BOARD01443

• Board Meeting Date: 5/21/2001
• Description: ISF Section - CWCB Instream Flow Quantification Program
• Board Meetings - Doc Type: Memo

suggests that the need to provide protection greater than, or less than, the amounts afforded' by thl! 2
single/mnltiple transect R2CROSS methodology may be necessaty in Certain circumstances. For instance,
on Deadhorse Creek and Hanging Lake, the Board found it necessary to provide a greater .level of
protection than the ainount afforded by an R2CROSS analysis based on the unique biologic, hydrologic,
and geologic natural environmnent of this watershed. : The Board has also found it necessary to apprOPriate.
flows less than the amounts resulting. from an R2CROSS analysis based on water availability limitations.
However, staff suggests that the need for a level of 'protection different than the amount afforded by an
R2CROSS analysis, and the identification of an appropriate alternative biologic quantification method,
should rest with the party making the recommendatioh. In the absence of adequate documentation and an
appropriate, alternative quantification method, staff recommends that the Board rely on the single/multiple
transect R2CROSS averaging technique, .

SUl!l!estlo!.ls for Develooinl! lSF Recommendatlon~ on Cool and Warm Water Stream Reaches
Colorado's ISF Program is also expanding dOWll$tream into cool and wannwater aquatic habitats.
Biologic instream flow recommendations can be difficult to quantifY in low elevation stream reaches
because the complex aquatic species assemblages that typically inhabit these reaches often have very
divergent streamflow requirements. In addition,' social and political issues may complicate the
appropriation of ISF water rights on cool and wann water stream reaches with significant water
development pressure. '

Staff suggests that the R2CROSS multiple transect; averaging methodology may also prove useful in
cool and wann water habitats. However, the adaptation of the R2CROSS method to these habitats may
require a modification of the hydraulic crite,na, which drive the biologic instream flow
recommendations, to reflect the streamflow requireJ;llents of the aquatic species that inhabit these cool
and wann water environments.

More sophisticated hydraulic models, like IFIMJPJiABSIM, may also be required on cool and wann .
water stream reaches with areas of high water develqpment pressure or when flow recommendations are
based on indicator species that are not dependent o~ riffle habitat types; i.e, species that dwell in pools,
backwaters, etc. :

At this time, staff recommends that the Board continue to evaluate biologic ISF recommendations on
cool and wann water stream reaches on a case-by-ca$e basis.

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