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INTRODUCTION <br />This report describes and presents the results of an analysis for the Colorado River <br />Water Conservation District (CRWCD) of certain issues associated with the operation of the <br />Colorado -Big Thompson Project. Principal among these issues were (1) the extent to which the <br />C -BT's east slope water rights and water supply have been utilized in the project operation, (2) <br />the extent to which "non- charge" water has been delivered to and diverted by east slope water <br />users, and (3) the effects on the flow in the Colorado River in the "15 -Mile Reach" of under- <br />' utilization of the C -BT's east slope water supply and the delivery of non - charge water. Less - <br />than -full utilization of the east slope water supply increases the Adams Tunnel diversions and <br />decreases the frequency and volumes of spill from Granby Reservoir on the west slope and, <br />consequently, reduces the flow in the Colorado River downstream from Granby Reservoir. <br />Decreased flow in the Colorado River in the 15 -Mile Reach impairs the accomplishment of the <br />' goals of the Recovery Program associated with several endangered fish species. The delivery <br />of non - charge water has become a significant part of the C -BT operation in recent years. It also <br />' increases the Adams Tunnel diversions and reduces the frequency and volumes of spill from <br />Granby Reservoir and the flow in the Colorado River below Granby Reservoir. <br />Our analysis was completed in four main steps. The first was study of the layout and <br />historical operation of the project. The second step consisted of evaluations of the water that <br />was available historically for diversion from the Big Thompson River under the C -BT water rights <br />and the historical deliveries and diversions of non - charge water. The third step consisted of an <br />evaluation of the operation of C -BT east slope facilities and the relationships among Adams <br />Tunnel diversions, in- priority diversions of Big Thompson River water, deliveries of non - charge <br />water, and storage in Carter Lake and Horsetooth Reservoirs. The purpose of the third step was <br />to show how the Adams Tunnel, Carter Lake, and Horsetooth Reservoir could be operated with <br />increased utilization of the C -BT east slope water source without reducing historical yields to C- <br />BT water users and how the deliveries of non - charge water affected Adams Tunnel diversions. <br />The fourth step was an evaluation of the effects of the C -BT Project's diversion of in- priority Big <br />Thompson River water and deliveries of non - charge water on the flow in the Colorado River in <br />the 15 -Mile Reach and on the Recovery Program for the endangered fish species in that reach. <br />Specifically, this latter evaluation focused on target flow shortages and Coordinated Facilities <br />Operations (CFOPS) spring peak flows. <br />Although background data covering the entire C -BT Project operation are tabulated and <br />discussed, the main focus of our analysis was on the water years 1983 -98. Hydrologic <br />conditions were generally favorable in this period, and reliable records are available for it. The <br />data compiled and used in this analysis were obtained from a variety of sources including the <br />U.S. Bureau of Reclamation's (USBR) daily operating records and annual operation plans, U.S. <br />Geological Survey (USGS) publications, and the Colorado Decision Support System (CDSS) <br />developed and maintained by the State of Colorado. The State Engineer's administrative water <br />year is used in this analysis; it begins in November of the preceding calendar year and extends <br />4 <br />