HydrionisGlobal Water Intelligence
Monitored reservoir

Hetch Hetchy Res A Hetch Hetchy CA

United States Geological Survey reservoir gauge 11275500 in CA, with published storage from 1930.

Storage record11275500

Storage now, against this reservoir's own record

367.46 Mm³
Stored, September 2026
64th
Percentile among 73 Seps on record
-1.7%
Against the Sep 1991–2020 average
+1.5%
Change over the 12 months to 2026-09

Storage in September 2026 was around the middle of the Sep record, ranked against 73 other Seps held for this reservoir.

Storage, 1930–2026

Monthly mean storage
873 monthly values, 1930 to 2026
Monthly mean storageMonthly mean storage: Hetch Hetchy Res A Hetch Hetchy CA in Mm³, from 1930.75 to 2026.6666666666667.0200400Mm³1930.751940.16666666666671949.33333333333331958.51967.66666666666671999.833333333333320092018.1666666666667Year

Monthly means of the publisher's daily record. Volume, not percentage of capacity — see below for why.

What the shaded periods are (0 events)

Not shown on this chart

  • The 1976 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
  • The 2022 drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
  • Yorkshire drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
  • Cantareira water crisis, São Paulo — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
  • Cape Town Day Zero — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
  • European drought — This event’s area does not contain this chart’s subject. Sharing a country is not sharing an event, so it is not drawn.
View the data behind this chart
240 of 873 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 873 would be longer than this page and unusable with a screen reader.
PeriodHetch Hetchy Res A Hetch Hetchy CA (Mm³)
October 1930129
May 1931117
August 193139.2
December 19313.65
April 193234.0
July 1932254
November 1932112
March 19330.472
June 1933227
October 1933141
January 193423.6
May 1934145
September 193497.6
December 19347.91
April 193546.2
August 1935229
November 1935112
March 193651.7
July 1936251
October 1936154
February 193725.2
June 1937209
September 1937194
January 1938150
May 1938311
August 1938415
December 1938259
April 1939237
July 1939340
November 193991.9
February 194048.3
June 1940427
October 1940265
January 1941176
May 1941335
September 1941366
December 1941276
April 1942268
August 1942407
November 1942267
March 1943218
July 1943424
October 1943306
February 1944122
June 1944390
September 1944320
January 1945152
April 1945121
August 1945401
December 1945282
March 1946253
July 1946416
November 1946183
February 194797.3
June 1947403
October 1947177
January 194875.5
May 1948183
September 1948348
December 1948189
April 194977.8
August 1949367
November 1949210
March 195078.0
July 1950428
October 1950282
February 1951397
May 1951422
September 1951360
January 1952196
April 195293.7
August 1952436
December 1952245
March 1953163
July 1953445
November 1953260
February 1954127
June 1954418
October 1954245
January 1955123
May 1955132
September 1955275
December 1955170
April 1956249
July 1956438
November 1956310
March 1957174
June 1957427
October 1957297
February 1958149
May 1958215
September 1958405
January 1959206
April 1959186
August 1959318
December 1959141
March 196062.9
July 1960348
November 1960158
February 196143.4
June 1961243
October 1961103
January 196256.8
May 1962281
August 1962419
December 1962215
April 1963181
July 1963441
November 1963295
March 1964149
June 1964380
October 1964255
February 1965259
May 1965179
September 1965404
January 1966254
April 1966262
August 1966337
December 1966191
March 1967122
July 1967439
October 1967349
February 1968145
June 1968350
September 1968236
January 1969139
May 1969270
August 1969433
December 1969268
April 1970286
July 1970434
November 1970232
March 1971146
June 1971387
October 1971275
February 197298.4
May 1972192
September 1972278
December 1995363
April 1996305
August 1996430
November 1996355
March 1997276
July 1997442
October 1997355
February 1998222
June 1998309
September 1998421
January 1999365
May 1999289
August 1999416
December 1999299
April 2000225
July 2000436
November 2000304
March 2001216
June 2001439
October 2001291
January 2002205
May 2002287
September 2002351
December 2002295
April 2003301
August 2003410
November 2003298
March 2004273
July 2004431
October 2004315
February 2005296
June 2005415
September 2005388
January 2006344
May 2006306
August 2006427
December 2006295
March 2007292
July 2007417
November 2007269
February 2008208
June 2008436
October 2008321
January 2009285
May 2009406
September 2009376
December 2009317
April 2010318
August 2010418
November 2010393
March 2011270
July 2011445
October 2011399
February 2012345
June 2012441
September 2012336
January 2013301
April 2013308
August 2013386
December 2013254
March 2014229
July 2014417
November 2014306
February 2015300
June 2015416
October 2015327
January 2016328
May 2016337
September 2016370
December 2016393
April 2017348
August 2017443
November 2017392
March 2018356
June 2018443
October 2018348
February 2019349
May 2019309
September 2019406
January 2020312
April 2020297
August 2020392
December 2020273
March 2021225
July 2021374
November 2021326
February 2022375
June 2022444
October 2022335
January 2023355
May 2023325
September 2023434
December 2023374
April 2024368
July 2024436
November 2024309
March 2025341
June 2025445
October 2025337
February 2026391
May 2026442
September 2026367

