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, . DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH 1015 I utes -1.9 x 106 tons/yr. An equilibrium be- tween sediment supply and transport occurs downstream from the mouth of the Yampa River and may exist for some distance upstream. Tbus,the. r.each..gf-!ctivc:ch311IleLd.-egradation is relatively short, no more than 6li Jiv.cLmiles. Thisresiilt'inrconseql.lence of the Ipcation of I the, r!:SerVo!i' just upstream from t~~_higlL ee~~~ent~Yielding.l'Ortion o~theli~I~~n. Mean annual flow of the Green River at the Ouray gage has not changed appreciably since the construction of Aaming Gorge Dam~ annual water discharge of the Green River at VIe ~ay, Utah, gage was 5,500 ft3/s from 1951 to 19,62 compared to 5,450 ft3/s from 1963 to -12B.1...The estimated mean annual sediml'nl (!is- charge at the Ouray gage, however, decreased 48% frQm 12.8 x 106 tons prior to 1962, to 6.61 x 106 tons during the post-reservoir period. The decrease in the mean annual sediment discharge at the Ouray gage, 6.2 x 106 tons, is significantly greater than the estimated annual quantity of material deposited in Aaming Gorge Reservoir, 3.6 x 106 tons. A comparison of mean annual sediment inflow and outflow to the Green River between the Jensen and Ourav ga~es (reach 2) rshows that, on an average, 2.4' x 106 tons/yr I have been deposited in this reach (Table 1). Substantial aggradation of reach 2. therefore, ~_as occurred since the construction of Fiiiilfng Qorge Dam. This aggradation probably ~- centrated near the downstream end of the reac!t. e two principal tributaries to reach 2, the hite and Duchesne Rivers 'oin the Green iver within a short distance upstream from the uray gage. These tributaries deliver to the Green River an estimated mean ann~iment discharge of 4.8 x 106 tons or 85% of the total quantity of material supplied to reach 2. Conse- quently, aggradation in reach 2 is most likely greatest in the reach immediately upstream from the Ouray gage. r Upstream from the mouth of the Duchesne"\ , I River. there is an approximate balance between:' [graphs taken of the channel prior to 1962 indi-r Ition of the riverbed is inhibited by the coarse i the supply and transport of sediment within.. cate that the size of bed material has not ~-material armor that has developed. ;' reach 2 over a period of years. Consequently, changed significantly. Entrainment of such Downstream from the canyon, the Gree~~e channel of the Green River appears to be in . I, ....f coarse bed material probably was only a few River flows through the wide Browns Park Val- eRuilibrium from the mouth of the Vampa River, *' ; days a year on an average before flows were ley for 40 river miles before entering Ladore downstream to the mouth of the Duchesne f regulated (Graf, 1980). As a result of flow regu- Canyon. The size of bed material decreases River under the hydraulic conditions that have' "-Iation, discharges larger than 5,000 fi3/s no through the reach, until the channel bed be- existed since flow regulation by Aaming Gorge' longer occur. Prior to 1962, a daily mean dis- comes entirely sand with a median diameter of Reservoir began in October 1982. i charge of 5,000 ft3/s was equaled or exceeded 0.40 mm in the last 15 river miles. Owing to Mean annual water discharge of the Green 10% of the time, and the maximum value for the channel degradation and tributary inflow, the River at the Green River, Utah, gage was 5,580 water years from 1951 to 1962 was 19,000 quantity of sediment transported increases rap- ft3/s from 1963 to 1981 compared to 5,830 ft3/s. Since 1962, discharges large enough to en- idly through the reach from between river miles ft3/s from 1944 to 1962. As shown previously train bed material and thus degrade the riverbed 34 to 50 downstream from Aaming Gorge Res- for the Green River gages at Jensen and Ouray, have occurred only very rarely as a result of ervoir. The Yampa River joins the Green River Aaming Gorge Reservoir has not had an appre- extreme floods on tributaries. Further degrada- 68 mi downstream of the reservoir and contrib- ciable effect on the mean annual flow of the 1120 440 1110 1100 1070 430 I I I I I I I I I IDAHO I I I I I ------~ ~ I : II' I : 1\ I . UTAH L-t .' EX PLANA TION .... GAGING STATION @380MEANANNUAL --- FLOW (FTJ/S) 17.0 SEDIMENT DISCHARGE (x 10 6 TONS/YR) 420 410 400 COLORADO 390 o 25 50 100 MilES 380 Figure 2. Mean annual runoff and sediment load prior to 1962 at selected gaging stations. 1060