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1 1.1 GENERAL CHAPTER 1 INTRODUCTION AND PURPOSE The primary purpose of this report is to summarize the preliminary evaluation of alternatives for supplying an annual average of 20,000 acre-feet of water to the mainstem Colorado River. This water will be made available to supplement spring peak flows in the reach of the Colorado River 1 15 miles upstream from its confluence with the Gunnison River to the GVIC Diversion Dam near Palisade, Colorado (the "15-Mile Reach"). A secondary purpose is to summarize the preliminary evaluation of alternatives for providing flow enhancements to the 15-Mile Reach during the late summer/early fall (July 15 through October 31) period. ' Providing this water is an identified element of the Final Programmatic Biological Opinion (U.S. Fish and Wildlife Service, December 20, 1999). The PBO specifies actions designed to offset the impacts of all water depletions that have historically occurred, and a specified level of depletions (up to 120,000 acre-feet) that will occur in the future, upstream from the lower terminus of the 15-Mile Reach. Persons or entities with projects upstream from the 15-Mile Reach requiring a federal permit or federal action that would invoke the jurisdiction of the U. S. Fish and Wildlife Service (Service) under §7 of the Endangered Species Act (a "federal nexus"), may rely on actions identified in the PBO as a Reasonable and Prudent Alternative (RPA) to avoid jeopardy to, or adverse modification of critical habitat for, the four Colorado River fish species listed under the Act. The PBO identifies the 15-Mile Reach as important for the survival and recovery of the listed 1 species. The PBO also identifies providing adequate flows in the 15-Mile Reach, in combination with other recovery actions, as important to achieving recovery of the listed species. In particular, with regard to this report, the Service has identified enhancement of spring peak flows ' in the 15-Mile Reach as assisting in habitat formation and maintenance. The Service identified target peak flows in the 15-Mile Reach of 12,900 to 26,600 cfs as important to mobilize 1 gravel/cobble substrate. Management activities to enhance spring peak flows are expected to result in an increased frequency of years in which flows exceed 12,900 cfs. The Service has also identified enhancement of flows in the late irrigation season (July through October) as important because flows in the 15-Mile Reach are significantly reduced by agricultural operations in the Grand Valley. ' One of the management elements identified in the PBO is the subject of this report. The PBO states: ' "The intent is to provide additional water up to approximately 20,000 acre-feet per year of water for spring peak flow enhancement, without diminishing project yield or causing project sponsors to incur significant costs. When additional p:\data\gen\Ocwcb\ 18133\report\phase-1 \chap-Ldoc 1-1