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01189 { 'HUMPB'CK CHUB H,'BrT.'T MODELLING 397 the typical minimum and maximum daily flows (Figure S) showed that the MLFF regime resulted in the crctllion of slTIall amounts of persistent suitable shorelin..' habitat thJl djJ not e,xist during the 'no action' period (Table VI). The: relative importance of changes in the a'.;:rage amount 01 suilable habitat versus changes in the amount of per- sistent suitable habitat to native fish populations is unknolm, Bowen el al. (1998) and Freemen el ai, (2001) showed thot abundance of yay native fish populations in the Tollapoosa Rivcr in the southeastern USA was cor- related with the amount of persistent shallow water habito!. Voldez and Ryel (1995) hypothesized that hourly var- iation in discharge: results in 'destabilization" ofnearshor~ habitats used by native fish that could have a negative effect on mains rem survival. In Grand Canyon. there are no data to quantify or tesl this assertion for native fish, but there has been a well documented and dramatic increase in the rainbow trout population in the c1ear-water"reach upstream of Lees Ferry since the early 1990s when hourly flow variation was /irst reduced (McKinney el 01,,2001), More recent sampling shows what looks like a large increase in rainbow trout throughout the canyon (L. Coggins, Grand Canyon Monitoring and Research Center (GCtvIRC). Flagstaff. AZ, personal communication, 200 I). While there is a reasonable consensus among Grand Canyon rt:searchers that reduced flow variation has caused an increase in the trout population, the overall effect on humpback chub and other native fish remains to be seen. It may be that the exotic response to reduced flow variation outweighs the direct habitat benefits for n~tive fish and results in an o.....erall negative effect on [he resources of most concern. A common theme in our results was the relatively high \ ariation among reaches in the response of suitable habi- tat availability to changes in discharge (Figures 3 and 4), We modelled more than 3.6km (19%) of the mainstem habitot used by the LCRlmainstem humpback chub aggregation, yet in spite of this relatively comprehensive cov- erage, it was difficult to make conclusive statements about the overall effect of impoundment on habitat availabil- ity, For example. reductions of flow from 425 to 226 m'/s. a change achieved during the LSSF experiment in 2000, improved habitat availability at four reaches, reduced it at two reaches, and had no effect on the remaining reach (Figure 3). Ex.tensive reach-to-reach variation was a dominanl characteristic of our results, so extrapolation to sec- tions that were nOt modelled is tenuous. Interestingly, Schmidt et 0/, (1999) documented high spotial variation in the erosion and deposition of sand in eddies after the 1996 e,'(pcrimenlJ] flood in this same section of river. Strong spatial voriation in responses driven by flow fields affect<d by local morphology may be a dominant theme in Gr4l.nd Canyon, and perhaps more caution must be exercised here than for other rivers when trying 10 make infer- ences abouf system-\vjdc responses on [he basis of site-specific results. Restoring elements of the natural hydrograph has been identified as a cornerstone in restoration of riverine eco- systems (National Research Council. 1992; Poff et ai" 1997) and Ihe approach has been recommended for restora- tion of the Colorado River in Grand Canyon (NationJl Research Council, 1996), The seasonally adjusted low st<ady flow regime identified in the Biological Opinion of 1994 was designed to mimic parts of the historical sea- sonol discharge pOllem with high. steady flows in May-June and low steady flows through the summer and fall (Valdez and Carothers. 1998). Low summer steady flows were hypothesized to increase growth and survival of young native /ish in the mainstem through a combination of stabilizing nearshore and backwater habitats and increasing water temperatures downstream from the LCR (Valdez el al., 1999), Habitat stabilization (Table VI) and temperature increases were achieved during the LSSF experiment, but it is wonh noting that the seasonal pal. tern in habitat availability documented in our analysis was anything but natural. The 226 m',s steady discharge from June to August 2000 resulted in greatly improved suitable shoreline habitat availability during months when suitable habitat availability was at minimum or near-minimum values prior to impoundment (Figure 6), It may be that this 'unnatural' strategy will be more effective than a more 'natural' one in a post-impoundment era that is characterized by several new and unnatural elements. such as fluctuating daily discharge, abundant piscivorous exotic /ish populations. and colder water temperatures, Changes in discharge from GCD have the potential to affect our ability to monitor /ish populations as well as affecting the populations directly through habitat effects, The range of discharges over which the 1993 electro- fishing samples were collected was relatively small. yct at reaches where the amount of suitable shoreline habitat area was sensitive to changes in discharge, these discharge differences could result in significant (twofold to four- fold) variation in CPE estimates (Figure 9), Our estimates of the amount of discharge-driven variation in CPE <S1imates is probably lower than what would be expect<d under the current sampling programme because diurnal flow variation under the MLFF regime is slightly larg<r relative to a yoar like 1993 whon interim flows were in effect (Table HI). It is wonh examining the sensitivity of suitable shoreline habitat area to changes in discharge at Copyright Itl 2004 John Wiley &. Sons. LId. Rh'a Res. AppJiC'. :20: 319-400 (2()()4)