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'.~ . r .. .' , ' 1120 DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH lOB 1110 1080 1070 1100 440 430 I I I I\ IDAHO I r" ..~,,\ " ( \v . (~ I : ,.\~ I ,13 -------1:: \, ) \ '\ " .(\"1 I : '---~Basin Boundary I r '___', ! \ -..I.\, Lt .~---- ~ ) ) ..J WYOMING 420 410 UTAH 400 COLORADO 390 o 25 50 100 MILES 380 Figure 1. Locations of principal water and sediment gaging stations. fish (Miller and others, 1982). These fISh have adapted to a riverbed with areas of both sand and gravel. Gravel-bed parts of the channel, es- pecially rimes, are essential for spawning. Sand- bed parts of the channel, especially backwater along banks or in the lee of bars, are utilized by the juvenile fISh. Either aggradation that results in aAII sand-bed channel or degradation that results in an all gravel-bed channel would elimi- nate essential habitat. Although the babitat of native fISh is sensitive to channel disequilibrium, appreciable channel aggradation or degradation also will have serious consequences for engineer- ing structures and the use of land adjacent to the river (Lane, 1955). HYDROLOGY OF THE GREEN RIVER BASIN The Green River drains -44,700 mi2 along the west flank of the Rocky Mountains in Wyoming, Colorado, and Utah (Fig. 1). It is one of the principal tributaries in the Colorado River basin. The main-stem Green River originates in the Wind River Mountains of Wyoming and Oows southerly to its confluence with the Colorado River near Moab, Utah. The principal tributar- ies of the Green River are the Blacks Fork, Yampa, Duchesne, White, Price, and San Raf- ael Rivers. Flow in the Green River has been regulated by Flaming Gorge ReselVoir since Oc- 1060 tober 1962. Fontenelle ReselVoir, also located on the main-stem Green River, is the second largest impoundment in the Green Rivj:r basin and was completed in April 1964. Tribu,taries to the Green River have numerous. smail im- poundments, especially in their headwaters. Ex- cept for the Duchesne River, however, the tributaries are generally free flowing and unregu- lated at present (1985). Several reselVoirs have been proposed and are being considered for the principal tributaries. The location of the long-term gaging stations in the Green River basin are shown in Figure 1. Water discharge has been recorded daily at most gages for several decades. Extensive records of suspended-sediment concentration also have been collected at the gages shown in Figure I. At those gages where the suspended-sediment concentration has been measured daily for five years or more, the mean annual sediment dis- charge was determined by averaging the series of annual values. At the other gages, the relation between sediment discharge and water discharge (sediment-discharge rating cUlVe) was computed from a least-squares regression of the log- transformed values. The mean annual sediment discharge was computed by the sediment- discharge rating curve and flow-duration method. The water- and sediment-discharge records were compiled to determine mean annual water and sediment budgets prior to construction of Flaming Gorge ReselVoir, for three reaches of the Green River. Reach 1 extends downstream from Flaming Gorge ReselVoir to the Jensen gage. Reach 2 extends downstream from the Jensen gage to the Ouray gage. Reach 3 extends downstream from the Ouray gage to the Green River, Utah, gage. The mean annual inflow and outflow of water and sediment to each reach are summar- ized in Table I. The periods of record used to compute the mean annual values are indicated. The mean annual water and sediment discharges at the Jensen and Green River, Utah, gages since regulation by Flaming Gorge Dam began were computed from the measured daily values. Sampling of suspended-sediment concentration in the Green River at the Ouray gage was dis- continued in September 1966, 4 yr after the be- ginning of reselVoir regulation. This record was extended by correlating tbe 4 yr of measured sediment loads with the sediment loads deter- mined for the Green River, Utah, gage. The coefficient of determination for the relation is 0.95. For some tributaries, the mean annual contri- butions of water and sediment to the Green River were computed from daily measurements collected after regulation of flow by Flaming