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ment of the effects of controlled flooding was also based on prelimi- nary findings from the 1996 con- trolled flood in the Grand Canyon (GCMRC 1997). Predictions regard- ing thermal-modification impacts were based on studies of changing the thermal regime at Flaming Gorge Dam on the Green River (Holden and Crist 1981). The effects of sedi- ment augmentation were assessed by comparing the characteristics of present river geomorphology and ecology to pre-dam conditions (Webb 1996). In our evaluation of the tech- nique of controlled floods, we as- sumed no sediment augmentation, but we did assume that frequent con- trolled floods are part of sediment augmentation because flooding is the only mechanism to redistribute sand from the channel bed to eddies and f channel margins. No single engineering technique yields desirable responses for every' ecosystem resource and process. Steady flows benefit some resources, such as the tailwater trout fishery, lower riparian-zone vegetation, and marshes, but restrictions on fluctu- ating flows still lead to dereriorarion of many resources. Disturbances caused by controlled floods are nec- essary to maintain some pre-dam relict resources, but controlled floods damage some post-dam artifact re- sources. Even during steady flow, sandbars undergo progressive ero- sion and the size and abundance of low-velocity nursery habitats for native fish decrease. Thus, occasional dam releases that exceed power-plant capacity are necessary to restore sandbar volume and rejuvenate nurs- ery habitats. Floods, however, dam- age some post-dam arrifacts, includ- ing marshes, waterbird habitat, and the endangered Kanab ambersnail population. In some cases, modification of ex- isting structures may permit in- creased flexibility in load-following hydroelectric power production. Sediment augmentation provides more frequent rejuvenation of eroded sandbars and might permit more wide-ranging, load-following dam operations. However, the increased amplirude of load-following releases would likely harm the tailwater trout fishery. There is a large potential for error in predicting the effects of imple- menting a full restoration strategy. Most research in the Grand Canyon has been conducted on a rransformed river, but the native endemic fish species evolved in a sediment-laden, light-limited, largely heterotrophic system. Food sources of pre-dam fish assemblages may have been linked to the decomposition of abundant woody debris, which is now mucb less abun- dant along the channel. Reconstruc- tion of the pre-dam trophic structure in the Grand Canyon therefore rep- resents a major scientific challenge in the development of a robust strat- egy for restoration. Complex resource tradeoffs exist under the five management goals (Table 5). We believe that the choice of an appropriate management goal can be developed only through soci- etal valuation of resources. For ex- ample, under the naturalized river strategy, biodiversity increases be- cause disturbance intensity and bio- geographic ptocesses are reduced. By contrast, strategies that create river conditions that are more like pre-dam conditions decrease bio- diversity and productiviry and in- crease the strength of physical con- trols on the ecosystem. These changes are likely to be more favorable to native than non-native fishes, such as the highly valued rainbow trout. The dichotomy between manag- ing for rei ice versus artifact resources is most obvious with respect to sand- bars and lower riparian-zone vegeta- tion. The two resources are interre- lated, with the management goals of maximum exposed sandbars and ro- bust lower riparian-zone vegetation being mutually exclusive. The ex- pansion of riparian vegetation, in- ;~I "'." ,.. ~I !1 :~-' : '.) .:/.. :< ;... ~. ,,:, :\ '0' .' ~ '. . \:,' I,,'. '.' l:- ~ iii.::;' ,..>~ ;: .: j. ~.1 r '. ~.... L Table S. Expected tradeoffs in ecosystem resources under five management strategies for the: Colorado River in the Grand Canyon. ~ .., Management strategy Traditional river management Expected to increase or stay the same as 1990 condition Uncertain effect Biological diversity Non-native fishes; marsh habitat; terrestrial habitat; non-native planes; migratory species; power revenues Expected to decrease from 1990 condition ,'. " ..1., , Sandbars; rapids; aquatic habitat; native fishes; boat- ing safety; recreational fishing !.. r '\.' ; ~_:~, Sandbars; rapids; power revenues Non-native plants and fishes; marsh habitat; terrestrial habitat; biological diversity; boating safety; recreational fishing Aquatic habitat; native fishes; migratory species Naturalized ecosystem 1:,.._ ~~ (~~ .,", :::t' Recreational fishing; power revenues Simulated natural ecosystem Sandbars; rapids; native fishes; native plants; boating safety Aquatic habitat; non-native fishes; marsh habitat; terrestrial habitat; non-native plants; migratory species; biological diversity .' .~ io Substantially restored river Sandbars; rapids; native fishes; native plants Aquatic habitat; non-native fishes; terrestrial habitat; non.native planes; biological diversity; boating safety Marsh habitat; migratory species; power revenues; recreational fishing Fully restored river Sandbars; rapids; native fishes; aquatic habitat; native plants Boating safety Non-native fishes; marsh habitat; terrestrial habitat; non-native planrs; migratory species; biological diversity; recreational fishing; power revenues September 1998 745