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INTRODUCTION Resource Engineering, Inc. (RESOURCE) has completed an analysis of the historic operation of the Orchard Mesa "Check" structure located on the Colorado River above it's confluence with the Gunnison River. The purposes of the investigation were to determine if recent changes in the State Engineer's administration of the Check and appurtenant water rights have resulted in increased water demand at the site and if so, what effect has the change had on upstream water rights. ; + . . In summary, the study indicates that the State Engineer's current administration of the Check does result in a new, increased demand on the Colorado River near Grand Junction. This demand, in turn causes an accelerated release of water from Green Mountain Reservoir which, in dry years, could deplete storage. reserves necessary to protect west slope beneficiaries under Senate Document 80. This operation could jeopardize numerous west slope water users who otherwise would have been protected throughout the summer by releases of water from Green Mountain Reservoir. The results of the investigation are discussed in more detail below. BACKGROUND INFORMA TION ~ 1; J; ;.. , There exist several large agricultural and industrial water rights on the Colorado River above it's confluence with the Gunnison River near Grand Junction, Colorado. Collectively these water rights total approximately 2,260 cfs and make up what is commonly referred to as the Cameo demand. The majority of the water rights are owned by the United States of America (USA), Grand Valley Water Users' Association (Association), Grand Valley Irrigation Company (GVIC) and Orchard Mesa Irrigation District (OMID). They are diverted at two headgates; the Grand Valley Diversion Dam (Roller Dam) and the Grand Valley Irrigation Canal. A schematic diagram of the subject water rights is shown in Figure 1. Colorado River streamflows at Cameo are often insufficient to satisfy the full agricultural and power demands during the late irrigation season. During dry years, these rights can place an administrative call on the river in early July through mid-October. The call is initiated by GVIC's 119.47 cfs irrigation right. This is the most junior right associated with the Cameo demand (see Figure I). Historically, GVIC's rights were satisfied in whole or part through implementation of a water conveyance structure commonly referred to as the Orchard Mesa "Check.'