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001.113 . . In this scenario, Elkhead Reservoir operating assumptions were modified from the baseline scenario to reflect a seasonally variable target recreation pool. In addition, the contemplated draft of the Juniper Project was modeled as an instream flow right at Juniper Canyon with a priority date of 1954. Steamboat Lake and Elkhead and Stagecoach reservoirs were modeled with storage decrees just senior to the instream flow right to reflect the probable future transfer of a portion of the Juniper Project right to those reservoirs. All other model assumptions were identical to those made for the baseline scenario. Elkhead Reservoir Elkhead Reservoir exhibits more seasonal cycling in storage contents under the with- project scenario. This is due in large part to the fact that basin water demands are increased to projected 2040 conditions. Releases are regularly made to supplement downstream junior demands. More releases are also made out of the City of Craig and Tri-State contract pools than in the baseline scenario. Reservoir releases are periodicany constrained by target recreation pools specified as a maximum drawdown of 10 feet during the summer months and 20 feet during the fall, winter and spring. Modeled reservoir contents of enlarged Elkhead Reservoir are shown in Figure 7 for the 53 year study period. Elkhead Creek Table 6 shows monthly flows below Elkhead Reservoir when the reservoir enlargement is represented in the model. The effect of the reservoir is to shift the outflow hydrograph towards the late summer and early fall months when releases are made to junior downstream water rights. Flows in Elkhe.ad Creek increase substantially over baseline conditions downstream of the reservoir. Flows are also slightly higher during the winter months compared to baseline conditions. Peak flows during April through June are lower than in the baseline scenario as the enlarged reservoir fills. The annual yield of Elkhead Creek below the reservoir is approximately 62,000 af, .or roughly 1,000 af less than in the baseline scenario. This is due to evaporative losses of roughly 2,400 af per year. Figure 8 shows a comparison of modeled flows in Elkhead Creek downstream of the reservoir under the baseline and with project scenarios. Yampa River - Maybell Table 7 shows modeled monthly flows of the Yampa River at Mayben when Elkhead Reservoir is enlarged to 44,900 af and under projected 2040 demand conditions. Figures 9A through 9L compare monthly flow exceedance curves at Maybell for baseline and post-project conditions. Table 8 presents the modeled 25th, 50th and 75th percentile flows at Maybell for the baseline condition. The greatest impacts to modeled baseline flows at Maybell occur during the months September through February. During these months, decreases in median flows range from 15 % to more than 34 % During September and October, median flows decrease by roughly 34% and 25%, respectively. Although modeled average flows at Maybell decrease during several months of the year under the with-project scenario, average flows in Elkhead Creek below the enlarged reservoir actually increase during these same months. These increases are due to additional water releases f~om the reservoir to meet downstream demands. Any decrease in modeled Maybell flows during these months is therefore attributed to the additional water usage defined in the model to represent projected 2040 demand conditions in the basin. 25 ill J~ ',_, ;;