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dam, such as regional geology and geomorphology, climate, tributary inflows of water and sediment, and human activities (Table 1). Changes in these factors have caused adjust- ments in channel geomorphology, alterations in riparian vegetation and fish assemblages, decreases in habi- tar availability for endangered fish, and changes in water temperature and quality. Deciding whether to restore or rehabilitate the Colorado River ecosystem requires an under- standing of the role of Glen Canyon Dam, in relation to othet factors, in causing ecosystem change and the potential to reverse these changes. Water discharge and sediment trans- port. The construction and opera- tion of Glen Canyon Dam reduced the frequency, magnitude, and dura- tion of floods through the Grand Canyon. Before the dam was con- structed, peak discharge occurred in late spring following snowmelt in the Rocky Mountains (Figure 2). The magnitude of the two-year recur- rence flood for the period 1921- 1962 was 2150 m'/s, and flows that exceeded 1250 m'/s were typically sustained for 30 days or more (Table 2). Short-duration floods occurred in September and October. Since the dam's completion in 1963, the magnitude of annual high flows is determined by the magni- tude of inflows and the elevation of the reservoir when these inflows oc- cur (Figure 3). Lake Powell did not fill to capacity until 1980, and dam releases were always less than the capacity of the Glen Canyon Dam power plant, which is approximately 891 m'/s. Large-magnitude dam re- leases occurred in 1980, and annu- ally between 1983 and 1986, be- cause the reservoir elevation was high and inflows were large. The two- year recurrence flood at Lees Ferry was 679 m'/s for the period 1963- 1996; flood flows of 1250 m'/s or more now occur less than 1 % of the rime (Garrett and Gellenbeck 1991). The 1996 controlled flood of 1272 m'/s was much smaller than typical pre-dam floods (Figure 2). Dam re- leases between 1964 and 1990 were characterized by large, hourly flow fluctuations resulting from load-fol- lowing hydroelectric power produc- tion in response to regional demand. September 1998 Figure 2. Hydrograph ofw3reryear 1996corn- 51 pared to pre-dam and 1Il8 post-dam hydrographs for the Colorado River I at Lees Ferry, Arizona. tf! (a)Pre-dam(1922-19621. I;; (b) Post-dam (1963- ,. 1995). The heavy black ~ line is the hydrograph i!O for 1996 and is the same on both panels. The dashed, solid, and dot- ~ red lines connect the ~ mean daily discharge values for each date be- low which 90%, 50%, and 10% of the years, respectively, occur. . 3000 2500 2000 1500 1000 soo 9OthPlrcmtl.., ,I .........\,\,1. II "1 I " ~ \ I 50th~rc.-rtJr. ~ '\ ,I,,, \ 1., 1996___ I \ 10\:t11Wca1t11e I ,- ~ I 1 ~ i'lll ~ '.. t'. ~; r, o o n-."-".":.:':" - - '-..::-:::..- so 100 150 200 250 DAYS, BEGINNING OCTOBER 1 350 b 3000 The average sus- ~ 2500 pended-sediment Ii! load of the Colorado ~ 2000 River at Lees Ferry !ii was approximately Ii 1500 6.0 X 1010 kg/yr be- i3 fore construction of J; 1000 the dam. An average I additional 1.8 x 1010 . soo kg/yr is contributed by tributaries down- stream from Lees Ferry; 70% of that amount comes from rhe Paria and Little Colorado Rivers (Andrews 1990). The magnitude of this annual sediment resupply varies greatly. In 1964 and 1965, after the dam was completed, the average annual sus- pended-sediment load of the Colo- rado River at Lees Ferry was only 0.000013 x 1010 kg/yr because Lake Powell traps all the sediment trans- ported from the upper Colorado River basin. o o River corridor geomorphology. Res- toration options are determined in part by the geomorphic attributes of the river corridor. The width of the Colorado River in the Grand Can- yon is constrained by bedrock, talus, and debris fans (Howard and Dolan 1981). Debris fans, which are com- posed of coarse debris supplied from steep tributaries, partially consrrict the channel and create rapids (Webb et al. 1989). Before the completion of Glen Canyon Dam, mainstem floods reworked debris fans and re- moved all but the largest boulders from the rapids (Howard and Dolan 1981, Kieffer 1985, Webb etal. 1989, 1996). Debris fans have increased in '00 ~.. 5Oth~I. Hlgfi l , . I..... l: 9Oth"'~t11. ~ /1"1,(1 '\' \ ,1/ " ....'r .....1...1"_,..,... , , ~~:'. ;~ ~r;; ~;:~ '. .,.... ", so 300 .'. f" i. . . }'" ~ 350 100 150 200 250 DAYS, BEGINNING OCTOBER 1 volume and thus narrowed adjacent rapids because the magnitude of post- dam floods has been too sma/J to transport the coarse debris delivered to the river since the dam was com- pleted. As rapids narrow, they pog tentially become more difficult to 1 navigate and pose safety hazards. . Thus, high dam releases might be used to rework accumulating coarse debris. Releases at maximum power- plant capacity rework parts of recent debris-flow deposits; larger dam re- leases, such as those that occurred between 1983 and 1986 and during the 1996 controlled flood, caused substantial debris-fan reworking, but they still did not entirely reverse the narrowing trend (Kieffer 1985, Webb et al. 1996). Un vegetated sandbars were a dis- tinctive landscape feature of the un- regulated river. Sandbars form in eddies that occur downstream from most debris fans (Schmidt 1990, Schmidt and Rubin 1995). These eddies have relatively low velocity and turbulence and are prominent sites of sand accumulation. Sand- bars are dynamic features subject to deposition during floods and ero- ! '. i'.. ~. . to' i ~: , ~. .' .,. . ", f;': ~t ~tf~ ;....<" 737