six depths, in five vertical sections. He <br />found that some parts of the cross section had <br /> <br />r....,.,1,.,0;",,1 S,.,,,;,,,tv of America Bulletin, December 1999 <br /> <br />concentrations that exceeded 1200 mgll for grains <br />between 0.125 mm and 0.175 nun. The distribu- <br />tion of suspended grains in transport was asym- <br />metric over the cross section, and the highest con- <br />centrations were transported through a high <br />velocity zone on the left side of the channel, near <br />the opposite bank from the inset deposit. Near the <br />right bank, velocities were much lower, but sand <br />grains finer than 0.088 nun were evenly distrib- <br />uted in the vertical and grains finer than 0.125 nun <br />occurred in concentrations exceeding 190 mgn. <br />Historical suspended sediment data shows that <br />a large proportion of the annual suspended load <br />was finer than 0.125 nun. The mean percentage <br />of particles finer than 0.125 mm in measured <br />samples was 91 %, as reported by.loms et al. <br />(1964) for 138 samples collected between 1951 <br />and 1957 at the cableway. <br /> <br />Rate and Process of Vertical Accretion <br /> <br />We confirmed our interpretation of the timing <br />and rate of vertical accretion by analyzing cross- <br /> <br />1763 <br />
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