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<br />Virginia has increasedjuvenile abun-
<br />dances of native striped bass~ Mim-
<br />icking short-duration flow spikes that
<br />are historically caused by summer
<br />thunderstorms in the regulated Pecos
<br />River of New Mexico has benefited
<br />the reprod uctive success of-the Pecos
<br />bl untnose shiner.
<br />We also recognize that there are
<br />. scientific limits to how precisely the
<br />natural flow regime for a particular
<br />river can be defined~ It is possible to
<br />have only an approximate knowl-
<br />edge of the hJstoric condition of a
<br />rivert both because some human ac-
<br />tivities may have preceded the instal-
<br />lation of flow gauges, and because
<br />climate cond-itions may have changed
<br />over the past century or- more. Fur.-
<br />thermorel in many rivers, year-to-
<br />year differences in the, timing and
<br />quantity of flow result in substantial
<br />variabHity around any average flow
<br />condition. Accordingly, managing
<br />for the R ave~age U condition can be
<br />misguided~ For example, in human-
<br />altered rivers that are managed for
<br />increplental improvements, restoriIlg
<br />a flow pattern that is simply propor-
<br />tional to the natW'al hydrograph in
<br />years with little. runoff may provide
<br />few if any ecological benefits, be-
<br />cause many geomorphic and . ecO-
<br />logical processe's show nonlinea'r re..
<br />sponses to flow~ Clearly, half of the
<br />peak discharge will not move half of
<br />the sediment, half of a migration" .
<br />motlvational flow will not motivate
<br />half of the fish, and half of an
<br />overbank flow wiIllnot inundate half
<br />of the floodplain. In such rivers, more
<br />ecological benefits would accrue
<br />from capitalizing on the natural be..
<br />tween~year variability' in flow. For
<br />example, in years with above-aver-
<br />age flow I "surplus" water could be
<br />used to exceed flow threshold$ that
<br />drive critical ge9p1.orphic and eco~
<br />logical processes.
<br />If full flow restoration is impos-
<br />sible, mimicking certain geomorphic
<br />processes may provide some ecologi-
<br />cal benefIts. Well-timed irrigation
<br />could stimulate recruitment of val~
<br />ued riparian trees such as cotton-
<br />woods (Friedman et al. 1995). Stra-
<br />tegically clearing vegetation from
<br />river banks could provide new
<br />sources of grav'el for sediment..
<br />starved regulated rivers with reduced
<br />peak flows (e.g., Ligan et aI. 1995).
<br />In all situations, managers will be
<br />
<br />December 1997
<br />
<br />required to make judgments about
<br />specific restoration goals and to work
<br />with appropriate components of the
<br />natural flowTegime to achieve those
<br />goals. Rec ognition of the natural flow
<br />varia bility an d' careful identification
<br />of key processes that are linked to
<br />various components of the flow re-
<br />gime are critical to making these
<br />judgments.
<br />Setting specific goals to restore a
<br />more natural" regime in rivers with
<br />. altered flows (or, equally important,
<br />to preserve unaltered flows in pristine
<br />rivers) should ideally be a cooperative
<br />process involving river scientists, re..
<br />source managers; and appropriate
<br />stakeholders. The details of this pro-
<br />cess will vary depending on the spe-
<br />cific obJec~ives for the river in ques..
<br />tiont the degree to which its flow
<br />regime and other environmen tal vari...
<br />abIes (e.g'J thermal regimet sediment
<br />supply) have been altered, and the
<br />social and economic constraints that
<br />are in play. Establishing specific cri..
<br />teria for flow restoration will be chal-
<br />lenging because our unders~anding
<br />of the interactions of individual floW
<br />components with geomorphic and
<br />ecological processes is incomplete.
<br />However, qua.ntitative, river-specific
<br />standards cant in principle, be devel-
<br />oped based on t~~ recon$truction of
<br />the natural flow regime (e.g., Rich-
<br />ter at al. 1997). Restoration actions
<br />based on such guidelines should be
<br />viewed as experiments to he moni..
<br />to~ed and evaluated-that is, adap-
<br />tive management-to provide criti-
<br />cal new knowledge for creative
<br />management of natural ecosystem
<br />variability (Ta bIe 3). .
<br />To manage rivers from this new
<br />perspective; some policy changes are
<br />needed. The narrow regulatory fo-
<br />cus on minimum flows and single
<br />species impe~es .::enlightened river
<br />management and restoration, as do
<br />the often conflicting mandates of the
<br />many agencies and organizations that
<br />are involved in the, process. Revi-
<br />sions of laws and regulatlonst. and
<br />redefinition of societal goals and poli-
<br />cies, are essential to eriabl~ man'agers
<br />to' use the best science to d.evelop ap-
<br />propriate management programs.
<br />Using science to guid.e ecosystem
<br />. management requires that basic and
<br />applied research address difficult
<br />questions in complex, real-world set-
<br />tings, in which experimental con-
<br />
<br />troIs and statistical replication are
<br />often impossib]e~ Too little attention
<br />and too few resources ha.ve been de-
<br />voted to clarifying how restoring
<br />specific components of the flow re-
<br />gime will benefit the entire ecosys..
<br />tern . Nevertheless lit is clear that.
<br />whenever possibleJ the natural river ~
<br />system sh~uld be allowed to repair
<br />and maintain itself. This approach is
<br />likely to be the most successful and
<br />the least expensive way to restore
<br />and maintain the ecological integrity
<br />afflow-altered rivers (Stanford et at
<br />1996)~ Although the most effective
<br />mix of human-aided and natural re-
<br />covery methods will vary with the
<br />river, we believe that e.xisting.knowl-
<br />edge makes a strong Cc;tse that resto r-
<br />ing natural flows.should be a corner...
<br />stone of our management approach
<br />to river ecosystems. .
<br />
<br />Acknowledgments
<br />
<br />We thank the following people for
<br />readIng and commenting on earlier
<br />versions of this p~p.er: Jack Schmidt,
<br />Lou Totht Mike Scott! David Wegner,
<br />Gary Meffe, Mary Power," Kurt
<br />Fausch, Jack Stanford, Bob Naimanj
<br />DOll Duff, John Epifanio, - Lori
<br />Robertson, Jeff Baumgartner t Tim
<br />Randle, David Harpman, Mike
<br />Armbruster J and Thomas' Payne.
<br />Members of the, Hydrop.ower Re--
<br />, form Coalition also offered construc-
<br />tive comments~ Excellent final re-
<br />views were provided by Greg Auble~
<br />Carter Johnson'. an anonymous re...
<br />viewer, and the editor of BioScience.
<br />Robin Abell contributed to the de-
<br />velopment of the timeline in Figure
<br />5} and graphics assistance was pro-
<br />vided by Teresa Peterson (Figure 3) f
<br />Matthew Chew (Figur~ 4) and Robin
<br />Abell and Jackie Howard (Figure 5).
<br />We also thank the national offices of
<br />Trout Unlimited and American Riv-
<br />ers for encouraging the expression of
<br />_ the ideas presented here. _We espe-
<br />cially thank the George Gund Foun-
<br />dation for providing 'a grant to hold
<br />a one-day workshop, and The Na~
<br />tUre Conserv~ncy for proyiding lo-
<br />gistical ,support for several of the
<br />authors prior to the workshop.
<br />
<br />References cited
<br />
<br />Abramovitz IN. 1996. Imperiled waters, im-
<br />poverished future: the decline of freshwa-
<br />
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