USGS Water-Year Summary for Site 12176500

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	00065 Gage height(Obs.)

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Water year 2025: 2025-10-01 to 2026-09-30

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USGS Water-Year Summary 2025

12176500 DIABLO RESERVOIR NEAR NEWHALEM, WA

LOCATION - Lat 48°42'56", long 121°07'52" referenced to North American Datum of 1927, in SE 1/4 sec.05, T.27 N., R.13 E., Whatcom County, WA, Hydrologic Unit 17110005, (unsurveyed), Ross Lake National Recreation Area, at Diablo Dam on Skagit River, 1.2 mi downstream from Thunder Creek, 6.0 mi northeast of Newhalem, and at mile 101.0.

DRAINAGE AREA - 1,125 mi². includes 400 mi² in Canada.

REVISIONS HISTORY - Gage data was previously published using the 'elevation, ft above NGVD29' parameter. This was in error as the gage references the as-built datum of the dam which predates NGVD29. Beginning 12/9/2020, published data is using the 'gage height' parameter and a conversion to NAVD88 is now referenced.

SURFACE-WATER RECORDS

PERIOD OF RECORD - Oct. 1929 to Sept. 1999 (published as "Diablo Reservoir"), Oct. 1999 to Sept. 2008 (published as "Diablo Reservoir near Nehalem, WA"), October 2008 to current year. Oct. 1929 to Sept. 1938, monthly change in reservoir contents published with records for Skagit River at Newhalem.

GAGE - Water stage recorder. Datum of gage is City of Seattle as-built datum for Diablo Dam and is 6.37 ft above NAVD of 1988 (USGS GNSS survey, June 2021). Prior to Oct. 1, 1964, at datum 0.28 ft. higher.

REMARKS - Reservoir is formed by concrete-arch dam, completed in 1930; storage began in October 1929. Usable storage, 8,820 acre-ft between elevations 1,195 ft, normal lower limit of operation, and 1,205 ft, top of taintor gates. Dead storage, below elevation 1,040 ft, 12,900 acre-ft. Crest of spillway is at elevation 1,187 ft. Water used by City of Seattle for power development at Diablo and Gorge powerplants. Capacity table furnished by City of Seattle. Figures given herein represent total contents. U.S. Geological Survey satellite telemeter at station.

EXTREMES FOR PERIOD OF RECORD - Maximum contents, 90,600 acre-ft, July 14, 1933, elevation, 1,206.5 ft; minimum contents not determined.