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<br />The N aturaI Flow Regime <br /> <br />A paradigm for river conservation and restoration <br /> <br />N. LeRoy Poff,l. David Allan, Mark B. Bain,lanles R. Karr, Karen L. Prestegaard, <br />Brian D. Rich~er, Richard E. Sparks, and Julie C. Str9mberg <br /> <br />Humans have long_been fasci- <br />nated by the dynamism of <br />free-flowing waters. Y~t we <br />have expended great effort to tame <br />rivers for transportation, water sup'" <br />ply, flood control. agrfculturet and <br />power generation. It Is now recog- <br />nized that harnessing of streams and <br />rivers comes at great cost; Many <br />. rivers no longer support socia!Jy val- <br />ued native .species or sustaIn healthy <br />ecosystems that provJde important <br />goods and services (Naiman et at <br />1995, NRC 1992). <br /> <br />N. LeRoy Poff is an asslstar~.t professor <br />in the Department of Biology, Colorado <br />State UniversitYt Fort Collins, CO 80523- <br />1878 and form~rly senior scientist at <br />Trout Unl1m!ted~ Arlington. VA 22209~ <br />J.- David Al1an1s aprofes~or at the School <br />of Natural Resources & Environment. <br />U ni versity of Mi~higan I Ann Arbor, MI <br />48109-1115. Mark B. Bain Is a research <br />scientist and associate professor at the <br />New York Cooperative fish & 'Wildlife <br />Research Unit of the Department of <br />Natural Resources, Cornell University; <br />Ithaca. NY 14853-3001. James R. Karr <br />is a professor in the departments of Fish- <br />eries and Zoologyt Box 357980, Univer- <br />sity of Washington, Seattle I W A 98195- <br />7980. Karen L. Pr~stegaard is an associate <br />professor in the Department of Geology f <br />University of Maryland, College Park, MD <br />20742. Brian D. Richter is national hy- <br />drologist In the Biohydrology Program. <br />_ The Nature Conservancy. Hayden, CO <br />81639. Richard E. Sparks is director of <br />the River Research Laboratories at the <br />Illinois Natural History Survey I Havana, <br />It 62644. Julie C. Stromberg is an asso.. <br />ciate professor in the Department of <br />Plant Biology r Arizona State. U nJverslty ~ <br />Tempel AZ 85281. @ 1991 American <br />Institute of Biological Sciences. <br /> <br />December 1997 <br /> <br />The ecological integrity <br />of river ecosystems <br />depends on ~~eir natural <br />dynamic character <br /> <br />1;he extensive ecological degrada- <br />tion and loss of biolqgical div~rsity <br />resulting from river exploitation is <br />eliciting widespread concern for con- <br />servation and restoratiqn of. healthy <br />r~ver ecosystems among scientists and <br />the lay pu blic alike (Allan and Fl~ cker <br />199.3, Hughes an~l.No$s 1992, Karr <br />et a.t 1985t TNC 1996. Williams ,et <br />al. 1996). Extirpation of species~ clo- <br />sures offisheries, groundwater deple- <br />tion. declines in w~ter quality and <br />availa,bility, and more frequent and <br />intense flooding are increasingly rec_.. <br />ognized as co~sequences of .current <br />river ~anagement and development <br />policies (Abramovitz 1996, Collier <br />et al. 1996t Naiman et al. 1995) ~ The <br />broad social support in the United <br />St,ates for the Endangered Species <br />Act, the recognition of the intrinsic <br />value of noncommercial native spe- <br />. cies, and the proliferation of water- <br />shed councils and .riverwatch teams <br />are evidence of society's interest in <br />maintaining the ecological integrity <br />and se,lf-sustaining productivity of <br />free..flowing river systems. <br />Society1s' ability to maintain and <br />restore the integrity of river ecosys... <br />terns requires that conservation and <br />management actions be firmly <br />grounded in scientific understand.. <br /> <br />ing. However. current management <br />approaches often fail to recognize <br />the fundamental scientific principle <br />that the integrity of flowing wa.ter <br />. systems depends largely on their natu- <br />ral dynamic character; as a resultt <br />these metho ds freq uently prevent suc... <br />cessful river conservation or restora- <br />tion. .Str~a.mt1ow qua.ntity and tim.. <br />ing are critical components of water <br />supplyt water qualitYI and the eco.. <br />logical integrity of river systems. In... <br />deed. streamflow, which is, strongly <br />correlated with many crit.ical physi- <br />cochemical c-haracteristics of rivers, <br />such as wate~ te~perature, channel <br />,geomorpholQ:gy~ and habitat diver- <br />si ty, can be consi dere d a "master <br />variableJJ that limits the distribution <br />and abundance of riverine sp-ecies <br />(Power et al. 1995, Resh et a1. .1988) <br />and'regulates the ecological integrity <br />of flowing water systems (Figure 1) ~ <br />Until recently, however, the impor- <br />tance of natural streamflow variabil- <br />ity in maintaining healthy aquatic <br />ecosystems has been virtually ignored <br />in a management context. , <br />HistoricaIly. .the II protectio_n" of <br />river ecosystems has been limited in <br />scope, emphasjzing water quqlity and <br />only one aspect of water quantity; <br />minimum flow. Water resources <br />management has also suffered from <br />the often incongruent perspectives <br />and fragment~d responsibilIty of <br />- agencies (for example, the US Army <br />Corps of Engineers and Bureau of <br />Reclamation are responsible for wa- <br />ter supply and flood controt the US <br />Environmental Protection Agency <br />and state environmental agencies for <br />water quality, and the US Fish & <br /> <br />769 <br />