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Post-McPhee Dam Hydrology The average annual yield of the Dolores River above McPhee dam is 349,900 acre-feet (see Table 5). Of this flow, approximately 136,100 acre-feet has historically been diverted by the Montezuma Valley Irrigation Company (MVIC), and 6,900 by other senior water right holders. An average of 206,900 acre-feet of water remains in the Dolores River to be captured by McPhee Reservoir. When in full operation, McPhee Reservoir would deplete the Dolores River by an additional 106,400 acre-feet (99,600 acre-feet project water + 5,400 acre-feet for evaporation + 1400 acre-feet bank: storage). Flows in the Dolores River below McPhee Dam would average 105,400 acre-feet a year. These flows would consist of 3,900 acre-feet released for down- stream water rights, 25,400 acre-feet of project water released specifically to improve the Dolores River fishery, and 76,100 acre-feet of surplus snowmelt runoff that could be spilled or released in anticipation of spills. The 1,600 acre-feet of water reserved for future fish and wildlife uses would be stored in McPhee Reservoir until the location and nature of use were detennined. Ofthe 76,1 ()() acre-feet of surplus snowmelt runoff, about 66,000 acre-feet would be released on a scheduled basis in anticipation of spills from mid- April to the end of June to provide whitewater boating opportunities in the river below McPhee Dam (USBR, 1977). Table 6 shows the effects of the project on streamflows during typical wet, nonnal, and dry years. The minimum flows in the river below McPhee Dam would be maintained at 78 cubic feet per second (ft3/s) during wet years (13 years of the Definite Plan ReIJort (DPR) 46-year study period or 28 percent), 50 ft3js during nonnal years (23 out of 46 years or 50 percent), and 20 ft3js during dry years (10 out of 46 years or 22 percent). In the DPR simulated operation, a wet year was considered to begin on May 1 and extend for 1 year if the end-of- April elevation of McPhee Reservoir exceeded 6,914 feet (82 percent of the active capacity). A dry year would begin on March 1 and extend for a year if the March 1 prediction of the elevation of McPhee Reservoir at the end of June was less than or equal to 6,892 feet (45 percent of the active capacity). A nonnal year was assumed to occur when the end-of- April elevation was less than 6,914 feet (82 percent of the active capacity) and the March 1 prediction of 18 the end-of-June elevation was greater than 6,892 feet (45 percent of the active capacity). The minimum flows are composed of releases for downstream users, spills, and the project releases to improve the stream fishery (USBR, 1977). In summary, historical diversions have reduced the average annual flow of the Dolores River by approximately 39 percent. When fully developed, McPhee Reservoir will further deplete Dolores River flows by an additional 30 percent. Thus, 69 percent of the historic flow of the Dolores River will be depleted by MVIC diversion and McPhee Reservoir. The Bureau. of Reclamation Operations Studies for McPhee Dam were limited to monthly flow values, Four offices of the Bureau of Reclamation were contacted in an attempt to locate a daily operations model for McPhee Reservoir. Finding none, the " PuIs" method was used to route an inflow hydrograph through McPhee Reservoir. This method assumes a constant discharge-storage relationship for the reservoir being assessed. For a given time interval, the change in storage is computed and outflow is detennined by the discharge-storage relationship. The purpose of this simplified routing method was to determine the 7 -day high flows that potentially could be released to the Dolores River below McPhee Dam. The 7-day high flows were determined to represent the channel forming/bank:-full flows as described in the Pre-McPhee Dam Hydrology section. Historic daily flows from the Dolores near McPhee streamflow station for the years 1939 to 1952 were utilized as inflows. Outflows were determined utilizing the "PuIs" method described above with the following assumptions: 1. The previous End Of Month (EOM) reservoir storage elevation from the USBR Reservoir Operation Study (Definite Plan Report, April 1977) was used for the beginning reservoir elevation for each month simulated. 2. The area capacity curve in the DPR was used for operations purposes. The reservoir was considered full at a stage of 6,928 feet and a capacity of 381,000 acre-feet. The spillway elevation was set at 6928 feet. The outlet works were essentially closed requiring all flows to go over the spillway.