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~ .:~ lj ~ ;\1 r.~ t.~ f? h~ .,.-. r::'.:=.: , ~" : ~ -: Table 2. Comparison of pre- and post-dam Colorado River resources downstream from Glen Canyon Dam. Changes occurred on widely differing time scales. Whereas physical changes (in flow, sediment load, and temperature) occurred rapidly afrer closure of the dam in 1963, geomorphic (e.g., debris-fan reworking and sandbar erosion and deposition) and biotic (e.g., trophic patterns and non-native species invasion) changes occurred more slowly and are ongoing. Ri....erine feature Hydrologic regime Before Cleo Canyon Dam Variable; two-year flood was caused by regional snowmelt that averaged 2150 m3/s between 1921 and 1962' Sediment load Variablej mean annual suspended scdiment load at Lr-cs Ferry was 6,0 x 1010 kg" Debris fans All but largest boulders from rapids frequently reworkedb,d.. River temperarure Varied seasonally, from near freezing in winter co 25-30 oC in summer'" Unvegetated sandbars Common; distinctive features associated with eddies downstream from debris fans~~,III Trophic srructure Thought to be heterotrophic because high sediment loads diminished light availability Eight native endemic speciesj 74% level of endemism is highesc among North American rivers; heaVily dependent on terrestrial food sources; some species extirpated, others endangeredf,q Fish assemblage After Glen Canyon Dam Regulated; two-year flood of 679 m3/s is less than the power-plant capaciry of 940 m3/s; large hourly fluctuations are associated with l03d-folJowing power production Virtually zero in dam releases; mean annual contribu- tion of 1.8 x 1010 kg from tributaries downstream from l.ees Ferrr,b,~ Debris flows continued, with consequent aggradation] of rapidsd.. Nearly constant 8-10"C because water is drawn from below thermal discontinuity in Lake Powell in summerj there is slight year-to-year variation in the temperature 4Y of the winter isothermal periodI.8.b,',J ~ '\)(~ A near-river riparian zone has been established that 'f consists of a marsh zone within the range of river stages regulated by power-plant operations'.n,o.p Autotrophic in dam rai]water and in nearshore or cobble-bars downstream',1 Warm-water fishes introduced to lake Mead and trout lO the rributaries by rhe 1930s; tailwarer trout fishery is highly valued \., " ~,: '. . ;:-- , L f t ft.: '~'i. ., ; ~ r , '1 ~. ,. I. t.: ,. t . ,.. i . ~. . , . f.' 'Andrews (1990). "Howard and Dolan (1981). <Randle et aL (1993). "Webb et al. (1989). 'Webb et al. (1996). 'Valdez and Ryel (1997). 'Marzolf et al. (1996). ~Stanford and Ward (1991). iStcvens et a!. (1997a). sion after flood recession (Rubin et al. 1990). Before the dam's comple- tion, both the size of the sediment- comprising eddy bars and the shape of these bars reflected the character- istics of sediment transported during recession from the annual spring peak as well as from lower-magnitude late- summer floods. Thus, pre-dam de- posits that still exist in the Grand Canyon are typically very fine sand mixed with silt and clay. 7000 ~ 8 .. ~ 6000 figure 3. Annual peak ~e sooo discharge of the Colo- n 4000 rado River at Lees Ferry, Arizona. Solid 3000 line is a weighted aver- u ~ 2000 . age. The year of corn- u . pJetion of Glen Can- I!; 1000r yon Dam (1963) and the magnitude of the o ' 1996 controlled flood 1920 are indicated. 738 ISteVens et al. (1997b). 'Schmidt (1990). ISchmidt et al. (1995). .Webb (1996). KTurner and Karpiscak (1980), 'Johnson (l991). pStcvens et al. (1995). 'Miller (19591. A sediment budget calculated for a 141 km reach immediately down- srream from Lees Ferry indicates that fine sediment accumulates in the Grand Canyon despite the fact that no sediment is released from Glen Canyon Dam (Randle et al. 1993). Accumularion occurs because the undammed Paria and Linle Colo- rado Rivers continue to contribute 8IIl~0II1llDll_1d I 1 '9t"~ftood \X... x)( x X ~ k I '~~'. ~ -"Ie x x...J< ~ x xX X "><.?'~ x !Ix xX 1930 1940 1950 1960 1970 1980 1990 2000 Yl'AR significant amounts of fine sediment to the Colorado River. This sedi- ment accumulates on the channel bed and in eddies because the bars along the river's margin have typi- cally eroded, not aggraded. At least 30% of all large, high-elevation sand- bars in the Colorado River decreased in size between 1965 and 1973; 32 % decreased in size between 1973 and 1991 (Kearsley et al. 1994). These decreases were caused by degradation and invasion by riparian vegetation. Although high discharges of wa- ter in 1983 caused a number of sand- bars to increase in size, almost all of these bars had decreased to pre-1983 sizes by 1991. River runners use large sandbars as campsites, and decreases in sandbar size cause decreases in campsite carrying capacity (Kearsley et al. 1994). Net post-dam erosion of sandbars may decrease with distance downstream from the dam (Webb BioScience Vol. 48 No.9 r t ~.': ," ,. , . r .: i l'f. ~f (: ~.. ' c, ~ . , ( .'