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' CHAPTER 2 RESERVOIR OPERATING STUDIES This chapter describes the reservoir operating studies conducted as part of the present study. These operating studies were accomplished using a computer model of the Yampa River basin and were done in several phases as the study progressed. The discussion below first presents an overview of these phases and then describes the basin computer model used to perform the operations studies. A brief description of two phases of the earlier studies is then presented. The chapter concludes with a detailed discussion of the final operating studies which are also presented in the Preliminary Biological and Environmental Assessments. OVERVIEW OF OPERATIONS STUDIES 1 IJ The conclusions of the predecessor study (the "Yampa River Basin Alternatives Feasibility Study") included a recommendation to move forward with the conversion of a portion of the Juniper Project water rights to an instream flow water right for the benefit of the endangered fishes and the present study began with this objective in mind. The initial administrative and operational concept underlying this conversion was to make releases from the enlarged Elkhead Reservoir to alleviate water supply shortages caused by the "call" of the relatively senior Juniper-based instream flow right located at Juniper Canyon. However, consideration of the existing subordination of the Juniper rights led to modification of this concept to one of subordinating the intream flow right to upstream junior depletions and then making augmentation releases from the enlarged Elkhead Reservoir to achieve the instream flow levels that would have been realized in Juniper Canyon with an unsubordinated intream flow water right. From the standpoint of modeled physical hydrology these two operations are indistinguishable. However, it became evident as the study progressed that the terms of existing transfers and subordination of the Juniper rights, along with concern related to protection of Colorado's compact entitlements, may make it difficult to obtain and administer a subordinated Juniper-based intream flow water right. Accordingly, the Colorado River Water Conservation District ("River District") and the Colorado Water Conservation Board ("CWCB") began negotiations for a Memorandum of Understanding that contemplated the development of an overall Yampa River Basin Water Management Plan. This water management plan would include contractual agreements for intream flow enhancement from releases of stored water and new intream flow water rights to protect natural flow and storage releases through the river reaches of interest. Under the plan, comparable intream flow benefits could be realized without the difficult conversion and administration of a Juniper-based intream flow right. The operating studies conducted during the present study reflect this evolution in thinking about how to best obtain and protect intream flows. Initial studies focused on quantification of the contemplated draft of the Juniper rights and on the possible reservoir storage and operations necessary to either mitigate shortages caused by the Juniper-based intream flow "call" or to supplement flows obtained through a subordinated intream right. For comparison, another 2-1