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<br />split-flows" on streams with coldwater fisheries around 1981. While the concept of seasonal split- a <br />flows is intuitively reasonable, the biological basis for CDOW's seasonal split flow has never been _ <br />fully documented. CDOW's biologic justification for recommending seasonal flows should be <br />researched and documented. <br /> <br />. <br /> <br />R2CROSS may also be useful in warmwater and cool water environments, if a relationship <br />can be established between the hydraulic paranleters estimated in the R2CROSS staging table and <br />the biologic requirements of the species that inhabit these types of environments. This relationship <br />may be explored through a review of the existing biologic literature (e.g.., USFWS habitat suitability <br />curves) or it may require additional research on warm and coolwater species. The CWCB and <br />CDOW staffs should investigate the possibilities of expanding the use ofR2CROSS into cool and <br />warmwater environments. <br /> <br />Finally, the CWCB and CDOW staffs should attempt to address public concerns that the <br />Board's decreed instream flow water rights, which were quantified utilizing the R2CROSS <br />methodology, may not be providing reasonable protection of the natural envirol1lIlent. This concern <br />is likely to be very difficult to either prove or disprove due to the subjectivity of the question and the <br />plethora of flow and non-flow related factors that may inhibit strict interpretation of the data. One <br />possibility for making this determination would be to design a study that would evaluate fish <br />population characteristics above and below major diversion structures that frequently reduce <br />strearnflows to the amounts of the Board's instream flow water rights. Differences in fish population <br />characteristics between natural flow, "control" sections (above diversions) and depleted flow, "test" <br />sections (below diversions) could be evaluated to determine the effectiveness of the Board's instream e <br />flow rights in preserving the natural environment to a reasonable degree. Other research possibilities <br />also exist, but staff time and budgetary constraints may prohibit an extensive study. <br /> <br />IFIMIl'HABSIM <br />IFIM/PHABSIM is a more general model than R2CROSS. Whereas R2CROSS is most <br />applicable for modeling rime and run habitats, IFIM/PHABSIM can be used to model almost any <br />stream habitat type. In addition, IFIM/PHABSIM habitat suitability indices exist for a variety life <br />stages of common aquatic species, including cold, cool, and warmwater fish species, reptiles, and <br />amphibians, as well as for recreational uses like wading and rafting. <br /> <br />However, instream flow recommendations based on IFIM/PHABSIM studies are generally <br />more expensive and time consuming than those based on R2CROSS studies. This extra expense can <br />be attributed to the additional data requirements of the IFIM model and the technical complexities <br />of running the model. The methodology has also been criticized because a relationship between <br />WUA and a population response has never been established and because it cannot model competition <br />between sympatric aquatic species. <br /> <br />In Colorado, IFIM/PHABSIM may a.n appropriate, and necessary, methodology for <br />developing recommendations on highly controversial stream segments and on streams with fish <br /> <br />13 <br /> <br />e <br />