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<br />c,
<br />
<br />
<br />Figure 5-Continued. Flow over the gravel bar at the mouth of the Paria River at a range of discharges. (G) Upstream view on about June 1,
<br />'928, Discharge of the Colorado River is approximately 110.000 cubic feet per second, Bar is almost completely overtopped by the Colorado
<br />River, and backwatered conditions extend far up the Paria River. At lower discharges, a riffle is present in the left-bank channel (in foreground).
<br />At 110,000 cubic feet per second, however, the riffle is washed out as the hydraulic control for the Lees Ferry Gage shifts farther downstream.
<br />Photograph taken by the U,S, Geological Survey, Source of this photograph: wall display in the Flagstaff, Arizona U,S Geological Survey office,
<br />
<br />Pre-Dam Hysteresis in the Water-Surface
<br />Slope in the Reach Upstream from the
<br />lees Ferry Gage
<br />
<br />In a one-dimensional sense, the longitudinal water-
<br />surface slope in a river at a given sub-critical discharge is
<br />governed by the geometry of the downstream hydraulic
<br />control, cross-sectional geometry, streamwise changes
<br />in cross-sectional geometry, and bed roughness (for
<br />example, see Chow, 1959), Thus. the longitudinal water-
<br />surface slope can vary as a function of the volume of
<br />sediment stored in pools because changes in the volume
<br />of stored sediment can affect the mean cross-sectional
<br />geometry, streamwise variation in cross-sectional
<br />geometry, and bed roughness, Analysis of the water-
<br />surface-slope data in the reach upstream from the Lees
<br />Ferry Gage indicates that prior to the construction of
<br />Glen Canyon Dam, seasonal scour and fill of the pools
<br />upstream from the gage (Leopold and Maddock, 1953;
<br />Colby, 1964; Howard and Dolan, 1981; Burkham, 1986;
<br />Topping and others, 2000; Grams and others, Utah State
<br />University, written commun., 2(02) caused hysteresis in
<br />the water-surface profile, with the water-surface slope
<br />
<br />being steeper during the rising limb than during the
<br />receding limb of the annual snowmelt flood (figs, 6C-E).
<br />Following the extensive scour of these pools during high
<br />dam releases in 1965 (Pemberton, 1976; Williams and
<br />Wolman, 1984; Burkham, 1986; Grams and others, Utah
<br />State University, written commun., 2(02), however, this
<br />secondary control on water-surface slope near Lees Ferry
<br />ceased to be important.
<br />The effect of the seasonal pre-dam scour and fill
<br />of the pools upstream from the Lees Ferry Gage on the
<br />water-surface slope is evident in figs, 6C-E. As shown
<br />in fig, 68 of Topping and others (2000), the bed at the
<br />Upper Cableway scoured each spring during the rising
<br />limb of the annual snowmelt flood. then filled during the
<br />receding limb (with only a slight lag between the time of
<br />the flood peak and the time of maximum scour), Colby
<br />(1964) and Topping and others (2000) concluded that
<br />scour and fill of the bed at the Upper Cableway was
<br />largely driven by the interaction between the !low and the
<br />geometry of the reach, and not by changes in the upstream
<br />sediment supply [as was the case at the Grand Canyon
<br />gaging station downstream (Topping and others, 2(00)].
<br />
<br />02244
<br />
<br />Physical Characteristics of the Lees Ferry Gaging Reach 15
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