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0/ 5 i t The Natural Flow Regiume A paradigm for river conservation and restoration N. LeRoy Poff, J. David Allan, Mark B. Bain, James R. Kan, Karen L. Prestegaard, Brian D. Richter, Richard E. Sparks, and Julie C. Stromberg umans have long been fasci- nated by the dynamism of free-flowing waters. Yet we have expended great effort to tame H i rivers for transportation, water sup- ply, flood control, agriculture, and power generation. It is now recog- nized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially val- ued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992). N. LeRoy Poff is an assistant professor in the Department of Biology, Colorado State University, Fort Collins, CO 80523- 1878 and formerly senior scientist at Trout Unlimited, Arlington, VA 22209. J. David Allan is a professor at the School of Natural Resources & Environment, University of Michigan, Ann Arbor, MI 48109-1115. Mark B. Bain is a research scientist and associate: professor at the New York Cooperative Fish & Wildlife Research Unit of the Department of Natural Resources, Cornell University, Ithaca, NY 14853-3001. James R. Karr is a professor in the departments of Fish- eries and Zoology, Box 357980, Univer- sity of Washington, Seattle, WA 98195- 7980. Karen L. Prestegaard is an associate professor in the Department of Geology, University of Maryland., College Park, MD 20742. Brian D. Richter is national hy- drologist in the Biohydrology Program, The Nature Conservancy, Hayden, CO 81639. Richard E. Sparks is director of the River Research Laboratories at the Illinois Natural History Survey, Havana, IL 62644. Julie C. Stromberg is an asso- ciate professor in the Department of Plant Biology, Arizona State University, Tempe, AZ 85281. () 1997 American Institute of Biological Sciences. December 1997 The ecological integrity of river ecosystems depends on their natural dynamic character The extensive ecological degrada- tion and loss of biological diversity resulting from river exploitation is eliciting widespread concern for con- servation and restoration of healthy river ecosystems among scientists and the lay public alike (Allan and Flecker 1993, Hughes and Noss 1992, Karr et al. 1985, TNC 1996, Williams et al. 1996). Extirpation of species, clo- sures of fisheries, groundwater deple- tion, declines in water quality and availability, and more frequent and intense flooding are increasingly rec- ognized as consequences of current river management and development policies (Abramovitz 1996, Collier et al. 1996, Naiman et al. 1995). The broad social support in the United States for the Endangered Species Act, the recognition of the intrinsic value of noncommercial native spe- cies, and the proliferation of water- shed councils and riverwatch teams are evidence of society's interest in maintaining the ecological integrity and self-sustaining productivity of free-flowing river systems. Society's ability to maintain and restore the integrity of river ecosys- tems requires that conservation and management actions be firmly grounded in scientific understand- ing. However, current management approaches often fail to recognize the fundamental scientific principle that the integrity of flowing water systems depends largely on their natu- ral dynamic character; as a result, these methods frequently prevent suc- cessful river conservation or restora- tion. Streamflow quantity and tim- ing are critical components of water supply, water quality, and the eco- logical integrity of river systems. In- deed, streamflow, which is strongly correlated with many critical physi- cochemical characteristics of rivers, such as water temperature, channel geomorphology, and habitat diver- sity, can be considered. a "master variable" that limits the distribution and abundance of riverine species (Power et al. 1995, Resh et al. 1988) and regulates the ecological integrity of flowing water systems (Figure 1). Until recently, however, the impor- tance of natural streamflow variabil- ity in maintaining healthy aquatic ecosystems has been virtually ignored in a management context. Historically, the "protection" of river ecosystems has been limited in scope, emphasizing water quality and only one aspect of water quantity: minimum flow. Water resources management has also suffered from the often incongruent perspectives and fragmented responsibility of agencies (for example, the US Army Corps of Engineers and Bureau of Reclamation are responsible for wa- ter supply and flood control, the US Environmental Protection Agency and state environmental agencies for water quality, and the US Fish & 769