7843

• UCREFRP Catalog Number: 7843
• Author: Stalnaker, C. B., et al.
• Title: The Instream Flow Incremental Methodology, A Primer for IFIM.
• USFW Year: 1995.
• USFW - Doc Type: Biological Report 29,
• Copyright Material: NO

10 BIOLOGICAL REpORT 29

Hydrologic Records

Of the many techniques available for standard-
setting related to fisheries, the easiest to use
requires data on the hydrologic records of a
stream. The use of stream gage records assumes
that measured flows support aquatic resources at
acceptable levels (Wesche and Rechard 1980).
This assumption only applies where streams are
essentially undeveloped or where the pattern of
development has been stable for a long period.
Eastern states increasingly face planning prob-
lems associated with undeveloped streams,
whereas most western states have streams al-
ready encumbered with sophisticated develop-
ment projects. In situations where stream flow is
depleted or regulated, the natural flow regime can
be reconstructed from gage records to account for
water diversions and stream modifications (an art
discussed by Bayha (1978); see also other stand-
ard techniques in Riggs (1968)). This approach is
satisfactory only if the analyst has information on,
or is willing to make assumptions about, the con-
dition of the fishery before development.
Even when pre-development data are avail-
able, it is difficult to predict future impacts on the
aquatic resources. On some developed streams,
channel structure and fish populations have ad-
justed to the new flow regime. Existing water
developments may have dampened chronic low- or
high-flow events, thus enhancing the fishery. De-
veloping a knowledge of post-project conditions
will require field investigations. In any case, se-
lecting flows from historical records in the pres-
ence of existing development is a limited long-
range planning technique.
Where it is possible to use historical records,
several questions arise, for example: Is it best to
recommend a flow based on natural or altered
conditions? What percentage of the historical
stream flow should be recommended? One solu-
tion is to use the 'aquatic base flow' (Larson 1981;
Kulik 1990). This technique selects the median
flow for the lowest flow month (typically August
or September) as adequate throughout the year,
unless additional flow releases are required to
meet the needs for spawning and incubation. An-
other planning scheme involves the use of median
monthly flows (Bovee 1982). This monthly flow
level is a surrogate for the natural annual pattern
of stream flows because it provides a flow that
typifies historical flows for each month.
A hydrologic technique that is inappropriate for
establishing instream flows for fish is the 7 -day-

10-year low flow (expressed as 7Q10). This statis-
tic was developed to ensure that water treatment
plants did not violate water quality standards
during droughts (Velz 1984). It establishes a very
low flow that must not be diminished in quality if
treated water is discharged into it. Thus, it re-
quires a high level of sewage treatment but does
not address the flow requirements of fish.

The Tennant Method

The most renowned of the long-range planning
tools for fisheries is that of Tennant (1976). In its
original form, the Tennant Method arrays flow
levels for seasonal periods based on percentages
of the mean annual flow. Tennant used 10 years
of personal observations in Montana and the mid-
west to categorize streams into varying quality
trout habitat based on recorded flow. He also
recommended that periodic high flows be provided
to remove silt, sediment, and other bed material.
The U.S.D.A. Forest Service has argued that an
annual high flow event is needed to protect the
channel structure in alluvial streams (U.S.D.A.
Forest Service 1984). Because Tennant had in
mind more of a scouring purpose, his approach
was not based on these morphological considera-
tions.
Table 2.2 shows Tennant's recommendations
for stream flow to support varying qualities offish
habitat based on his observations of how to best
mimic nature's hydrology. Some states recognize
that they cannot apply Tennant's recommenda-
tions to their own streams without adjustments.
In these cases, changes are made for the species
ofinterest and the types of streams in a particular
state.
Tennant's method and other desk-top tools an-
ticipate that hydrologic records are available;
when they are not, instream flows can still be
recommended based on a surrogate indicator.
Drainage area is an example of such an indicator
for managed streams. In one drainage area tech-
nique, a minimum instream flow value, or base
flow, of 0.5 cubic feet cler second per square mile
(cfsm; 0.0055 cms/km ) of drainage area is recom-
mended for the summer months. Higher flows in
fall and spring are used to accommodate the
spawning and incubation of anadromous species
(Larson 1981). Use of this technique for non-
anadromous species would, of course, require a
different set of rules.
These simple, rule-of-thumb techniques
are very useful in the development of long-range