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cluding marshes, occurred through colonization of previously bare sand- bars; consequently, restoration of barren sandbars must inevitably oc- cur at the expense of riparian vegeta- tion. Management complications arise because endangered Kanab ambersnails and sourhwestern wil- low flycatchers have colonized new native and non-native lower ripar- ian-zone vegetation. If flooding is crucial to the recovery of flood- adapted species such as the hump- back chub but the absence of floods is crucial to the conservation of ter- restrial endangered species in new habitats, then managers face an in- tractable dilemma. As if the varying impacrs of the assorted management strategies on different ecosystem components do not sufficiently challenge scientists in their attempts to advise managers, their impacts on resources also change longitudinally in the Grand Canyon. These longitudinal differ- ences occur beca use different reaches of the river have different geomor- phic characteristics that strongly in- fluence the depositional and ero- sional effects of flooding: the sediment budget changes down- stream with additional tributary in- puts; the population structures of native and non-native fish and ripar- ian vegerarion change downstream with changing temperature, geomor- phology, sediment transport, food supply, and biogeographic influ- ences; and some endangered species congregate at, or exhibit high fidel- ity to, specific sites. By necessity, some goals may apply only ro spe- cific reaches or sites. Science and societal choice about river-corridor resources Scientific research in the Grand Can- yon demonstrates strong linkages berween dam operations and the re- sponses of individual resources of the river ecosystem. Specific engi- neering actions cause ecosystem changes that enhance some resources at the expense of others. Although scientists may learn to predict with increasing precision the outcome 0 various actions on ecosystem func- tion, they cannot determine whether society will accept these changes. For example, riparian marshes, the 746 most productive and biologically di- verse habitat, would be eliminated if a broad range of pre-dam physical processes were restored to the river. Reduction in marsh area could re- duce wintering waterfowl and south- western willow flycarcher popula- tions in the Grand Canyon. Is this loss of biodiversity acceptable if in- creased riparian habitat in the Grand Canyon offsets losses of riparian habitat elsewhere in rhe region? Would the loss of that biodiversity be in conflict wirh the legislation enabling Grand Canyon National Park, which requires managemen "to preserve and protect [the park] for future generations"? Optimization strategies are ofren suggesred as a way to balance multi- objective resource decisions, bur such strategies are not appropriare for all management goals (Carorhers and Brown 1991). Pursuit of an optimi- zation strategy that seeks to identify the greatesr improvement in relict and arrifact resources while harming few of these resources is appropriate for the goal of creating a naturalized river but is inappropriate for the goal of full resroration. Optimiza- tion demands a detailed undersrand- ing of a complex ecosysrem, and dam- operating plans may need ro be revised repeatedly in response to changes in the relative composition of riverine resources. Although many studies have demonstrated the re- sponse trend to different dam release parterns, few provide sufficiently precise information on which to base an optimization strategy. Indeed, the' monitoring and research program necessary to implement this optimi- zation straregy may be costly and invasive to the wilderness character of the Grand Canyon. The choice of management goals and approaches involves value-laden decisions that include economic ef- fect~ and implicarions for other soci- etal values (Marzolf 1991). The op- tions facing society include protecting biodiversity; reestablishing a pre-dam landscape with lower diversity, abun- dance, and sranding mass of biota; and establishing the sense of wild- ness with some relarively natural amenities. Deciding among these choices is further hindered because some altered habitats are occupied by endangered species that have ex- panded or shifred their range and by some non~native species that are now valued by society. I The public must choose the direc- ~ tion for future management of rhe Colorado River in the Grand Can- yon. A proliferation of new scientific investigations to predict positive and negative effects of different dam op- eration strategies can refine ecosys- tem management opportunities, but values, not science, underlie the ,choice of a management goal for the ~. ver. The public is best served if I cientists clearly communicate and efine the implications of different . anagement scenarios. This infor- mation needs to be presented to soci- ety at large, and an informed debate on the most desirable management strategy should then proceed. Only after such a strategy is identified can scientists know where best to direct future scientific investigation, or whether such investigation is even warranted. ~ , q~ ~ ?~~ ~ f. '" ~'.., if If ~~ '. " ; ~,;: .. , ~. Acknowledgments Most of our research in rhe Grand Canyon was funded by rhe Glen Canyon Environmental Studies pro- gram of the US Bureau of Reclama- tion; David L. Wegner managed this program and supported our efforts. This manuscript was reviewed by Julio L. Betancourr, Rebecca Chasan, Penny Firth, Robert M. Hirsch, Theodore S. Melis, Seth R. Reice, and Frederic H. Wagner. References cited Anderson LS, Ruffner GA. 1987. Effects of post-Glen Canyon flow regime on the old high water line plant community along the Colorado River in Grand Canyon. Pages 271-286 in Glen Canyon Environ- mental Studies Technica.l Reporr. Salt lake City (UT): US Bureau of Reclamation. Andrews ED. 1990. The Colorado River: A perspective from Lees Fere}', Aeizon.a. Pages 304-310 in Surface-Water Hydrol- ogy. The Geology of North America, Vol. 0-1. Sp('ci.:1J Publication. Boulder (CO): Geological Society of America. Brown BT, Stevens LE. 1992. Winter abun- dance, age strucrure, and distribution of bald eagles along the Colorado River, Arizona. Southwestern Naturalist 37: 404-408. Carothers SW, Aitchison SW, Johnson RR. 1979. Natural resources, white-water rec- reation, and river management alterna- tives on the Colorado River. Grand Can- yon National Park, Arizona. Pages 253-260 in Proceedings of the First Confer- , ~ , i .' .- ,~'. ::. <.... < ;1': :'~ . BioScience Vol. 48 No.9 t- .~: I t ';'.