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2.3. Previous Studies <br />In 1990, the CRWCD initiated a study to consider the effect of the Juniper water right on instream <br />flows and other water uses. The Yampa Steering Committee was set up to guide the study. The <br />first Phase of this study was to estimate future demands for water in the basin, identify potential <br />storage opportunities, and determine the requirements for stored water in order to satisfy Juniper <br />' Project demands. This study treated the demands for the Juniper right as an instream flow. <br />Hydrosphere (1993) summarized the results of this study. <br />' In the second phase of the study, Hydrosphere (1995a) simulated the use of flow targets for the <br />Yampa River, rather than using the Juniper water rights. This study further looked at the <br />feasibility of enlarging Elkhead reservoir. The flow targets used for this study were preliminary <br />' targets proposed by the USFWS. <br />Since that time, there have been several refinements of flow targets as discussed in Chapter 3 of <br />this report. Hydrosphere completed additional studies, which are summarized in Appendix A of <br />this report. These studies have looked at flows recommended by the USFWS, instream flow <br />filings by the CWCB and various alternative target flows. <br />Hydrosphere (1995) also completed a Reconnaissance Evaluation of Yampa River Diversion <br />Structures. There are hundreds of diversion structures along the Yampa used mostly to divert <br />water for irrigation. At the time of the study, these structures were believed to inhibit migration of <br />the Colorado pikeminnow and other endangered fish. The objectives of the study were to: <br />1. develop an inventory of problematic mainstem Yampa diversion structures in river <br />reaches where endangered fish species are likely to occur; <br />2. make a reconnaissance -level evaluation of the potential effects of such structures on <br />flow conditions, fish passage, and aquatic and riparian habitats; <br />3. make preliminary recommendations concerning the structures thought to be most <br />problematic; <br />4. describe, in a preliminary way, potential alternatives to reduce any adverse effects of <br />these structures or their maintenance on endangered fishes. <br />The study found that the Maybell Canal and K. Diamond /Patrick Sweeney Ditch diversion <br />structures may represent significant obstacles to fish passage and recommended that a <br />biological study, involving radio telemetry, be considered for these two structures (Hydrosphere <br />1995b). Subsequent studies by Miller and Rees (1997) and Modde et al. (1999) found that the <br />existing structures do not appear to pose a challenge to the migration of Colorado pikeminnow. <br />Colorado pikeminnow typically migrate after spawning during periods of higher flows, when the <br />existing man -made and natural obstacles do not impede fish passage. During lower flows, when <br />these structures present potential barriers to fish passage, pikeminnow typically are less mobile. <br />However, if these structures were modified, fish passage could be impeded at higher flows. <br />Replacing a sloping rock structure with a vertical concrete structure, for example, could preclude <br />fish from migrating above the structure at any flow. <br />7 <br />