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<br />Green River. The estimated mean annual
<br />sediment discharge at the Green River, Utah,
<br />.g"age, however, has decreased 48% from 17~
<br />WOto lU!3 x 106 tons. The decrease in the
<br />mean annual sediment discharge at the Green
<br />River, Utah, gage, 8.1 x 106 tons/yr, is much
<br />greater than the estimalpn aRRllal 'lua~
<br />.material deposited in F1amin~ Gor2e Reservoir.
<br />3.6 x 106 tons. The comparison of mean annual
<br />sediment inflow and outflow in Table I shows
<br />that 2 x 106 tons/yr, on an average, have ac-
<br />cumulated in reach 3 since 1962. Tributaries
<br />deliver - 4.2 x 106 tons/yr to reach 3. The
<br />principal tributary, Price River, joins the
<br />Green River 18 mi upstream from the Green
<br />River, Utah, gage and supplies an estimated
<br />mean annual load of 2.18 x 106 tons. The other
<br />/~ tributaries deliver sediment throughout reach 3.
<br />Therefore, a radation of the Green River
<br />annel robably occurs along t e entire en
<br />...Q[Ieach 3. .
<br />The discussion has emphasized the down-
<br />stream impact on channel equilibrium (sediment
<br />budget) of the Green River due to flow regula-
<br />tion at Flaming Gorge Reservoir. Immediately
<br />downstream from the dam, potential transport
<br />of sand-sized material greatly exceeds availabil-
<br />ity. Tributaries draining areas of relatively large
<br />sediment yields, however, significantly increase
<br />the sediment load of the Green River within the
<br />first 68 river miles downstream from the reser-
<br />voir. As a result, the supply and transport of
<br />sediment probably are in eauilibrium down-
<br />. stream from the mouth of the Yampa River. The
<br />zone of equilibrium under present (1985) condi-
<br />tions probably extends downstream to the
<br />'~_.llIouth of the DJlCh(,ctll: ~ whIch joins the
<br />Green River at river mile 166. Within this reach
<br />of the Green River, there has been no net ac-
<br />cumulation or depletion of sediment. Down-
<br />stream from the mouth of the Duchesne River,
<br />the mean annual supply of sediment from up-
<br />stream and tributaries has exceeded the trans-
<br />port. As a result, there is a long-term net
<br />accumulation of sediment. The zone of aggrada-
<br />tion probably extends downstream on the Green
<br />River to its confluence with the Colorado, al-
<br />though there are no gage records of annual
<br />water and sediment discharge downstream from
<br />Green River, Utah, to confirm this conclusion.
<br />The principal tributary to the Green River
<br />downstream from Green River, Utah, is the San
<br />Rafael River. loms and others (1965) estimated
<br />that the San Rafael River supplied on an aver-
<br />age in excess of I x 106 tons/yr to the Green
<br />River, but relatively small mean annual water
<br />discharge of -140 ft3/s.
<br />Two aspects concerning the downstream se-
<br />quence of degradation, equilibrium, and aggra-
<br />dation that have existed since flow regulation
<br />
<br />E. D. ANDREWS
<br />
<br />began in 1962 are especially noteworthy. First,
<br />the reach of channel degradation has a much
<br />more limited extent than do either the reaches of
<br />equilibrium or of aggradation. Second, the abso-
<br />lute magnitude of the disequilibrium between
<br />sediment inflow and outflow throughout the ag-
<br />-grading reach is as large as or larger than within
<br />the degrading reach. The aggrading reach of the
<br />Green River is much longer than the degrading
<br />reach, and tbe volume of accumulated sediment
<br />is much larger. In terms of channel equilibrium,
<br />the greatest impact of Flaming Gorge D~m is
<br />not immediately downstream, but instead sev-
<br />eral hundred miles downstream. This effect is a
<br />direct result of the location of Flaming Gorge
<br />Reservoir within the drainage basin downstream
<br />from those parts of the basin with large water
<br />yield, but upstream of those parts of the basin
<br />with large sediment yields.
<br />The sediment-transporting characteristics of a
<br />river are altered in complex and manifold as-
<br />pects by a storage reservoir. Principally, a stor-
<br />age reservoir may change the magnitude and
<br />frequency of river flows. as well as the quantity
<br />of sediment transported by a given discharge due
<br />to alteration of the channel morphology and/or
<br />the availability of sediment within the channel.
