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CHANNEL NARROWING BY VERTICAL ACCRETION, GREEN RIVER A Peak Flows ~ ~ 2000 1 BOO 1600 1400 ~ _1200 ~ Ul 2 M'1000 ~ .s BOO o 600 400 200 o o (J) co ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ ~ ~ m m ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 8 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ N B Mean flows 350 300 250 --' Q) ~ 'Ul 200 .r:.fr ~ .s 150 o 100 50 o o ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ 0 ~ ~ ~ ~ m m ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 8 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ N c 1000 800 - 1895-1929 . . . . 1930-1962 --. 1963-1996 Q) ~ 'Ul 600 cu_ .r:.M ~ .s 400 o 200 o o 50 100 150 200 250 300 350 DAYS AFTER OCTOBER 1 Figure 3. Hydrologic history of the Green River at Green River, Utah (station 09315000), showing (A) peak flows (vertical bars), with the 2- and 5-year recurrence interval floods for each of three time periods shown by solid and dashed lines respectively; (B) mean annual discharge (vertical bars), with means for each of three time periods shown by a solid line; and (C) a com- posite hydrograph for each of three time periods. A natural climatic shift occurred about 1930, which led to a reduction in mean annual discharge, and a dam-induced shift occurred in 1963, which did not change mean annual discharge but did reduce flood magnitude. Note that the magnitude of change for both peak discharge A and mean annual flow B was greater for the post-1930 period than for the postdam period. Also note the increase in base flow for much of the year, following dam completion. Geolm,ical Societv of America Bulletin, December 1999 ments is now lower than it was in the past. Allred (1997) demonstrated that the magnitude of scour and fill at the present cableway during passage of the 1995 spring snowmelt flood of 844 m3/s was matched by compensating changes elsewhere within a O.8-km reach near the gage such that mean bed elevation for the entire measured reach did not change during the flood. In order to describe channel bed response to flooding, we identified all historical discharge measurements made within 7 days of a previous measurement when the elevation of the thalweg or the mean bed changed by more than 0.15 m; we also noted the discharge on that day. Mea- surements meeting the above criteria were used to create a histogram of discharges causing bed scour or fill. The threshold discharges that typically cause scour and fill are less than the magnitude of the present 1.25-year recurrence flood (Fig. 6), based on analysis of these dischargemeasurement data. There is little difference imong the frequency distributions for scour or fill and for thalweg or mean bed elevation. In all cases, the modal incre- ment of discharge that causes bed elevation change of at least 0.15 mis between 85 and 142 m3/s. Some bed fluctuations have occurred dur- ing discharges as low as between 57 and 85 m3/s. Channel Cross Section Change Presently, the Green River channel is about 25 m narrower at the old cableway than it was prior to 1928, but the elevation of the channel has not changed appreciably. The wide, pre-1930 chan- nel was photographed (Fig. 7) and/or surveyed (Fig. 8) in 1911, 1912, and 1928, and channel narrowing at this site began after these years. Bank accretion occurred on the left bank, which is the inside of the channel bend (Fig. 1); channel migration toward the right bank is inhibited by bedrock. 1\\10 distinct topographic surfaces now exist in the area of bank accretion. An intermedi- ate-elevation surface is now densely vegetated with saltcedar and Russian olives, and a low- elevation surface that includes a natural levee is now densely covered with willows (Fig. 7d). The Green River channel is about 15 m nar- rower at the present cableway than it was in 1930 when measurements were first made at this site. Bank accretion has entirely occurred on the right bank where an intermediate-elevation topographic surface is densely covered by saltcedar and Russ- ian olives (Fig. 9). There is no low-elevation wil- low-covered topographic surface at the cable- way, although that surface is present nearby within the reach. Channel narrowing began at the present cable- way soon after the measurement site was estab- 1761