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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 />