WSP01154

• File Number: 8062
• Description: Federal Reserved Water Rights
• State: CO
• Basin: Statewide
• Date: 3/1/1995
• Author: USDOC
• Title: Instream Flow Incremental Methodology - A Primer for IFIM
• Water Supply Pro - Doc Type: Report/Study

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THE lNSTREAM FLOW I;\ICREl-II"'7A1. MF:TIIOU)OLOGY 11

Table 2.2. Instream flow analysis based on the Tennant method (Tennant 1976),

Percent of mean annual flow

Health of habitat

October-March

Flushing or maximum
Optimum
Outstanding
Excellent
Good
Fair
Poor
Severe degradation

200
60-100
40
30
20
10
10
<10

ApriL-September

200
60-100
60
50
40
30
10
<10

planning recommendations, though they may be
criticized for technical reasons (Kulik 1990), A
more difficult question arises when a problem is
cast as long.range planning but is clearly destined
to become an intense negotiation within a very
short time. This change sometimes develops be-
cause decision makers do not understand in-
stream flow analysis and believe that a simple
one-time answer will accommodate a complex pro-
ject, At other times, policy requires a level of
analytic effort commensurate with some larger
public purpose, While the call goes out for a speedy
recommendation, the expectation is for a sophis.
ticated answer.

Mid-Range Techniques: A
Little More Than Basic
Standard-Setting But Not
Quite Incrementalism

Modified Tennant Approach

At the lower end of stream flow quantification
problems for fIsheries, where the controversy is
not intense but time is nevertheless a constraint,
a specially tailored Tennant approach might be
applied, This approach calls for the repetition of
all of Tennant's steps, The analyst would begin by
observing habitats known to be important in the
species' life history and by studying the stream
during flows approximating various percentages
of the mean annual flow, After collecting data on
cross-sectional width, depth, and velocity of the
stream at each flow, a set of recommendations
could be made to resemble the set shown in Ta.
ble 2,2, The difference would be that the new table
would reflect the empirical observations of the

analyst, instead of Tennant, and would be tailored
specifically to the species and stream of interest.

Wetted Perimeter Technique

The wetted perimeter technique (Nelson 1980)
is another method frequently used with some suc-
cess, in Montana and elsewhere, In this hydraulic
approach, a desired low-flow value is chosen from
a habitat index that incorporates stream channel
characteristics (Trihey and Stalnaker 1985), The
wetted perimeter technique selects the narrOwest
wetted bottom of the stream cross section that is
estimated to protect the minimum habitat needs.
The relation of wetted perimeter to cross section
is shown in Fig. 2,1,
The analyst selects an area assumed to be criti-
cal for the stream's functioning (typically a rime)
as an index of habitat for the rest of the stream.
When a riffle is used in the analysis, the assump.
tion is that minimum flow slltisfies the needs for
food production, fish passage, and spawning. The
usual procedure is to choose the break or 'point of
diminishing returns' in the stream's wetted pe-
rimeter versus discharge relation as a surrogate for
minimally acceptable habitat. This inflection point
represents that flow above which the rate of wetted
perimeter gain begins to slow. Once this level of
flow is estimated, other habitat areas, such as pools
and runs, are also assumed to be satisfactorily
protected, Because the shape of the channel can
influence the results of the analysis, this technique
is usually applied to streams with cross sections
that are wide, shallow, and relatively rectangular.
Other fisheries-related standard.setting meth.
ods in this middle ground include the Arkansas
Method (Filipek et al. 1987), Hoppe Method
(Hoppe 1975), and Texas Method (Mathews and