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<br />1978) i IFIM combines two models, a
<br />biological one that describes the physi-
<br />cal habitat preferences of fishes (and
<br />occasionally macroinvertebrates) in
<br />terms of depth, velocity, and substrate,
<br />and a hydraulic one that estimates
<br />how the availability of habitat for
<br />fish varies with discharge. IFIM has
<br />been widely used as an organiza..
<br />tional ,framework for formulating
<br />and evaluating alternative water
<br />management options related to pro..
<br />duction of one or a few fish species
<br />(Stalnaker et al~ 1995). >
<br />As a predictive tool for ecological
<br />management. the IFIM modeling
<br />approach has been criticized both in
<br />terms of the statistical validity of its
<br />physical habitat characterizations
<br />(WilUams 1996) and the limited re-
<br />alism of its biological assumptions
<br />(Castleberry et al. 1996). Field tests
<br />of its predictions have yielded mixed
<br />results (Morehardt 1986). Although
<br />this approach continues to evolve,
<br />both by adding biological realism
<br />_ (Va~ Winkle et al. 1993) and by
<br />expanding the range of habitats
<br />modEHed (Stalnaker et al. 1995) t in
<br />practice it is often :used only to estab~
<br />lish minimum flows for rc important II
<br />(i.e., game or imperiled) fish species.
<br />But current understanding of river
<br />ecology clearly indicates that fish
<br />and other aquatic organisms require.
<br />, habitat features that cannot be main- .
<br />t@ine.d by minimum flows alone '(see
<br />Stalnaker 1990). A range of flows is
<br />necessary to scour and revitalize
<br />gravel beds, to import wood and
<br />organic matter from the floodplaint
<br />an4 to provide access to prod ueti ve
<br />riparian wetlands (Figure 4). Inter-
<br />annual variation in these flow peaks
<br />is also critical for maintaining chan..
<br />nel and riparian dynamics. For ex..
<br />ample, imposition of only a fixed
<br />hIgh-flow level each year would sim-~
<br />ply result in the equilibration of in- C
<br />channel and floodplain habitats to
<br />these constant peak flows.
<br />Moreover I a focus on one or a few
<br />species and on minimum flows fails
<br />to recognize that what is f. goodU for'
<br />the ecosystem 'may not consistently
<br />benefit individual species. and that
<br />what is good for individual species
<br />may not be of benefit to the ecosys-
<br />tem. Long-term studies of naturally
<br />variable systems show that some spe-
<br />cies do best in wet years, that other
<br />species do best in dry.years. and that
<br />
<br />'780,
<br />
<br />overall biologtcal ~iverslty and eca..
<br />system function b,enefit from these
<br />variations in species success (Tilman
<br />, et aL 199 4) ~ Inde.ed, experience in
<br />river restoratio'n ~learly shows the
<br />impossibUity of si;multaneous]y en-
<br />gineering optimal eonditions for all
<br />species (Sparks 1~92, 1995, Toth
<br />1995). A holistic ~iew that attt~mpts
<br />to restore natural variability in eco-
<br />logical proces-ses apd species success
<br />(and that acknowfedges the tremen-
<br />dou~ uncertainty ~hat is inherent ~n
<br />attempting to mec~anjstlcally model
<br />all species in the eqosystem) is neces~
<br />sary for ecosystem; management and
<br />restoration (Fran~lin' 1993).
<br />
<br />and native species. Thus, to protect
<br />pristine or nearly pristine systems, It
<br />is necessary to preserve the natural
<br />h'ydrologic cycle by safeguarding
<br />against upstream river development
<br />and damaging land uses that modify
<br />runoff and sed~ment supply in the
<br />watershed.
<br />Most rivers are highly modified,
<br />of course, and so the greatest chal..
<br />lenges lie in managing and restoring ,
<br />flyers that are also used to satisfy
<br />hum~n needs. Can reestablishing the
<br />natural flow regime serve as a useful
<br />management an9. restoration goal?
<br />We beUeve that it can, although to
<br />,varying degrees, depending on the
<br />present extent of human interven-
<br />, Managi~g towafd a natural tiOD ,and flow alteration affecting a
<br />flow regime-; . particular river. Recognizing the
<br />natural variability of river flow and
<br />The first step toward better incarpo- explicitly incorporating the five com..
<br />rating flow regim~ into the manage- ponents of the natural flow regime
<br />ment of river ecosystems is to recog- (i.e., magnitudel frequencYt duration.
<br />nize that extensive: human alteration timingJ and rate of change) into a
<br />of river flow has Jresulted in wlde- broader framewc)rk for ecosystem
<br />spread ,geomorph~c and ecological management would constitute a
<br />changes in these e<;:bsystems. The his.. ,. majo( .'advance over most present'
<br />tory of river "use'!$ also a history of . management, which focuses on mini-
<br />flow alteration (Figure 5). The early mum flows and onjust a few species.
<br />establishment of tI;1e US Army Co:r;ps Such recognition would also can-
<br />of Engineers is testimony to the.iI?- tribute to the developing science of
<br />portance that the patlon gave to de.. stream restoration in heavily altered
<br />velopJng navigabl~ water routes and wat~rs~eds, wheret all too often.
<br />to controlling recu:rrent large floods. physical channel features (e.g., bars
<br />However, growing understanding of al}d _woody d~bris) are re-~reated
<br />the ~colog1cal imp~acts of flow alter-. ,without regard to restoring the flow
<br />ation has led to ~ shift toward an regime that will help to maintain
<br />appreciation of t~e merits of free- these re-created features.
<br />flowing rivers. F orjexample; the Wild '.J ust as rivers have been i.ncremen"
<br />and Scenic Rivers Act of 1968 recog.... tally modifiedt they can be incre-
<br />ni~ed, that the flow of certain rivers mentally_- restored, with resulting
<br />should be protected as a national Improvements to many phystcal and
<br />resource, and the recent blossoming biological processes~ A Ust of recent
<br />of natural flow re~toration projects efforts to restore various components
<br />(Table -3) may he~ald the beginning of a nat~ral flow regime (that is, to
<br />of efforts to undolsome of the dam- "naturalize" river flow) demon-
<br />ag~ of past flow al~erations. The next strates the scope for suc~ess (Table
<br />century holds pro;mise as an era for ,3). Many of the projects summarized
<br />renegotiating hurpan relationships in Table 3 'represent only partial steps
<br />with rivers, in whicp lessons from past toward full fl.ow restorationJ but they
<br />experience are used to direct wise and 'have had demons~rable ecological
<br />informed action in: the future. benefits. For. example, . high flood
<br />A large body; of evidence has flows followed by mimicked natural
<br />shown that the n~tural flow regime rates of flow decline in the Oldman
<br />of virtually all ri:vers Is Inheren~ly River of Alberta, Canada, resulted in
<br />variable, and that this variability' is a massive cottonwood recruitment
<br />cri~ical, to ecosysfem function and that ~xtended for morethan 500 km
<br />native 'biodlversit~. As we have al- downstr~am from the Old~an Dam.
<br />ready discussed~ fivers with highly' Dampening of the unnatural flow
<br />altered and regulated flows lose their fluctuations caused by hydroelectric
<br />ability' to support~ natural processes generation on the, Roanoke River in
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<br />BioScience Vol. 47 No. 11
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