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<br />and mass wasting of upstream sides lopes
<br />provide material for floodplain deposits,
<br />Low-gradient reaches of many streams, and
<br />especially large rivers, have geomorphic
<br />settings that often produce relatively large
<br />floodplains and valuable wetlands.
<br />Floodplain habitats provide cover, nest-
<br />ing, spawning, and rearing for fish and
<br />wildlife. Floodplains also play an impor-
<br />tant part in the transfer of sediments and
<br />nutrients that maintain stream productiv-
<br />ity (Sed ell et al. 1989), If the stream and its
<br />associated floodplain are separated from
<br />water by improper flow management, both
<br />will change over time because the original
<br />dynamic balance between flows and flood-
<br />plains has been altered,
<br />For floodplain ecosystems, timing and
<br />duration of flooding is particularly impor-
<br />tant. Seasonal flooding affects seed dis-
<br />persal. seedling survival, and growth of
<br />many plant species that occupy channel
<br />banks and floodplains (e,g" cottonwoods
<br />and willows) (Platts 1979). Flooding dur-
<br />ing the growing season apparently has a
<br />greater effect on floodplain productivity
<br />than does an equal amount of flooding dur-
<br />ing the nongrowing season (Junk et al.
<br />1989),
<br />Floodplains receive a wide range of nu-
<br />trients, organic matter, and fine soil par-
<br />ticles during overbank flows, Floodplain
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<br />nutrients can, however, establish their own
<br />cycles because organisms and environ-
<br />mental conditions differ considerably from
<br />those of the main stream (Vannote et al.
<br />1980; Minshall et at. 1983), Floodplains also
<br />import, store, produce, and recycle mate-
<br />rials used in downstream food chains, thus
<br />providing energy flow to detrital food webs
<br />(Vannote et at 1980),
<br />Riparian vegetation is a major factor af-
<br />fecting floodplains, fisheries habitat, and
<br />channel characteristics (Platts 1979), The
<br />fundamental importance of vegetation to
<br />long-term channel stability and form is
<br />usually the weakest part of most flow anal-
<br />yses, Corridors of riparian vegetation along
<br />streams influence light, temperature, and
<br />organic input; provide cover; and control
<br />bank morphology (Larsen et al. 1986), Nat-
<br />ural flooding that maintains the riparian
<br />system in a productive growth stage, if re-
<br />duced, can enable non riparian species to
<br />invade riparian zones and floodplains. Al-
<br />though extreme events may play an im-
<br />portant role in shaping channels, Wolman
<br />and Miller (1960) indicate that the less ex-
<br />treme and more frequent flooding events
<br />(considered as bankfull) are probably most
<br />influential. In high desert streams, Platts
<br />et at. (1985) found that large storm events
<br />dominated the channel-forming process.
<br />
<br />A CONCEPTUAL APPROACH FOR FLOW DETERMINATIONS
<br />
<br />Maintenance of stream ecosystems rests
<br />on streamflow management practices that
<br />protect physical processes which, in turn,
<br />influence biological systems. Consequent-
<br />ly, multiple flow regimes are needed in
<br />most streams to protect multiple resources.
<br />We use U,S, Geological Survey (USGS)
<br />stream gage data from the Salmon River at
<br />White bird, Idaho (Table 1), to illustrate our
<br />flow regime concepts, Mean monthly flows
<br />are derived from 76 years of daily records
<br />(Figure 1), Instream flows for fish, such as
<br />the Tennant (1975) method used here, and
<br />out-of-channel flow requirements are all
<br />illustrated at this site,
<br />The Whitebird reach of the Salmon Riv-
<br />er can be classified using Cupp's (1989)
<br />method for valley segments as an allu-
<br />viated mountain valley that is deeply en-
<br />trenched in mountainous side-walls with
<br />a relatively wide floodplain and alluvial I
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<br />colluvial deposition, There are local inclu-
<br />sions of steep competent hillslopes with
<br />steep colluvial complexes,
<br />Four potential flow requirements are il-
<br />lustrated in Figure 1; procedural method-
<br />ologies for evaluating these flows are sum-
<br />marized in Table 2, Flow magnitude
<br />increases from .flow regime 1 (fish main-
<br />tenance), flow regime 2 (channel mainte-
<br />nance), flow regime 3 (riparian mainte-
<br />nance), to flow regime 4 (valley process
<br />maintenance).
<br />
<br />Fishery Flows
<br />
<br />The PHABSIM, part of IFlM (Bovee 1982),
<br />is the most commonly used model for
<br />quantifying instream flow habitat needs of
<br />selected fish species (Orth 1987), The mod-
<br />el allows resource managers to predict what
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