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<br />Oil28~O
<br />
<br />represent discharges within the QLS to Q,o
<br />range (where QLS and Q,o indicate flows
<br />that are equalled or exceeded, on average,
<br />once every 1.5 and 10 years, respectively),
<br />HEC-2 modeling predicts the discharge
<br />needed to flood riparian habitat and the
<br />floodplain. High flow frequency of occur-
<br />rence (or return period) identifies when,
<br />in time, those discharges occur,
<br />Valley floor gradient and width, eleva-
<br />tion, fluvial processes, and soil parent ma-
<br />terial govern riparian type and the extent
<br />of the riparian zone (Platts et aL 1985).
<br />Kondolf et aL (1987) reported that in a large
<br />V-shaped glacial valley. the width of the
<br />riparian strip is highly variable, whereas
<br />on alluvial fan deposits the riparian strip
<br />is relatively uniform, It follows that not all
<br />valley types support riparian vegetation,
<br />Steep-sided, V-shaped valleys lack flood-
<br />plains or even terraces that can support
<br />riparian habitat. Consequently, out-of-
<br />channel flows are not needed for riparian
<br />or valley maintenance in these types of
<br />valleys,
<br />
<br />Valley Maintenance Flows
<br />
<br />Climatic variabiiity. expressed through
<br />the magnitude and frequency of high-flow
<br />events and modified by vegetation, deter-
<br />mines valley form and condition in both
<br />natural and artificial systems (Platts et aL
<br />1985), Large hydrologic events affect val-
<br />ley sides, whereas smaller flow events af-
<br />fect channels and floodplains (Platts et aL
<br />1985); both, over the long term, affect val-
<br />ley form.
<br />The steeper the valley slope, the greater
<br />
<br />Knowledge of fluvial-geomorphic pro-
<br />cesses that create and maintain streams and
<br />how aquatic and terrestrial ecosystems
<br />function synergistically is fundamental to
<br />identifying flows that maintain fish habi-
<br />tats and, ultimately, fish biomass and di-
<br />versity, Protecting these parameters with
<br />multiple flow recommendations is neces-
<br />sary because of these ecological linkages,
<br />Single purpose in stream flows alone,
<br />whether derived from the PHABSIM or
<br />other methodologies, provide only short-
<br />term protection for fish populations, Di-
<br />version or storage of bankfull and flood
<br />
<br />I M. T, Hill et aL
<br />
<br />the stream power of valley water flow
<br />(Lanka et aL 1987), Also, the narrower the
<br />valley width, the less horizontal distance
<br />the flows cover, Changes in valley form
<br />and slope generally represent long-term
<br />adjustments because immen~e quantities of
<br />materials are redistributed and vegetation
<br />patterns changed (Lotspeich and Platts_,
<br />1982). In many valleys, form and condition
<br />are still under the influence of Pleistocene
<br />events (Platts 1979). Other processes must
<br />continue for decades and centuries to over-
<br />ride these prehistoric settings,
<br />We identify valley-forming flows as
<br />those peak discharges that usually exceed
<br />Q" (Figure 4). This flow regime is more
<br />difficult to establish because most valleys
<br />have been formed by forces other than flu-
<br />vial processes, such as glaciers, faults, and
<br />lava flows (Lotspeich and Platts 1982), Es-
<br />tablishing this flow must be done with cau-
<br />tion, Valleys created over long periods
<br />(thousands of years) by historic flood flows
<br />may now be occupied by roads, homes.
<br />businesses, and other property, Further-
<br />more, because of the infrequent occurrence
<br />of >Q" flows, relatively short-term gaging
<br />station records, and measurement errors
<br />associated with large flows, our ability to
<br />accurately determine QZ5 is limited in many
<br />areas.
<br />Not all valley types are dependent upon
<br />the energy from flood flows to maintain
<br />their geomorphology ,In wide alluvial fan-
<br />type valleys, the energy associated with
<br />flood flows has no affect on valley geo-
<br />morphology but it is the channel flow that
<br />induces meandering, scouring, and filling.
<br />
<br />SUMMARY
<br />
<br />flows in different valley bottom types will
<br />result in habitat alterations and a reduction
<br />in fish populations and diversity,
<br />The concepts we present in this paper
<br />examine broad interactions of fluvial-geo-
<br />morphic processes, riverine-riparian hab-
<br />itat, and their geographic setting. We rec-
<br />ognize the limitations inherent in using
<br />our conceptual method, For example, most
<br />tributaries lack historical flow records that
<br />define average annual discharge and there
<br />are frequently no data available on the du-
<br />ration and frequency of flow events. Chan-
<br />nel maintenance flows vary by channel
<br />
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