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

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i ' records. The differences evidently are rounding and the small volumes of Wind River water in the CDSS records. ' The available USBR and state records indicate that C -BT Project's actual in- priority diversions of Big Thompson River water were less than the volumes indicated in this analysis. ' Consequently, this analysis probably overstates the actual in- priority diversions. Some of the discrepancies could be inaccuracies in the data and imprecisions in project operation. Although our analysis did not cover 1999, we note that the USBR reported that 55,000 acre -feet of in- priority Big Thompson River water was stored in Carter Lake and Horsetooth Reservoir in 1999. ' DIVERTIBLE BIG THOMPSON RIVER WATER The water that was available for diversion under the C -BT water rights during 1983 -98 ' was estimated through daily calculations of the native flows in the river and comparisons of these native flows to the actual recorded diversions into the 15 major ditches and 1 municipal ' pipeline on the river. The native flows that were in excess of these diversions were considered as being surplus to the other water users on the river and available for C -BT Project diversions, provided that river calls were not being made by water users on the South Platte. The ' calculations and comparisons were made in an EXCEL spreadsheet beginning with calculations for the gaging station at the canyon mouth and working downstream past all the points of diversion. The native water at the gaging station at the canyon mouth was calculated by (1) ' adding the upstream diversions into the Olympus and Dille Tunnels to the gaged flow at the canyon mouth, (2) subtracting the Adams Tunnel diversion, and (3) adjusting for changes in storage in Marys Lake and Lake Estes. Thus, it can be noted that the calculated native flows are essentially "virgin" or "undepleted" flows in that they are the flows that would have occurred but for the upstream operation of the C -BT Project. Although changes in storage in Marys Lake and Lake Estes were generally small in relation to the native flows, the adjustments for their changes in storage helped to make the native flow estimates more consistent. Once the native ' flow was calculated, the flow passing each downstream point of diversion was calculated as the flow passing the immediate upstream point of diversion plus the stream gain in the intervening reach minus the diversion at the current point of diversion. The divertible flow was then ' calculated as the smaller of either the native flow at the canyon mouth less the required 45 cfs bypass at the Dille diversion facility or the minimum flow passing any of the downstream points of diversion. Thus, the native water that was available for C -BT Project diversions was limited by both the native water at the Dille diversion facility exceeding the required minimum bypass and the smallest amount of native water passing any of downstream points of diversion. Had the C- ' BT Project diverted native water at greater rates, either sanding problems would have occurred at the Dille diversion facility or existing water users would not have been able to make their historical diversions. Negative values occurred in the calculations for the native flow passing downstream points of diversion on days when C -BT Project water was being diverted by the water users since the total historical diversions, including C -BT Project water as well as river ' water, were used in the comparisons. Obviously, divertible native flows did not exist on days when negative values occurred. 10