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<br />10 RIOLOt;\CAL REPORT 29
<br />
<br />Hydrologic Records
<br />
<br />Of the many techniques available for standard.
<br />setting related to fisheries, the easiest to use
<br />requires data on the hydrologic records of a
<br />stream. The use of stream gage records assumes
<br />that measured flows support aquatic resources at
<br />acceptable levels (Wesche and Rechard 1980),
<br />This assumption only applies where streams are
<br />essentially undeveloped or where the pattern of
<br />development has been stable for a long period,
<br />Eastern states increasmgly face plannmg prob.
<br />lems associated with undeveloped streams,
<br />whereas most western states have streams al-
<br />ready encumbered with sophisticated develop-
<br />ment projects. In sltuatlOns where stream flow IS
<br />depleted or regulated, the natural flow regime can
<br />be reconstructed from gage records to account for
<br />water diversions and stream modifications (an art
<br />discussed by Bayha (1978); see also other stand-
<br />ard techniques in Riggs (968)). This approach is
<br />satisfactory only if the analyst has information on,
<br />or is willing to make assumptions about, the con.
<br />dition of the fishery before development.
<br />Even when pre.development data are avail-
<br />able, it is dIfficult to predict future impacts on the
<br />aquatic resources, On same developed streams,
<br />channel structure and fish populations have ad-
<br />justed to the new flow regime. Existing water
<br />developments may have dampened chronic low- or
<br />high-flow events, thus enhancing the f.shery. De-
<br />veloping a knowledge of post.project conditions
<br />"ill require field .nvestlgatlOns, In any case, se-
<br />lecting flows from historical records in the pres-
<br />ence of existing development is a limited long-
<br />range planning technique.
<br />Where it is possible to use historical records,
<br />several questions arise, for example: Is it best' to
<br />recommend a flow based on natural or altered
<br />conditions? What percentage of the historical
<br />stream flow should be recommended? One solu-
<br />tion is to use the 'aquatic base flow' (Larson 1981;
<br />Kulik 1990). This technique selects the median
<br />flow for the lowest flow month (typically August
<br />or September) as adequate throughout the year,
<br />unless additional flow releases are required to
<br />meet the needs for spawning and incubation, An-
<br />other planning scheme involves the use of median
<br />monthly flows (Bovee 1982), This monthly flow
<br />level is a surrogate for the natural annual pattern
<br />of stream nows because it provides a flow that
<br />typifies historical flows for each month,
<br />A hydrologic technique that is inappropriate for
<br />establishing instream flows for fish is the 7-day-
<br />
<br />lO-year low flow (expressed as 7Q 10), This statis-
<br />tic was developed to ensure that water treatment
<br />plants did not violate water quality standards
<br />during droughts (Velz 1984), It establishes a very
<br />low flow that must not be diminished in quality if
<br />treated water is discharged into it. Thus, it re-
<br />quires a high level of sewage treatment but does
<br />not address the flow requirements of fish,
<br />
<br />The Tennant Method
<br />
<br />The most renowned of the long-range planning
<br />tools for fisheries is that of Tennant (1976). In its
<br />original form, the Tennant Method arrays flow
<br />levels for seasonal periods based on percentages
<br />of the mean annual flow, Tennant used 10 years
<br />of personal observations in Montana and the mid-
<br />west to categorize streams into varying quality
<br />trout habitat based on r~corded flow, He also
<br />recommended that periodic high flows be pro\ided
<br />to remOve silt, sediment, and other bed material.
<br />The U,S,D.A, Forest Service has argued that an
<br />annual high flow event is needed to protect the
<br />channel structure in alluvial streams (U,S,D,A,
<br />Forest Service 1984), Because Tennant had in
<br />mind more of a scouring purpose, his approach
<br />was not based on these morphological considera-
<br />tions.
<br />Table 2.2 shows Tennant's recommendations
<br />for stream flow to support varying qualities offish
<br />habitat based on his observations of how to best
<br />mimic nature's hydrology, Some states recognize
<br />that they cannot apply Tennant's recommenda- ~
<br />tions to their own streams without adjustments,
<br />In these cases, changes are made for the species .'
<br />ofinterest and the types of streams in a particular '\
<br />state. '
<br />Tennant's method and other desk-top tools an.
<br />ticipate that hydrologic records are available;
<br />when they are not, instream flows can still be
<br />recommended based on a surrogate indicator,
<br />Drainage area is an example of such an indicator
<br />for managed streams. In one drainage area tech-
<br />nique. a minimum instream flow value, or base
<br />flow, of 0,5 cubic feet l"r second per square mile
<br />(cfsm; 0,0055 cmslkm ) of drainage area is recom.
<br />mended for the summer months, Higher flows in
<br />fall and spring are used to accommodate the
<br />spawning and incubation of anadromous species
<br />(Larson 1981l. Use of this technique for non-
<br />anadromous species would, of course, require a
<br />different set of rules.
<br />These simple. rule-of-thumb techniques
<br />are very useful in the development of long-range
<br />
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