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. . r .~ 1022 Adjustment oC the Green River Channel in Browns Park Bankfull-channel width of the Green River through Browns Park was measured at 24 cross sections spaced evenly throughout a reach of 22 river miles, using large-scale aerial'photographs taken in 1951 and 1980. The channel was nar- rower at all locations except one. On an average, channel width decreased by 13% from 560 to 485 ft during the period. As described above, the effective discharge of the Green River in Browns Park decreased by 63% to 2,750 ft3/s from 7,450 ft3/s as a result of flow regulation by Flaming Gorge Reservoir. At some time since the 1951 aerial photo- graphs were taken, the Green River in Browns Park began building a new flood plain -4.0 ft lower than the previous one. The lower 110od- plain elevation is due both to a decrease in bank- full depth associated with the decreased effective discharge and to degradation of the riverbed. Given the measured river slope, bankfull width, and bed. material-size distribution, stage-dis- charge relations were computed for the pre- and posl-reservoir periods, using lhe Engelund- Hansen roughness equation (Engelllnd and Hansen, 1967). This analysis indicated that the bankfull depth has decreased from 4.7 ft to 3.1 ft. The mean channel degradation therefore has been -2.4 ft. An estimated 9.5 x 106 tons of sand-sized bed material was eroded from the 22 mi of alluvial channel within Browns Park be- tween 195 I and 1980, when the aerial photo- graphs were taken. All of the degradation probably occurred since 1962, when the up- stream supply of sediment was substantially de- creased by Flaming Gorge Reservoir. Adjustment of channel width to the regulated l10w and decreased sediment supply to the Green River through Browns Park is far from complete. The estimated quasi-equilibrium channel width associated with the decreased ef- fective discharge as computed by equation 3 is 340 ft. This estimate is significantly less than the measured value of 485 ft. On the basis of the comparison of aerial photographs, as well as cross sections surveyed in 1983, the present channel primarily is the result of riverbed degra- dation. Deposition of new bank material does not appear to have been a significant factor con- tributing to the decrease in channel width. Rather, the present, smaller channel was formed by entrenchment within the pre-1962 channel. Given the deficit in sediment supply compared to transport, net deposition of material along the banks would be inconsistent. E. D. ANDREWS Adjustment oC the Green River Channel Downstream Crom Jensen, Utah A few miles downstream from the Jensen gage, the Green River enters an alluvial valley within which it flows for nearly 60 river miles. Bankfull-channel width in this reach was mea- sured at 15 cross sections, using large-scale aerial photographs taken in 1964 and 1978. Although the earlier series oC photographs were taken 2 yr after flow regulation had begun, they probably show a good representation of lhe pre-reservoir river channel. The channel was narroWer at all locations examined except at one in 1978 com- pared to 1964. On an average, bankfull-channel width decreased by 13% from 700 flto 610 ft. The computed effective discharge of the Green River at the Jensen, Utah, gage decreased by 44% from 20,500 to 11,500 fl3/s during fhe post-reservoir period, water years 1966-1981. Adjustment of Green River channel width downstream from the Jensen, Utah, gage was incomplete in 1978. The estimated quasi- equilibrium channel width associated with the decreased effective discharge as computed by equation 3 is 524 ft. This estimate is substan- tially less than the measured value of 610ft. If a constant rate of channel narrowing is assum-ect;-- -lQ]~~~!be required for the channel wi![t!:i~ attain t~ed value associated with the p-.Qst-reser~ive discharge. At many cross sections, the bankfull channel has become nar- rower as a result of accretion of material along one or both banks. Although the accretion of bank material was common, the most significant process narrowing the channel occurred where a distributary channel has filled with bed material, and the mid-channel bar has become attached to the bank. Concomitantly, a thick vegetation' cover has become established on these areas. Both series of aerial photographs show the Green River at approximately the same dis- charge, 6,000 to 7,000 ft3/s. The nllmh..r allo areaLex~~_~~ gLI!!!41;bJl!!nel J~~rs was apprecia- bly greater in 1978 than in 1964. in spite of the facTthat some bars jJav~.become attachedw ----... --" - banks. Adjustment oC the Green River Olannel Downstream from Green River, Utah An alluvial reach located downstream from the Green River, Utah, gage was selected to in- vestigate changes in bankfull-channel width in reach 3, the aggrading part of the river. large- scale aerial photographs of this reach were taken in 1952 and 1981. Bankfull-channel width was measured at 14 cross sections located in a reach of nearly 15 river miles. The channel was nar- rower at all cross sections in 1981 compared to 1952. Qn an average, the bankfull-channel width decreased by 10% from 5 I 'i 10 46Ut. - -~,..::..--- -. '- Adjustment oC channel width in the Green River downstream Crom the Green River, Utah, gage to the flow regulation by Flaming Gorge Reservoir appears to be nearly complete...1Ee _.!,':xp.ected cj}anneLwidth nnn..r quasheq.l!i!ibl:iY..m--- g)l1ditions, given)_~e decreased effe~j~~~s- charge, is 450 ft. This value is only slightly less than the measured value oC 465 ft. SUMMARY AND CONCLUSIONS The contribution of runoff and sediment per unit area to the channel network varies greatly within the Green River basin. Furthermore, the principal source areas of runoff and sediment are different. A majority of the basin-wide runoff is supplied by the relatively high-elevation areas (> 10,000 fl) near the rim of the basin. These areas, however, have very small sediment yields. The most important sediment-contributing areas are far downstream, within the middle- and lower-elevation parts oC the basin. The downstream effects of Flaming Gorge Reservoir are profoundly affected by its location in the drainage basin relative to the principal runoff and sediment-contributing areas. Com- pared to the farthest downstream gage, the Green River at Green River, Utah, the area up- stream from Flaming Gorge Reservoir, prior to its construction, contributed 37% of the annual runoff but only 21 % of the sediment load from 37% of the drainage area. Consequently, the res- ervoir controls a proportional share of the basin runoff but traps only a moderate proportion of the basin-sediment yield. The downstream ef- fects of Flaming Gorge Reservoir on the Green River channel would be markedly different if , either a larger or smaller proportion of the basin- j sediment yield were trapped within the lreservoir. Pre- and post-reservoir sediment budgets were computed for three reaches of the Green River, using measured daily water and sediment discharges. Prior to the construction of a dam in Flaming Gorge, a quasi-equilibrium condition appears to have existed downstream in the Green River channel; that is, over a period of years, the transport of sediment out of a given river reach equaled the supply of sediment into the reach. Since reservoir regulation began in . 1962, the mean annual sediment discharge at