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I pirical analysis of channel responses to dams, such as those described in the case studies, to develop a hybrid theoretical/empirical means of pre- dicting the biologically significant morphological changes that are likely to occur after dam construc- tion. We do not recommend trying to preserve the geomorphic processes in a biological vacuum. One would be well advised to know the physical habitat requirements of species or assemblages deemed important by their value (either economic or aes- thetic) or danger to humans, by their perceived vulnerability to human- induced changes, or by the strength of their role in maintaining func- tional ecosystems. Human-defined important species might include fish that support subsistence, commer- cial, or recreational fisheries; dis- ease vectors such as blackflies (Sirnuliidae);, or species with cul- tural or spiritual significance to lo- cal peoples. Vulnerable species would include rare or endangered species and those with narrow, spe- cialized ecological niches. Species with strong roles in ecosystem func- tion would be identified as strong interactors (Paine 1980) whose ab- sence would significantly change the ecosystem. The primary biological research goal during our hypothetical two- year program would be to identify the life-history bottlenecks of a few of these important species and the nature and degree of dependence of these bottlenecks on features of the morphology or hydraulics of the river (Power et al. page 159 this issue). For although our goal would be to mimic the preexisting geomor- phology and hydraulic conditions, in most cases changes are inevitable. If we know which features are criti- cal to the important species, we can concentrate on preserving those fea- tures at the possible expense of fea- tures deemed less important. Mitigation While predicting the geomorphic and biological changes resulting from a dam may be difficult, mitigating them may be even harder. Much of the difficulty is a result of the al- tered sediment supply: greatly re- duced immediately below the dam but increasing downstream as a re- sult of tributary and bank inputs (Meade et al. 1990). Ideally, one could develop a schedule of flows that would transport the post-dam sediment load with a frequency and a morphological effect that are simi- lar to those of pre-dam sediment transport. Often, it will not be pos- sible to develop such a schedule, and geomorphic compromises must be made. To avoid such compromises, it may be necessary, though expen- sive, to add sediment below the dam. This procedure has been carried out successfully on the Sacramento River in California (CDWR 1992). Ulti- mately, the best means to preserve the morphology and biota of rivers is to design reservoirs that pass sedi- ment on a regular basis, consistent with the frequency of natural sedi- ment transport events. Although passing sediment through small res- ervoirs that can economically be emptied every year is relatively easy, there are no means yet developed for passing sediment through larger reservoirs. In the United States there has been little economic incentive to pursue such technology, because reservoir sedimentation is not a great problem.' In contrast, there are ex- treme reservoir sedimentation prob- lems in China, and although the Chinese are actively pursuing solu- tions, the problem is not yet solved.' However, some technologies may be developed for passing sediment that may make sense from a strictly economic perspective, but they may be deleterious to the downstream ecosystem. The type of approach that we and others (e.g., Petts 1991) are recom- mending-trying to preserve a dy- namic, geomorphically living stream-is part of a growing trend in stream ecology away from an emphasis on one (or a few) species or piecemeal mitigation and toward more holistic and catchment-based management (e.g., Cummins 1992, 'J. Linsioni, 1994, personal communication. US Army Corps of Engineers, Seattle, WA. 'G. Jin, 1994, .personal communication. Anhui Water Resources Research Institute, Anhui, China. Moyle and Yoshiyama 1994, Ward and Stanford 1989). Trying to main- tain a river's natural geomorphic processes in the absence of sediment passage or supplementation might not always be feasible, but it should be the starting point from which one can retreat to more intrusive engi- neering solutions. For example, on the McKenzie, the high flows necessary to main- tain the river's complexity would flood existing towns and farms. As this outcome is likely to be deemed too high a price to pay, even for salmon, some other solution is nec- essary. One possible technique would be to notch the river banks and remove the trees at places where avulsions or meander cut-offs are likely to occur (without damage to existing structures), so that a bank- full flood event would result in a channel shift and the introduction of coarse sediment into the stream, thus renewing the process of midchannel bar/island formation. Conclusions Rivers are subject to a variety of abuses such as pollution, reduced flows, and altered timing of flows. They suffer obvious and severe mor- phological affronts such as channeli- zation, bank protection (e.g., rip- rapping), and aggregate mining. While the geomorphological adjust- ments of rivers below dams are of- ten less obvious, they are equally important. Although usually not as locally severe as, say, a sewage out- fall, the scope of the problem is immense, due to the vast number of dams that have been built or pro- posed and the tendency for the changes to persist indefinitely. Preserving the physical habitat below a dam is necessary, but may not be sufficient, to protect the eco- system (changes in temperature, water quality, or blocked migrations may be of overriding importance). We have chosen to emphasize the foundation of the stream ecosys- tem-its physical structure and hy- draulics-not only because it is sen- sitive to disturbance, but because, when a dam is built, there may be a greater opportunity to understand and predict the changes in a river's geomorphology than in its ecology. 190 BioScience Vol. 45 No. 3