<br />Each of these factors varies downstream from
<br />the reservoir. The following discussions describe
<br />adjustments of (I) the relation between sediment
<br />transport rate and water discharge for various
<br />size fractions; (2) the magnitude of effective
<br />water discharge; and (3) bankfull channel di-
<br />mensions in the vicinity of the Jensen and Green
<br />River, Utah, gaging stations.
<br />
<br />Sediment Transport Rate
<br />
<br />It has been shown that the mean annual sed-
<br />iment discharge at the Jensen gage has decreased
<br />by 54% from 6.92 x 106 to 3.21 x 106 tons since
<br />1962. This change was determined from mea-
<br />sured daily values of suspended-sediment con-
<br />centration and water discharge. During the
<br />period of record at the Jensen gage, water years
<br />1947-1979, the size distribution of suspended
<br />sediment was determined for 218 of the daily
<br />concentration samples, 161 before October
<br />1962 and 57 after October 1962. The daily sed-
<br />iment transport (lk) for a given size fraction, k,
<br />was computed from the measured percentage of
<br />sediment in a size fraction, Pk, the total concen-
<br />tration, C, and the daily mean discharge, Q:
<br />
<br />Ik = 0.0027 (Pk)(C) (Q). (I)
<br />
<br />Daily sediment transport rates were deter-
<br />mined for 6 size fractions-<0.004 mm,
<br />0.004-0.016 mm, 0.016-0.0625 mm, 0.0625-
<br />0.125 mm, 0.125-0.250 mm, and 0.250-0.500
<br />
<br />mm, as well as for all sand-sized material and all
<br />material for the pre- and post-reservoir periods
<br />at the Green River near the Jensen, Utah, gage.
<br />The transport rate of suspended sediment in 4
<br />size fractions, 0.004-0.016 mm, 0.0625-0.125
<br />mm, sand-sized, and all material measured dur-
<br />ing the pre- and post-reservoir periods, are plot-
<br />ted in Figure 3 versus the associated water
<br />discharge. Regardless of particle size, no appre-
<br />ciable difference in the suspended-sediment
<br />transport rate at a given discharge between the
<br />pre- and post-reservoir periods is apparent at the
<br />Jensen gage.
<br />For each sediment-sized fraction, a least-
<br />squares linear regression was fit to the log-
<br />transformed values of water discharge and daily
<br />sediment transport rate measured during the pre-
<br />and post-reservoir periods. The regression equa-
<br />tions are summarized in Table 2. The regression
<br />equations for the pre- and post-reservoir periods
<br />were compared, using the F-test, to detect
<br />whether a statistically significant change in the
<br />relation between sediment transport rate and
<br />water discharge has occurred. The results of this
<br />analysis are summarized in Table 2. The level of
<br />confidence at which there is no significant dif-
<br />ference between the pre- and post-reservoir pe-
<br />riods varies somewhat, but without any ap-
<br />parent trend, among the several size fractions.
<br />For sand-sized sediment, there is no significant
<br />difference in the relation between transport rate
<br />and water discharge during the pre- and post-
<br />reservoir periods at the 90th percentile level. For
<br />all sediment sizes, there is no significant differ-
<br />ence in the pre- and post-reservoir transport rela-
<br />tions at the 75th percentile level of confidence.
<br />These tests are quite strict. It is thus concluded
<br />that the sediment-transport rate at a given dis-
<br />charge has changed very lillie, if at all, at the
<br />Jensen gage as a result of flow regulation and
<br />sediment storage by Flaming Gorge Reservoir.
<br />Any changes in the sediment-transport relations
<br />probably are limited to those particles <0.062
<br />mm in diameter.
<br />This analysis is in good agreement with the
<br />pre- and post-reservoir sediment budgets sum-
<br />marized in Table 1. Although the mean annual
<br />sediment load decreased by 54% after 1962, the
<br />inflow and outflow of sediment to the reach
<br />upstream from the Jensen gage have remained
<br />in approximate equilibrium. Consequently, the
<br />quantity of sediment stored in the reach up-
<br />stream has remained nearly constant. Assuming
<br />the size distribution of sediment has not changed
<br />appreciably, the transport rate of sediment at a
<br />given discharge should be unaffected by pres-
<br />ence and operation of Flaming Gorge Reservoir.
<br />At the Green River, Utah, gage, the size dis-
<br />tribution of suspended sediment was determined
<br />for 286 of the daily concentration samples, 220
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