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USGS Water-Year Summary 2024
11275500 Hetch Hetchy Reservoir at Hetch Hetchy, CA
LOCATION - Lat 37°56'52", long 119°47'13" referenced to North American Datum of 1927, in NW 1/4 NW 1/4 sec.16, T.1 N., R.20 E., Tuolumne County, CA, Hydrologic Unit 18040009, Yosemite National Park, near center of O'Shaughnessy Dam on Tuolumne River at Hetch Hetchy, 1.5 mi downstream from Falls Creek, and 5.9 mi (air line) northeast of the town of Mather.
DRAINAGE AREA - 455 mi².
REVISIONS HISTORY - WSP 1930: Drainage area.
SURFACE-WATER RECORDS
PERIOD OF RECORD - May 1923 to current year. Prior to October 1930 monthend contents published in WSP 1315-A.
GAGE - Water-stage recorder installed March 1995. Datum of gage is 1.84 ft above NGVD of 1929. Prior to Oct. 1, 1927, nonrecording gage at same site and datum. Oct. 1, 1927, to July 9, 1972, water-stage recorder at same site and datum. Prior to October 1974, datum published as at mean sea level.
REMARKS - Reservoir is formed by concrete gravity-type dam, completed to crest gage height 3,726.5 ft in 1923 and raised to 3,812.0 ft in 1937. Storage began Apr. 6, 1923. Ten-foot drum gates were installed on spillway in 1949. Capacity, 360,400 acre-ft, between gage heights 3,512.0 ft, bottom outlet, and 3,806.0 ft, top of drum-type spillway gates. Water is diverted from reservoir through tunnel to Robert C. Kirkwood Powerplant 15 mi downstream. Flow is diverted from powerplant tailrace in a closed conduit through Hetch Hetchy Aqueduct to Moccasin Powerplant with flows in excess of aqueduct capacity being spilled to the river. At Moccasin Creek Diversion Dam, water re-enters Hetch Hetchy Aqueduct and flows into Crystal Springs Reservoir. Surplus water is spilled into Don Pedro Reservoir (station 11287500) at Red Mountain Bar. Flow downriver is for State Department of Fish and Game and Raker Act requirements. Records, excluding extremes, represent contents at 2400 hours. See schematic diagram of Tuolumne River Basin available from the California Water Science Center.
EXTREMES FOR PERIOD OF RECORD - (at 0800 hours) Maximum contents, 369,100 acre-ft, Dec. 3, 1950, gage height, 3,810.4 ft; no contents at times in 1929-31.
