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<br />CHANNEL NARROWING BY VERTICAL ACCRETION, GREEN RIVER
<br />
<br />
<br />Flaming Gorge Dam
<br />
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<br />110"15' 00'
<br />
<br />Channel change was
<br />evaluated in this
<br />reach based on
<br />aerial photography
<br />
<br />--
<br />
<br />N
<br />
<br />Figure 1. Map of the
<br />study area.
<br />
<br />10000' 00' W
<br />
<br />39007' 30'
<br />
<br />39" 00' 00"
<br />
<br />t
<br />
<br />/
<br />'v<.:l:>""-":':~~~~~_'_~'
<br />
<br />.,.._.-../ ~ Old cab,~Lay )"\.'''''''_",
<br />'j i \Green .
<br />",- \ River OJSeFt '-'--..--.-.,.,.
<br />/..._..._... <~~~:~~~~.~.,.,.. 5 km
<br />
<br />...;
<br />
<br />acterize and quantify channel change in a 26.4-
<br />Ian alluvial reach between the Book Cliffs and the
<br />present cableway. Channel features were mapped
<br />directly onto stereo photos with Mylar overlays
<br />using a stereoscope to aid in visual identification
<br />of geomorphic features. Mapped attributes in-
<br />cluded: (I) geomorphic feature (e.g., channel-
<br />bank deposit, midchannel bar, secondary chan-
<br />nel), and (2) location, either within or beyond the
<br />active channel, based on breaks in slope and the
<br />extent of vegetation. Digital photographic scaling
<br />techniques were used to transfer attributes from
<br />photo overlays to a common I: 12 000 scale base
<br />map by aligning several common fixed points
<br />such as road intersections or diversion structures
<br />on the photographs and the base maps. Margins of
<br />the mapped features then were digitized directly
<br />into a geographic information system for quanti-
<br />tative analysis.
<br />The area of different map units was measured
<br />in each photo series and changes between time
<br />
<br />38052' 30' N
<br />
<br />periods were calculated. Mean channel width
<br />was determined by dividing the total active
<br />channel area by the length of the main channel
<br />in the mapped reach. We compared channel
<br />widths for different time periods by computing
<br />the total area of all units designated as within
<br />the channel, regardless of whether they were in-
<br />undated at the time of photography. Errors were
<br />estimated based on the width of the trace line at
<br />the scale of the map, and that error was assumed
<br />to potentially exist on each side of the channel.
<br />This method of error estimation assumed that
<br />we analyzed the aerial photographs correctly,
<br />and that error was solely introduced by data
<br />transfer procedures.
<br />
<br />Stratigraphy and Dendrochronology
<br />
<br />Stratigraphy was analyzed by excavating part
<br />of the lower-elevation surface near the present ca-
<br />bleway. The excavated deposit is very similar to
<br />
<br />(1".nl,...,,;,.,,1 ~,...";,,.tv nf Am..ri,.", Rnlletin. December 1999
<br />
<br />other low-elevation deposits throughout the reach.
<br />Using methods developed by Hereford (1984),
<br />root-crown elevations of seven buried saltcedar
<br />were identified in the excavation. Slabs were re-
<br />moved throughout the root crown areas and were
<br />sanded and polished. Tree rings were counted to
<br />determine the age of each tree at different eleva.
<br />tions, and the age of each alluvial layer adjacent to
<br />a root crown was assigned the same age as the tree
<br />at that elevation. Comparisons were made be-
<br />tween cross-section data and tree-ring data to cor-
<br />roborate conclusions about the timing of bank
<br />aggradation.
<br />
<br />RESULTS
<br />
<br />Hydrology
<br />
<br />Climatic change, transbasin diversions, and
<br />dams have reduced the magnitude and duration
<br />of floods and the magnitude of the total annual
<br />streamflow. The gaging record of the Green
<br />River at Green River is sufficiently long to distin-
<br />guish hydrologic changes that occurred early in
<br />the twentieth century from those that occurred af-
<br />ter completion of Flaming Gorge Dam. Stockton
<br />and Jacoby (1976) determined that the Green
<br />River at Green River, Utah, had higher total an-
<br />nual runoff between 1907 and 1932 than during
<br />any other period since the early 1600s, based on
<br />analysis of tree rings. They also found similar
<br />trends for the Colorado River near Cisco, Utah,
<br />and further downstream, although the ending
<br />year of this wet period differed from site to site.
<br />Hereford and Webb (1992) similarly showed a
<br />change in climate that occurred in the southern
<br />part of the Colorado Plateau in about 1930, based
<br />on rainfall data. Thus, we identified 1930 as the
<br />beginning of a natural climatic shift, and we ana-
<br />lyzed the hydrologic records before and after
<br />1930 to facilitate comparison with records else-
<br />where in the Colorado River basin.
<br />The mean annual discharge of the Green River
<br />at Green River, Utah, is now about 28% less than
<br />it was between 1895 and 1929 (Fig. 3b). There is
<br />no statistical difference between values for the 32
<br />years immediately before construction of Flam-
<br />ing Gorge Dam and the 33 years after comple-
<br />tion, as was demonstrated by Thompson (1984).
<br />Transbasin diversions out of the Green River
<br />drainage began in 1914 and were typically be-
<br />tween 2 and 4 cubic meters per second between
<br />1917 and 1957 (Ioms et al., 1965). There was no
<br />major change in the magnitude of these diver-
<br />sions in 1930.
<br />The magnitude of the annual instantaneous
<br />peak discharge, however, was lower between
<br />1963 and 1996 than in either of the periods be-
<br />fore completion of Flaming Gorge Dam (Fig. 3a).
<br />
<br />1759
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