Environmental Engineering Reference
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on some fraction of an annual, seasonal, or monthly low or they can include a large number of low
characteristics.
4.6.2.1.1 Montana (Tennant) Method
Perhaps the most widely used methods for establishing EFRs and instream low requirements are
the regression methods (Pyrce 2004), such as the Montana method developed by Tennant (1975,
1976). This method has the advantage that it can be implemented solely based on historical low
data, assuming that the stream morphology from Tennant's studies is similar to those streams that
are targeted. The Montana method was developed based on the measurements of width, average
depth, and average velocity in streams in Montana, Wyoming, and Nebraska.
Tennant found that the quality of instream habitats changed more rapidly from no low to a low
of 10% of the average than in any higher range (Table 4.7). Tennant (1975, 1976) then concluded that
10% of the average annual low is the minimum instantaneous low needed to sustain short-term
survival. The conditions at this low low included:
•
The depths and velocities were signiicantly reduced.
•
The substrate was exposed.
•
The gravel bars were dewatered.
•
The streambank cover was diminished.
•
The ish were crowded into pools.
•
The rifles were too shallow for larger ish to pass.
Tennant determined that a low of 30% of the average annual low was required to maintain a
good habitat for aquatic life and that an optimal habitat was provided by lows of 60%-100% of
the average annual low and lushing lows were 200% of the average annual low. Therefore, the
method could be implemented based on an analysis of annual lows to determine the minimal, good,
optimum, or lushing lows.
4.6.2.1.2 Lyons Method
The Lyons method (Bounds and Lyons 1979) was derived from the Tennant method (1976), devel-
oped for the cold-water isheries of Montana and other western states, but developed and validated
TABLE 4.7
Instream Flow Recommendations by Tennant Method
Recommended Base Flow Regimes
Description of Flows
October-March (%)
April-September (%)
Flushing or maximum
200
Optimum range
60-100
Outstanding
40
60
Excellent
30
50
Good
20
40
Fair or degrading
10
30
Poor or minimum
10
10
Severe degradation
0-10
Note:
All lows are deined as a percentage fraction of the average low.
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