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'001309 . . Draft Task 9 Technical Memorandum Refinement of Stagecoach Reservoir Inflows Model inflows to Stagecoach Reservoir were developed in a manner similar to the methods used to develop Elkhead Reservoir inflows, This method is summarized as follows: 1. Assemble data for USGS gage 9237500. The period of record for this gage is 1940 to 1944 and from 1957 to 1988. The gage was extended by regression against the Steamboat Springs gage on the Yampa River to cover the entire 53 year study period between 1930 and 1982. In 1989 Stagecoach Reservoir was completed and a new gage (9237450) was installed immediately upstream of the reservoir. The regression equation used to extend the record of gage 9237500 was: Annual Discharge Annual Discharge Gage 9237500 extension: Gage 9237500 = 0.2521 * (Gage 9239500) - 17444 R2 = 0.889 Average Annual Error = 1.11 % The estimated average annual flow at the site of the Stagecoach Reservoir dam is 62,077 af. 2. The estimated average annual yield was disaggregated into monthly flows using the average monthly distribution over the period of record for gage 9237500. 3. Because gage 9237500 is located very close to the location of the Stagecoach dam it was not necessary to transpose the gage data upstream or downstream. In addition to numerous irrigation ditches there are several small reservoirs located above Stagecoach Reservoir which affect the natural hydrology of the Yampa River. Included are Stillwater Reservoir and Yamcolo Reservoir. In developing the inflows used in the basin model an assumption was made that these ditches and reservoirs would continue to operate as they have historically and will continue to result in a similar pattern and amount of net depletion to the basin. There has been recent speculation that depletions above Stagecoach Reservoir are increasing and that the historical inflows may over-represent future inflows. While recent gage data (9237450) show lower inflows, the period of record is very short and covers only relatively dry years. This issue has been discussed with Division 6 staff, however a defensible alternative for estimating Stagecoach Reservoir inflows has not been developed. Continued monitoring of Stagecoach inflows in the future may reveal more about this issue. The Upper Yampa Water Conservancy District has investigated the possibility of supplementing inflows to Stagecoach Reservoir by means ofa diversion from Morrison Creek into the Little Morrison Creek drainage and then into the reservoir. To date this plan has not been developed. Morrison Creek is a tributary flowing into the Yampa River at a point just below Stagecoach Dam. The average annual yield of Morrison Creek is approximately 59,000 5 Iii: ~;,~