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. " . ;- DOWNSTREAM EFFECTS OF RESERVOIR ON RIVER, COLORADO AND UTAH 1021 300.000 !llO.ooo D PRE-RESERVOIR ~ POST-RESERVOIR 250.000 400.000 ii: t '" z g 0 " JOO.OOO 9 !ii' ~ 0 '" '" 200.000 500.000 200.000 iC >- Ui z 0 t: 0 <l: 150.000 0 -' >- Z w :i i5 w VI 100.000 50.000 100.000 DPRE-RESERVOIR ~POST.RESERVOIR 10.(00 20.000 :I).em 4O.em 5O.em DISCHARGE IFT''/51 o o Figure 10. Comparison of the quantity of bed material transported by increments of discharge during the pre- and post-reservoir periods computed for the Green River in the vicinity of the Green River, 40,000 Utah, gage. 10.000 20.000 30.000 DISCHARGE IFT'/SI Figure 9. Comparison of the quantity of bed material transported by Increments of discharge during the pre- and post-reservoir periods computed for the Green River in the vicinity of the Jensen, Utah, gage. extent of channel migration were determined di- rectly from the photographs. (2) Changes in the channel pattern were described. (3) The nODd plain constructed during the pre- and post- reservoir periods was identified on photographs. Subsequently, the change in nODd-plain eleva- tion was surveyed during a field inspection. Leopold and Maddock (1953) found that the variation of the hydraulic variables-mean ve- locity (u), width (w), and mean depth (d), with increasing bankfull discharge (Qu) in the down- stream direction-could be described broadly by a set of simple power equations: These relations are called the "downstream hy- draulic geometry" of a river. Langbein (1964), Engelund and Hansen (1967), and Parker (1978, 1979) showed that the width-versus- discharge equation is the most consistent rela- tion. The coefficient of determination (r2) for the width equation typically is 0.95 or larger, and the exponent value, b, rarely differs much from 0.5. Underlying the hydraulic geometry analysis there is the assumption or determination that the hydraulic variables have attained a quasi-equilibrium adjustment to the magnitude and frequency of nows that have occurred over a period of years. A significant deviation in the value of a hy- draulic variable from the expected value is evi-. dence that the particular variable is not in adjustment with the prevailing range and dura- tion of sediment and water discharges. Given the prior bankfull width (WI)' the prior bankfull dis- ii = k QBm, (2) and w = a QBb, (3) d=cQuf. (4) charge (QI), and estimated future bankfull dis- charge (~), the future bankfull width (w2) can be eslimated by equation 3, assuming that a( = a2. This is not an unreasonable assumption as long as such factors as channel slope, composi- tion and density of bank vegetation, bed- material-size distribution, and suspended-sedi- ment concentration versus discharge relations remain unchanged. If the measured bankfull width after a large change in the effective dis- charge differs significantly from the estimated value, then one may conclude that the adjust- ment of channel width is incomplete. Although the Green River has been affected by the spread of tamarisk during the past cen- tury, riparian vegetation has not been a major cause of any channel changes since 1951. Graf (1978) examined channel changes at 18 cross sections first photographed by river explorers prior to 1915 and found that thick stands of tamarisk had grown along the banks in many reaches. Concomitantly, channel width has de- creased by an average of 27%. These data indi- cate, however, that all of this change occurred before 1951.