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<br />002832
<br />
<br />l
<br />lnnel
<br />
<br />well-known streamflow approaches into a
<br />unified methodology that recognizes flow
<br />requirements for fish, riparian habitat,
<br />floodplains, and channel morphology, Es-
<br />tablishing streamflows only on the basis of
<br />fish needs may result in the degradation
<br />of the stream channel. alteration of geo-
<br />morphological processes, reduction or al-
<br />teration of riparian vegetation, and may
<br />cause changes in floodplain function, We
<br />review physical processes that lead to the
<br />ecological linkage between instream and
<br />out-of-stream resources and the depend-
<br />ency of riverine resources on streamflows.
<br />The US Fish and Wildlife Services' In-
<br />stream Flow Incremental Methodology
<br />(IFIM) (Bovee 1982) and the Physical Hab-
<br />itat Simulation system (PHABSIM) (Mil-
<br />hous et at. 1984) have many limitations as
<br />habitat-based models for the instream flow
<br />needs of fish (Annear and Conder 1984;
<br />Mathur et at. 1985; Orth 1987), Although
<br />it wouid be convenient for fish popula-
<br />tions to be limited by three or four envi-
<br />ronmental factors, such situations are the
<br />exception rather than the rule, In the sim-
<br />plest example of limitations within IFlM,
<br />trout populations often fluctuate consid-
<br />erably and in a manner that is apparently
<br />
<br />egimes
<br />Le con.
<br />zones,
<br />'01 fish
<br />leeded
<br />suring
<br />flows,
<br />tion of
<br />Using
<br />:e how
<br />ltion.
<br />
<br />independent of direct simultaneous envi-
<br />ronmental control (Platts and Nelson 1988),
<br />Seldom do we measure variables that truly
<br />affect fish populations. Models that fail to
<br />account for the natural fluctuations in an-
<br />imal populations are destined to be only
<br />coincidentally accurate (Platts and Nelson
<br />1988). Broader thinking and more ecolog-
<br />ically centered approaches are needed
<br />when managing streamflows. However, as
<br />we explain later in this paper, PHABSIM-
<br />derived fish flows playa key role in the
<br />overall flow evaluation,
<br />Multiple flow regimes are needed to
<br />maintain biotic and abiotic resources with-
<br />in a river ecosystem, The four flow groups
<br />we examine are (1) flood flows that form
<br />floodplain and valley features; (2) over-
<br />bank flows that maintain surrounding ri-
<br />parian habitats, adjacent upland habitats,
<br />water tables, and soil saturation zones; (3)
<br />in-channel flows that keep immediate
<br />stream banks and channels functioning; and
<br />(4) in-channel flows that meet critical fish
<br />requirements. When natural flow patterns
<br />are altered, we must look beyond imme-
<br />diate fish needs to determine how stream-
<br />flows affect channels, transport sediments,
<br />and influence vegetation.
<br />
<br />STREAM PROCESSES
<br />
<br />Watersheds reflect the long-term influ-
<br />ence of geology, climate, and topography
<br />as well as shorter.term influences of veg-
<br />etation (Chorley et at. 1984), Flows result-
<br />ing from climatic conditions create and
<br />maintain stream.forming processes. When
<br />natural flow patterns are changed, fluvial
<br />processes change, and condition of the val-
<br />ley, the stream, and all other ecological
<br />components must change as a consequence
<br />(Lotspeich 1980), ,
<br />To understand stream processes, one
<br />must first consider a watershed in four di.
<br />mensions (Ward and Stanford 1989). These
<br />are the longitudinal dimension from head-
<br />waters to mouth, the lateral dimension ex-
<br />tending beyond the channel boundaries,
<br />and a vertical dimension resulting from
<br />out-of-channel flows moving downward
<br />into the soil and groundwater. Each of these
<br />dimensions must then be analyzed in a
<br />temporal dimension.
<br />To determine which flow patterns are
<br />needed to maintain a stream system, one
<br />
<br />,y, in-
<br />
<br />nel flows nec-
<br />)n and flood-
<br />Bazzaz 1977;
<br />, 1989; Strom-
<br />ever, no mod-
<br />,uggested that
<br />)(-stream flow
<br />~sources. Con-
<br />'gement typi-
<br />-itical resource
<br />Lltaneous pro-
<br />s,
<br />: a conceptual
<br />raluating both
<br />flow require-
<br />streamflow
<br />We combine
<br />
<br />I M. T. Hill et al.
<br />
<br />must match the respective valley bottom
<br />type, riparian type, and floodplain and
<br />channel type to the hydrologic processes
<br />that control form and function, Typically,
<br />steep high elevation streams flowing
<br />through V-shaped valleys lack floodplains
<br />or even riparian habitat. Other valley types
<br />create streams with riparian habitat but lack
<br />floodplains, The fluvial-geomorphic pro-
<br />cesses vary by valley type (Leopold et at.
<br />1964),
<br />An assemblage of geomorphic processes
<br />develop characteristic landforms as they
<br />construct the valley and its stream system
<br />(Strahler 1957), Flowing water erodes,
<br />transports, and deposits sediment and con-
<br />trols vegetation species and growth in gen-
<br />erally predictable ways (Morisawa 1968),
<br />Thus, valley type can be determined
<br />through land classification, historical anal-
<br />ysis, and hydrological approaches (Lot-
<br />speich and Platts 1982),
<br />The temporal distribution of flow, inter-
<br />acting with geology, topography, and veg-
<br />
<br />199 I I~
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