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<br />1,000,000
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<br />DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH
<br />
<br />1019
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<br />I T
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<br />WATER DISCHARGE (FP/SI
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<br />Figure 4. Measured suspended-sediment transport rates for various-sized material as a
<br />function of water discharge at the Green River at Green River, Utah, gage during the pre-
<br />reservoir (open triangles) and post-reservoir (solid dots) periods. A. 0.004-0.016 mm frac-
<br />tion. B. 0.0625-O.125mm fraction. C. Sand-size fraction. D. All sizes.
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<br />transported by an increment of discharge was
<br />determined by summing over the several size
<br />fractions.
<br />The quantities of sediment transported by dif-
<br />ferent increments of discbarge during the pre-
<br />
<br />and post-reservoir periods are compared in
<br />Figure 8 for tbe Browns Park reach, in Figure 9
<br />for alluvial sections in the vicinity of the Jensen
<br />gage, and in Figure 10 for alluvial sections in the
<br />vicinity of the Green River, Utah, gage. The
<br />
<br />effective discharge (modal value) for the Green
<br />River in the Browns Park reach was computed
<br />to be 7,450 ft3/s during the pre-reservoir period,
<br />water years 1951-1962. This discharge was
<br />equaled or exceeded 5.5% of the time, or 20.0
<br />days/yr on an average. During the post-
<br />reservoir period, water years 1966-1981, the ef-
<br />fective discharge decreased by 63% to 2,750
<br />ft3/s. This discharge has a duration of 27% of
<br />the time, or 99 days/yr on an average. The effec-
<br />tive discharge (modal value) for the Green River
<br />near Jensen gage was 20,500 ft3/s during the
<br />pre-reservoir period, water years 1947-1962.
<br />This discharge was equaled or exceeded 3% of
<br />the time, or II days/yr on an average. During
<br />the post-reservoir period, water years 1966-
<br />1981, the effective discharge decreased by 44%
<br />to 11,500 ft3/s. This discharge has a duration of
<br />7.5% of the time or 27.4 days/yr. The effective
<br />discharge of the Green River in the vicinity of
<br />the Green River, Utah, gage was 26,500 ft3/s
<br />during the pre-reservoir period, water years
<br />1944-1962. The effective discharge was equaled
<br />or exceeded 2.8% of the time or 10.2 days/yr.
<br />During the post-reservoir period, water years
<br />1966-1981, the effective discharge decreased by
<br />23% to 20,500 ft3/s. The duration of effective
<br />discharge has decreased slightly to 2.4% of the
<br />time or 8.8 days/yr.
<br />Magnitude of the effective discharge has de~
<br />creased significantly in all three reaches as a con-
<br />sequence of flow regulation; The percentage
<br />change is greatest attbe farthest upstream reach
<br />and decreases downstream as tbe influence of
<br />the reservoir diminishes. Nevertheless, the effect
<br />is still significant 290 river miles downstream at
<br />the Green River, Utah, gage, where the contrib-
<br />uting area is 2.7 times that of the reservoir. No
<br />directly comparable studies showing the down-
<br />stream influence of a reservoir are known.
<br />Gregory and Park (1974), however, described
<br />an example in which the decrease in channel
<br />area extended downstream to a point where the
<br />contributing area was four times tbat at the
<br />reservoir.
<br />
<br />ADJUSTMENT OF BANKFULL-
<br />CHANNEL DIMENSIONS
<br />
<br />Computed effective discharges were shown to
<br />be in excellent agreement with measured bank-
<br />full discharges at unaltered watersheds with self-
<br />formed channels in the Yampa River basin
<br />(Andrews, 1980). The bankfull characteristics of
<br />these self-formed channels therefore appeared to
<br />be determined by the effective discharge over a
<br />period of years. As shown above, the effective
<br />discharge computed for selected reaches of the
<br />Green River downstream from Flaming Gorge
<br />'Reservoir has decreased significantly. Conse-
<br />quently, it is to be expected that the bankfull
<br />
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