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2.8. Stillwater Reservoir No. 1 <br />Stillwater Reservoir No. 1 (WDID 583540) is the most upstream of the major reservoirs <br />in the Yampa River (Bear River) drainage. It is owned by the Bear River Reservoir Co. <br />and is used to provide supplemental irrigation water supplies to a number of individuals <br />served by several of the major direct flow structures in the upper Bear River. The <br />reservoir has a decreed capacity of 6,392 acre-feet and carries an appropriation date of <br />January 9, 1935 (State Administration Number = 33782.31054), making it one of the <br />most senior storage decrees in the Yampa River basin. Although it is reported that the <br />outlet for the reservoir is at the bottom of the reservoir, the reservoir company and the <br />water commissioner consider the active storage to be approximately 5,175 acre-feet. This <br />is further evidenced by the reservoir shareholder list which allocates 5,175 acre-feet <br />among 22 shareholders. One share is considered to be equal to one acre-foot of storage. <br />An elevation-capacity table was obtained from the Division 6 engineer who also provided <br />a regression equation relating the reservoir water surface area as a function of the <br />reservoir capacity: <br />Surface Area = 7.0*(Capacity)^0.35 <br />According to the historical observed storage contents of the reservoir and interviews with <br />the water commissioner, it appears that the reservoir can fill to its capacity in average and <br />above average runoff years, but does not fill in below average years, largely because of <br />limited physical supply in the tributary basin above the reservoir and in part, because of <br />high loss rates through the reservoir dam embankment. Because it is the highest reservoir <br />in the system, the water commissioner informally attempts to store as much of the <br />available runoff as possible in Stillwater Reservoir No. 1, on the assumption that if the <br />lower reservoirs do not fill in priority, releases can be made from the Stillwater Reservoir <br />to satisfy unfulfilled storage downstream. During the summer irrigation season, the Water <br />Commissioner normally sets the release from Stillwater Reservoir No. 1 at a relatively <br />constant rate of flow and then regulates the deliveries to the various shareholders using <br />the storage in the downstream Yamcolo Reservoir. <br />Because of its remote location, the outlet is normally closed in the fall and the reservoir is <br />allowed to store all inflow during the winter (although there may be very little gain in <br />storage because of seepage losses being approximately equal to the inflow during the <br />winter months). After the reservoir has achieved it maximum possible fill in the spring, <br />the available storage is allocated among the shareholders pro rata to their ownership <br />interest. According to the water commissioner, the reservoir company has implemented <br />an informal rule that each shareholder can carry over no more than 40 percent of his <br />reservoir water remaining in storage on September 30 of each year, the remainder being <br />available for reallocation to all shareholders the following year. <br />Using reservoir and ditch ownership data provided by the water commissioner, the <br />individually owned storage accounts in Stillwater Reservoir No. 1 were grouped <br />according to the ditch structures which serve the irrigated land owned by those <br />individuals. The active storage in the reservoir is approximately allocated to the <br />following ditch structures: <br />Yampa River Basin Information 2-9 <br />