USGS Water-Year Summary for Site 15090000

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	00060 Discharge(Mean)

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Water year 2023: 2022-10-01 to 2023-09-30

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USGS Water-Year Summary 2023

15090000 GREEN LAKE NEAR SITKA, AK

LOCATION - Lat 56°59'14", long 135°06'37" referenced to North American Datum of 1927, in SW 1/4 SW 1/4 NE 1/4 sec.29, T.56 S., R.65 E., City & Borough of Sitka, AK, Hydrologic Unit 19010212, (Port Alexander D-4 quad), on Baranof Island, in Tongass National Forest, 0.4 mi upstream from mouth at Silver Bay, and 9.4 mi southeast of Sitka.

DRAINAGE AREA - 28.1 mi².

REVISIONS HISTORY - WSP 1372: 1916, 1917, 1922 (monthly discharge). WDR AK-84-1: Drainage area. WDR AK-86-1: 1984, 1985 (month-end reservoir contents, change in month-end and yearly contents, adjusted mean monthly discharges, and extremes). WRD AK-00-01: 1998-1999 (M m). GAGE. (MANU)--Staff gage on upstream face of dam. Datum of gage is at mean low water, which is about 5 ft below sea level. Totalizing MWH meters are on the two turbines in Green Lake powerhouse. September 1915 to September 1925, recording gage at site of present day dam, elevation of gage was 220 ft above sea level, by barometer; prior to December 27, 1916 at datum 1 ft higher. Water years 1983-88, nonrecording remote lake-level indicator at Blue Lake powerhouse (6 mi northwest of gage).

SURFACE-WATER RECORDS

PERIOD OF RECORD - September 1915 to September 1925 (published as "Green Lake Outlet"); monthly discharges only published in WSP 1372. October 1983 to current year (month end reservoir contents and monthly discharges).

COOPERATION - Daily reservoir elevations and MWH power generation provided by City and Borough of Sitka.

REMARKS - Reservoir is formed by concrete arch dam located at the outlet of Green Lake, construction began in 1978 and was completed in 1982. Total and usable capacity below spillway crest elevation of 395 ft is 88,000 and 75,000 acre-ft, respectively. Reservoir is used for power. Discharge released through the turbines is computed from relation between discharge, head, and power generation; release flow empties directly into Silver Bay and is not returned to stream. Spill is computed from a theoretical relation between discharge and stage above the crest of the 100 ft wide spillway. Turbine and spillway ratings and reservoir capacity table furnished by City and Borough of Sitka in 1983. Corrected reservoir capacity table furnished in April 1987.

EXTREMES FOR PERIOD OF RECORD -

Maximum Contents Observed: Elevation: 401.3 ft., Storage: 94,620 Ac-ft.; November 01, 2020. Maximum daily discharge,

5,910 ft³/s, November 01, 2020.

Minimum Contents Observed: Elevation 296.0 ft., Storage: 17,500 Ac-ft.; April 15, 2021. No flow released, February 5-8, 1987, November 27-29, 1988, June 19, 2004, January 18 and 25, 2009, July 10, 2011, April 10, 16, May 13, 15, and 18, October 17-21, November 18, December 18, 2016, January 4, and June 20-21, 2017, June 18, September 29 -30, 2018; May 12, 13, 15 – 18, 22, 23, 25 – 27 and September 16 – 17, 2020; April 21^st – July 8^th, July 11^th – 17^th, 2021.

AVERAGE DISCHARGE FOR PERIOD OF RECORD -

47 years (water years, 1916-25, 1985-2021), 331 ft³/s, 156.1 in/yr., 239,810 acre-ft/yr. Mean discharge for water years 1985-2021 adjusted for change in contents of Green Lake.