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Virginia has increased juvenile abun- dances of native striped bass. Mim- icking short-duration flow spikes that are historically caused by summer thunderstorms in the regulated Pecos River of New Mexico has benefited the reproductive success of the Pecos bluntnose shiner: We also recognize that there are scientific limits to how precisely the natural flow regime for a particular river can be defined. It is possible to have only an approximate knowl- edge of the historic condition of a river, both because some human ac- tivities may have preceded the instal- lation of flow gauges, and because climate conditions may have changed over the past century or more. Fur- thermore, in many rivers, year-to- year differences in the timing and quantity of flow result in substantial variability around any average flow condition. Accordingly, managing for the "average" condition can be misguided. For example, in human- altered rivers that are managed for incremental improvements, restoring a flow pattern that is simply propor- tional to the natural hydrograph in years with little runoff may provide few if any ecological benefits, be- cause many geomorphic and eco- logical processes show nonlinear re- sponses to flow. Clearly, half of the peak discharge will not move half of the sediment, half of a migration- motivational flow will not motivate half of the fish, and half of an overbank flow will not inundate half of the floodplain. In such rivers, more ecological benefits would accrue from capitalizing on the natural be- tween-year variability in flow. For example, in years with above-aver- age flow, "surplus" water could be used to exceed flow thresholds that drive critical geomorphic and eco- logical processes. If full flow restoration is impos- sible, mimicking certain geomorphic processes may provide some ecologi- cal benefits. Well-timed irrigation could stimulate recruitment of val- ued riparian trees such as cotton- woods (Friedman et al. 1995). Stra- tegically clearing vegetation from river banks could provide new sources of gravel for sediment- starved regulated rivers with reduced peak flows (e.g., Ligon et al. 1995). In all situations, managers will be required to make judgments about specific restoration goals and to work with appropriate components of the natural flow regime to achieve those goals. Recognition of the natural flow variability and careful identification of key processes that are linked to various components of the flow re- gime are critical to making these judgments. Setting specific goals to restore a more natural regime in rivers with altered flows (or, equally important, to preserve unaltered flows in pristine rivers) should ideally be a cooperative process involving river scientists, re- source managers, and appropriate stakeholders. The details of this pro- cess will vary depending on the spe- cific objectives for the river in ques- tion, the degree to which its flow regime and other environmental vari- ables (e.g., thermal regime, sediment supply) have been altered, and the social and economic constraints that are in play. Establishing specific cri- teria for flow restoration will be chal- lenging because our understanding of the interactions of individual flow components with geomorphic and ecological processes is incomplete. However, quantitative, river-specific standards can, in principle, be devel- oped based on the reconstruction of the natural flow regime (e.g., Rich- ter et al. 1997). Restoration actions based on such guidelines should be viewed as experiments to be moni- tored and evaluated-that is, adap- tive management-to provide criti- cal new knowledge for creative management of natural ecosystem variability (Table 3). To manage rivers from this new perspective, some policy changes are needed. The narrow regulatory fo- cus on minimum flows and single species impedes enlightened river management and restoration, as do the often conflicting mandates of the many agencies and organizations that are involved in the process. Revi- sions of laws and regulations, and redefinition of societal goals and poli- cies, are essential to enable managers to use the best science to develop ap- propriate management programs. Using science to guide ecosystem management requires that basic and applied research address difficult questions in complex, real-world set- tings, in which experimental con- trols and statistical replication are often impossible. Too little attention and too few resources have been de- voted to clarifying hove restoring specific components of the flow re- gime will benefit the entire ecosys- tem. Nevertheless, it is clear that, whenever possible, the natural river system should be allowed to repair and maintain itself. This approach is likely to be the most successful and the least expensive way to restore and maintain the ecological integrity of flow-altered rivers (Stanford et al. 1996). Although the most effective mix of human-aided and natural re- covery methods will vary with the river, we believe that existing knowl- edge makes a strong case that restor- ing natural flows should be a corner- stone of our management approach to river ecosystems. Acknowledgments We thank the following people for reading and commenting on earlier versions of this paper: Jack Schmidt, Lou Toth, Mike Scott, David Wegner, Gary Meffe, Mary Power, Kurt Fausch, Jack Stanford, Bob Naiman, Don Duff, John Epifanio, Lori Robertson, Jeff Baumgartner, Tim Randle, David Harpman, Mike Armbruster, and Thomas Payne. Members of the Hydropower Re- form Coalition also offered construc- tive comments. Excellent final re- views were provided by Greg Auble, Carter Johnson, an anonymous re- viewer, and the editor of BioScience. Robin Abell contributed to the de- velopment of the timeline in Figure 5, and graphics assistance was pro- vided by Teresa Peterson (Figure 3), Matthew Chew (Figure 4) and Robin Abell and Jackie Howard (Figure 5). We also thank the national offices of Trout Unlimited and American Riv- ers for encouraging the expression of the ideas presented here. We espe- cially thank the George Gund Foun- dation for providing a grant to hold a one-day workshop, and The Na- ture Conservancy for providing lo- gistical support for several of the authors prior to the workshop. References cited Abramovitz JN. 1996. Imperiled waters, im- poverished future: the decline of freshwa- December 1997 781