Analysis of Colorado-Big Thompson Project Operation With Respect to East Slope Diversions and Non-Charge Deliveries

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J Reach were evaluated using another EXCEL spreadsheet. This spreadsheet simulates the operation of Granby Reservoir on a monthly basis with different volumes of Adams Tunnel diversion. Changes in the historical volumes and patterns of transbasin diversion translate into changed storage volumes in Granby Reservoir, which accumulate until the reservoir spills. In general, decreased Adams Tunnel diversions result in increased spills from Granby Reservoir in terms of both frequency and volume. When spills are increased, the flow in the Colorado River downstream from Granby Reservoir is increased which may, in turn, decrease the target flow shortages in the 15 -Mile Reach. The inputs in the spreadsheet were flows storable in Granby Reservoir from the Colorado River and Willow Creek and the Adams Tunnel diversions calculated in the three runs described in the previous section of this report. The storage capacity in Granby Reservoir was limited to 539,800 acre -feet of total capacity. Spills from Granby Reservoir were allowed to occur in the month when the reservoir filled; i.e., no effort was made in the spreadsheet to release water in anticipation of a physical spill. Run A, which as previously explained is closest to the historical situation, was used as the baseline case. In evaluating the 15 -Mile Reach impacts, it was necessary to look at daily values for the flow entering the reach. The flow at this location has been gaged directly since October 1990. Daily values were derived for the prior times from the Cameo and Plateau Creek gaging records and Government Highline and Grand Valley Canal diversion records with allowances for the returns from the Orchard Mesa Power Canal. The target flows were calculated using the criteria specified by the U.S. Fish and Wildlife Service in its May 1995 report entitled Relationships Between Flow and Rare Fish Habitat in the `15 -Mile Reach' of the Upper Colorado River. These target flows were determined based on whether the individual years were high flow years (top 25 percent), above - average flow years (next 25 percent), below- average flow years (next 30 percent), or low flow years (bottom 20 percent). During May through July, the target flows are expressed as 10 -day volumes. When calculated this way, the target flows ranged from about 1.5 million acre -feet in dry years to about 3 million acre -feet in wet years. The historical target flow shortages were calculated as the differences between the target flows and the historical flows. In our simulations, the flow in the 15 -Mile Reach was calculated as the historical flow plus any increases in spill from Granby Reservoir (or minus any decreases in spill from Granby Reservoir), and the corresponding target flow shortages were calculated as the differences between the target flows and the simulated flows. In the simulations, the flows in the 15 -Mile Reach increased when spills increased, and the target flow shortages decreased if the historical flows were less than the target flows. If the historical flows were larger than the target flows, then shortages in the target flows did not occur historically and, therefore, were not decreased. Table 13 shows the results of this part of our analysis. Under Run A, the Adams Tunnel ' transbasin diversions averaged 224,541 acre -feet annually, spills from Granby Reservoir averaged 20,356 acre -feet annually, and the target flow shortages averaged 431,279 acre -feet ' annually. In Run B, when the C -BT Project increases diversions of in- priority east slope water and delivers both charge and non - charge water, the Adams Tunnel diversions are reduced by 1 16