WSP04021

• File Number: 8062
• Description: Federal Reserved Water Rights
• State: CO
• Basin: Statewide
• Date: 9/1/1979
• Author: R Barry Nehring
• Title: Evaluation of Instream Flow Methods and Determination of Water Quantity Needs for Streams in the State of Colorado - September 1979
• Water Supply Pro - Doc Type: Report/Study

19

OQQ138

Montana (Tennant) Method

In most instances the Montana Method gave a recommendat~9~
(Table 7) approximating the recommendations of the oth~r_thr~e_
methods. The one notable disparity was on the Lake Fork of ' the
Gunnison River where no plausible explanation existed for the
difference between the Montana Method flow recommendation and the
other three recommendations.

Flushing flows are defined by Stalnaker and Arnette (1976,
p. 12) as "That discharge (natural or man-caused) of sufficient
magnitude and duration to remove fines from the stream bottom_
gravel to maintain intragravel permeability." In trout streams
this is necessary to maintain the viabiiity of spawning beds. ~
survey of the ~ging histories indicates that all of the ~treams_
included in this study have discharges that meet or exceed flushing
,flows during peak run-off in an average water year.

_Minimum flow recommendations of 30% of the average flow are
recommended by Tennant (1975) as flows that will maintain adeg~a~e
habitat for most forms of aquatic life over a long (months) period
of time. To establish how close the 30% value used in the Montana
Method is to the other methods' recommendations, all minimum flows
have been converted to percentage of average discharge (Table 8).

The most remarkable part of the data is how similar the single
and multiple R-2 Cross and IFG4 recommendations for an individual
stream are as compared to the Montana Method. Wesche (1974) found
that available cover is reduced at its greatest rate in the range of
25% to 27% of the average discharge. Using this as one parameter,
he recommended that for summer rearing flows the average discharge
not be allowed to fall below 25%. The average percentage of average
discharge for the 18 study reaches were 28.4, 26.4, and 27.9 for
the Single Transect R-2 Cross, Multiple Transect R-2 Cross, and
IFG4 Methods, respectively.

Based on the findings in this evaluation and the almost identical
findings of Wesche (1974), when synthetic methods of analysis are
used, 25% of the average flow should be the minimum acceptable level
for summer rearing flows for trout. This level is regarded as a
common denominator between methodologies. It was used to determine
a synthetic minimum flow recommendation for all Class I (U.S. Fish
and Wildlife Service classification) streams with adequate U.S.G.S.
gaging histories in the State of Colorado. These recommendations
are presented in Appendix B.