BOARD00970

• Board Meeting Date: 7/22/1993
• Description: Agenda or Table of Contents, Minutes, Memos. Partial.
• Board Meetings - Doc Type: Meeting

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QUANTIFICATION SUPPLEMENT - UTILE DRY CREEK
July 1, 1993
PAGE 4

respectively. A value of 70%2 is selected as adequate for a maintenance flow. 70% of
3340,3313, and 1478 is 2338, 2319, and 1034. The flow that maintains 2338, 2319, and 1034
square feet of adult habitat is 2.8 cfs, 2.9 cfs and 0.65 cfs respectively for the three species
of concern. The average of these three flow values is 2.12 cfs.

A similar approach to Figure 6 yields inflection points at 3.7 cfs, 3.7 cfs, and 1 cfs for
the three species' adult life stages. The corresponding habitat values for these points are
3036, 3012, and 1556 square feet, respectively. 70% of these habitat values are 2125 square
feet, 2108 square feet, and 1089 square feet. The flows which maintain that level of habitat
are 3.3 cfs for creek chubs, 3.2 cfs for fathead minnows, and 0.6 cfs for white suckers. The
average of these three flows is 2.37 cfs.

A close examination of all the curves in Figure 5 and Figure 6 one might observe that
the shapes of the individual curves and the magnitude of the values for habitat are very
similar. Based on that observation, it would be appropriate to "combine" the two IFIM sites
by averaging the values from the above analysis. The average of 2.12 cfs and 2.37 cfs is 2.24
. cfs.

In the April, 1993 report an assumption that protecting the adult life stages of the
fish species would, in effect, preserve the populations. This assumption can be examined
more closely by looking at the curves for the other life stages that were modelled. At the
Greenwood Village site a flow recommendation of 2.2 cfs preserves 93% of optimum creek
chub juvenile habitat, and 80%, 85%, and 72% of optimum white sucker spawning, fry, and
juvenile habitat. Likewise at the Cherry Hills Village site a flow of 2.2 cfs maintains 75%,
63%, 97%, and 99% of optimum creek chub juvenile, and white sucker spawning, fry, and
juvenile habitat, respectively. The term optimum in this context means the flow at which
the flow vs. WUA curve starts to flatten out.

To summarize, the above described approach is what the Division of Wildlife would
call a conservative approach to flow recommendations for fish species like creek chubs,
fathead minnows, and white suckers. Since these species are quite common and widely

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2The 70% value is supported by published literature and by
unpublished data sets where IFIM and R2CROSS (or R2CROSS-like)
methodologies were compared. The level of protection that comes
from an R2CROSS analysis is approximately 70% of the "optimum"
level of habitat from IFIM output. The point on the curve where
the 70% is applied is a matter of interpretation and professional
judgement. In this case (as is discussed above), a conservative
approach was utilized.