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<br />DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH
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<br />ACKNOWLEDGMENTS
<br />
<br />I 1 I
<br />
<br />downstream gages has decreased substantially.
<br />The mean annual sediment discharge has de-
<br />creased by 54% from 6.92 x 106 to 3.21 x 106
<br />tons at the Jensen gage, by 48% from 12.8 x 106
<br />to 6.62 x IOIj tons at the Ouray gage, and by
<br />48% from 17.0 x 106 to 8.83 x 1.06 tons at the
<br />Green River, Utah, gage. The decrease in mean
<br />annual sediment discharge at the Ouray and
<br />Green River, Utah, gages far exceeds the quan-
<br />tity of sediment trapped in the reservoir.
<br />Post-reservoir annual sediment budgets show
<br />that a majority of the Green River channel is no
<br />longer in quasi-equilibrium. Three distinctlongi-
<br />tudinal zones involving channel degradation,
<br />quasi-equilibrium, and aggradation were identi-
<br />fied. Beginning immediately downstream from
<br />the dam and extending downstream 68 river
<br />miles, sediment transport out of the reach ex-
<br />ceeds the tributary contribution, and the channel
<br />, is degrading. The length of the degrading reach,
<br />: however, is relatively limited, due to the large
<br />: quantity of sediment supplied by tributaries. .
<br />'- In the reach between river miles 68 and 166
<br />downstream from Aaming Gorge Reservoir, the
<br />quantity of sediment supplied to the reach from
<br />upstream plus tributaries approximately equals
<br />the transport of sediment out of the reach over a
<br />period of years. This reach appears to be in
<br />quasi-equilibrium, as there is no net accumula-
<br />tion or depletion of bed material. Downstream
<br />from river mile 166 to the mouth (river mile
<br />412), the supply of sediment from upstream and
<br />tributary innow exceeds the transport of sedi-
<br />ment out of the reach by 5.4 x 106 tons/yr on an
<br />average.
<br />The decrease in mean annual sediment trans-
<br />\ port at the Jensen and Green River, Utah, gages
<br />\ since 1962 is due entirely to a decrease in the
<br />\ magnitude of river nows that are equaled or
<br />1 exceeded <30% of the time. Daily mean water
<br />"discharges with a duration of 5% or less have
<br />decreased in magnitude by 25% during the post-
<br />reservoir period at both the Jensen and Green
<br />River, Utah, gages. The magnitude of daily
<br />mean discharges with a duration >30%, how-
<br />ever, has increased to the extent that the mean
<br />annual runoff measured during the pre- and
<br />post-reservoir periods is virtually unchanged at
<br />both gages. The decrease in annual sediment
<br />transport thus results from a more uniform an-
<br />
<br />
<br />nual hydrograph rather than from a decrease in
<br />the annual runoff.
<br />The quantity of sediment in transport at a
<br />given discharge does not appear to have been
<br />affected by Aaming Gorge Reservoir. For both
<br />the Jensen and Green River, Utah, gages, the
<br />variation of daily suspended-sediment transport
<br />rate as a function of water discharge was com-
<br />puted for each size fraction from <0.004 mm to
<br />the 0.250-0.500 mm fraction by a least-squares
<br />regression of the log-transform data during the
<br />pre- and post-reservoir periods. No statistically
<br />significant change in the relations was detected
<br />by the f-statistic at the 95th-percentile level of
<br />confidence.
<br />Over a period of years, the bankfull dimen-
<br />sions of an alluvial channel adJiustto the magni-
<br />tude and duration of discharges. The effective
<br />discharge is the increment of discharge that
<br />transports the largest quantity of sediment over a
<br />period of years. Computed en'ective discharges
<br />for 15 alluvial reaches in the Yampa River basin
<br />were found to be nearly identical to the bankful
<br />discharge (Andrews, 1980). Therefore, the
<br />bankfull-channel dimensions appear to be ad-
<br />justed to the effective discharge.
<br />The effective discharges that occurred during
<br />the pre- and post-reservoir periods were com-
<br />puted for three reaches downstream from Aam-
<br />ing Gorge Reservoir. Since 1962, the effective
<br />discharge of the Green River has decreased from
<br />7,450 to 2,750 ft3/s, in Browns Park, 20,500 to
<br />11,500 ft3/s in the vicinity of the Jensen gage,
<br />and 26,500 to 20,500 ft3/s in the vicinity of the
<br />Green River, Utah, gage.
<br />An analysis of aerial photographs and inspec-
<br />tion of channel morphology indicate that the
<br />bankfull-channel width of the Green River has
<br />decreased in response to the decreased effective
<br />discharge caused by now regulution. On an av-
<br />erage, bankfull-channel width has decreased by
<br />13% to 485 ft from 560 ft through Browns Park,
<br />13% to 610 ft from 700 ft downstream from the
<br />Jensen gage, and 10% to 465 ft from 515 ft
<br />downstream from the Green River, Utah, gage.
<br />Although the decrease in bankfull-channel
<br />width has been significant in all reaches, achan-
<br />nel width consistent with the prevailing effective
<br />discharge exists only in the reach downstream
<br />from the Green River, Utah, gage.
<br />
<br />\'"
<br />
<br />1023
<br />
<br />Robert Hirsch and Brent Troutman contrib-
<br />uted to this investigation in many ways. Their
<br />thoughtful discussions and critical comments
<br />concerning tbe analysis of long-term trends in
<br />sediment transport rates bave been greatly ap-
<br />preciated. Robert Meade, John Lewin, and
<br />Donald Hillier reviewed drafts of the manu-
<br />script. Their suggestions bave been especially
<br />helpful.
<br />
<br />Rf:FERENCES CITED
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<br />
<br />MANU''''''" RECUVro.. THE SoCIEn MA' 20.1985
<br />REVISE.D M4Nuscllln Rr,CElVED Fultu....' 12, 1986
<br />I.lANUSCIl'" Accurw FOIu... 14. 1986
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<br />Printed in U.S.A.
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