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<br />.. <br /> <br />Figure 10. Changes in channel <br />characteristics over time. (A) Plot <br />of changes in measured channel <br />width extracted from U.S. Geo- <br />logical Survey (USGS) discharge <br />measurements made between <br />1930 and 1993; (8) plot of <br />changes in secondary channel <br />area detennined from aerial pho- <br />tographs taken at five different <br />times; (C) plot of mean channel <br />width determined from aerial <br />photographs. Note the similarity <br />in the overall trends for all three <br />plots. The dotted lines in 8 and C <br />represent possible naITOwing tra, <br />jectories suggested by the tempo- <br />rally rich USGS data. <br /> <br />-' <br /> <br />CHANNEL NARROWING BY VERTICAL ACCRETION, GREEN RIVER <br /> <br />110 <br />J: <br />I-~ <br />9 .5. 105 <br />3: <br /> <br />L5c- <br />a: ~ 0.4 <br /><~ <br /> <br />i= ~ 155 <br />oE <br />3: ~ 145 <br /> <br />the findings of Graf (1978) and Friedman et al. <br />(1996a, 1996b). However, our data also demon- <br />strate that riparian vegetation can only facilitate <br />vertical accretion at times when flood magni- <br />tudes are low enough so as not to remobilize <br />channel bars. <br /> <br />Processes of Channel Narrowing <br /> <br />Channel narrowing at the present cableway <br />and throughout the study reach occurred by <br />processes of bar stabilization and vertical accre- <br />tion. In some areas, narrowing occurred by filling <br />of secondary channels and conversion of a multi- <br />threaded channel into one with a single thread. <br />Our comparison of historical aerial photographs <br />shows that inset alluvial deposits have formed in <br />both straight and curving reaches, and can form <br /> <br />120 <br /> <br /> <br />PRE-DAM POST-DAM <br /> <br />115 <br /> <br />A MEASURED CHANNEL WIDTH - FROM USGS DISCHARGE <br /> <br /> <br />~ ~".. ."". ~ <br /> <br /> <br />. <br /> <br />. <br /> <br />100 <br /> <br />95 <br />0.6 <br /> <br /> <br />B SECONDARY CHANNEL AREA, IN SQUARE KILOMETERS - ROM AERIAL PHOTOS <br /> <br /> <br />0.5 <br /> <br />0.3 <br /> <br />0.2 <br />175 <br /> <br />165 <br /> <br />C MEAN CHANNEL WIDTH. FROM AERIAL PHOTOS <br /> <br />. <br /> <br /> <br />1960 <br />YEAR <br /> <br />1970 <br /> <br />1980 <br /> <br />1990 <br /> <br />2000 <br /> <br />135 <br /> <br />125 <br />1920 <br /> <br />1930 <br /> <br />1940 <br /> <br />1950 <br /> <br />Woodyer et al.(1979). Mayers and Schmidt <br />(Utah State University, unpublished data) found <br />similar evidence along the Green River in the <br />Uinta Basin, located about 320 kIn upstream <br />from the town of Green River. <br />Like other rivers with high suspended sedi- <br />ment loads, the vertically-constructed alluvial <br />benches of the Green River increase in elevation <br />with time relative to the channel bed. We found <br />no evidence in the study area for the type of cata- <br />strophic floodplain stripping that Nanson (1986) <br />described in Australian rivers. We also did not <br />find evidence for the rewidening of the Green <br />River channel in the study area by the large <br />floods of the 1980s reported by Lyons et aI. <br />(1992), and the rewidening that they measured <br />was probably less than the error of their analyti- <br />cal technique. In fact, we found no evidence of <br /> <br />TABLE 3. HYDRAULIC GEOMETRY OFTHE GREEN RIVER AT THE PRESENT GAGING STATION <br />NEAR GREEN RIVER, UTAH <br /> <br />Time period Width equation R2 Mean depth equation R2 Mean velocity equation R2 <br /> <br />1930-1938 71.2000.067 0.65 0.2400.390 0.94 0.0600.543 0.97 <br />1939-1957 76.7400.057 0.61 0.2200.408 0.96 0.06 0 0.536 0.97 <br />1963-1993 73.19 0 0.054 0.56 0.30 0 0.368 0.93 0.05 0 0.578 0.95 <br /> <br />Note: Q-discharge in m3/sec; R-correlation coefficient. <br /> <br />on the inside or outside of bends. We mapped the <br />conversion of entire channel bars into floodplain <br />areas, presumably by the same vertical accretion <br />processes we measured at the present cableway. <br />Although we observed horizontal bedding that <br />characterizes vertical accretion throughout the <br />study area, we do not know that all lower eleva- <br />tion surfaces formed at one time or that interme- <br />diate benches formed at another. For example, at <br />the present cableway, the entire deposit formed in <br />the second phase of narrowing, but the surface <br />correlates with intermediate elevation surfaces <br />elsewhere. <br />Thus, the Green River is probably constructing <br />two distinct topographic surfaces in the present <br />hydrologic regime, similar to what Pizzuto <br />(1994) found to occur on the Powder River. Sim- <br />ilar processes have also been described by <br /> <br />Geological Society of America Bulletin, December 1999 1767 <br />