The reservoir's year

Seasonal storage profile
Each calendar month's reference average, against the most recent year
Seasonal storage profileSeasonal storage profile: 1991–2020 average, Most recent 12 months in Mm³, from 1 to 12.0200400Mm³123456789101112Month1991–2020 average 1: 309 Mm³1991–2020 average 2: 300 Mm³1991–2020 average 3: 280 Mm³1991–2020 average 4: 285 Mm³1991–2020 average 5: 348 Mm³1991–2020 average 6: 420 Mm³1991–2020 average 7: 433 Mm³1991–2020 average 8: 409 Mm³1991–2020 average 9: 374 Mm³1991–2020 average 10: 345 Mm³1991–2020 average 11: 326 Mm³1991–2020 average 12: 313 Mm³Most recent 12 months 1: 396 Mm³Most recent 12 months 2: 391 Mm³Most recent 12 months 3: 395 Mm³Most recent 12 months 4: 427 Mm³Most recent 12 months 5: 442 Mm³Most recent 12 months 6: 445 Mm³Most recent 12 months 7: 431 Mm³Most recent 12 months 8: 400 Mm³Most recent 12 months 9: 367 Mm³Most recent 12 months 10: 337 Mm³Most recent 12 months 11: 333 Mm³Most recent 12 months 12: 347 Mm³
  • 1991–2020 average
  • Most recent 12 months

Twelve separate averages, one per calendar month. A January average describes Januaries only. Months are plotted 1–12.

View the data behind this chart
Underlying data
Period1991–2020 average (Mm³)Most recent 12 months (Mm³)
Jan309396
Feb300391
Mar280395
Apr285427
May348442
Jun420445
Jul433431
Aug409400
Sep374367
Oct345337
Nov326333
Dec313347

Why this page shows a volume and not a percentage

Rate and persistence

+0.47 Mm³
Mean change per month over 12 months
0 of 24
Recent months below the 20th percentile for their own month

Rates are computed only across an unbroken run of months, so a rate is never divided by a period in which some of the change was not observed. Percentile counts compare each month with the same calendar month in every other year held.

Location

Hetch Hetchy Res A Hetch Hetchy CA
Gauge location
Hetch Hetchy Res A Hetch Hetchy CAHetch Hetchy Res A Hetch Hetchy CA — 367.46 Mm³Hetch Hetchy Res A Hetch Hetchy CA — 367.46 Mm³

About this data

About this data

Everything below is a property of the data rather than a rating of it. There is deliberately no single confidence score: how much a gap matters depends on what you are about to do with the series, and only you know that.

Source
United States Geological Survey — National Water Information System · US Government work — public domain
How the values came to existMeasured
Period held1930–2026
Completeness75.0% of the periods in that span are held. Gaps are drawn as breaks and never interpolated.
Known gaps
  • Storage is a managed quantity. A falling line can be a rule curve, a scheduled release or a transfer to another system rather than a shortage of water, and nothing here distinguishes them.
  • No capacity figure on a stated basis is held for this reservoir, so no percentage of capacity is computed.
Continuity75% of the months between 1930 and 2026 are held. Monthly means of the publisher's own daily record.
Last read from the publisher2026-10-01
RedistributionPermitted by the upstream licences
Commercial reusePermitted by the upstream licences. No Hydrionis access tier can widen this.
MethodologyHow this was produced

Use this data

Use this data

No bulk download is offered here, which is a licence position rather than a product one: at least one contributing publisher does not permit redistribution. The values can be read on the page, cited, and pointed to at source.

Attribution and citation

Suggested citation for Hydrionis

Hydrionis (2026), Hetch Hetchy Res A Hetch Hetchy CA reservoir records, accessed 2026-10-05, hydrionis.com/reservoirs/reservoir-usgs-11275500/

Cite Hydrionis where you rely on the compilation, the normalisation, the eligibility judgements or a chart. Where you use an unmodified published value, the publisher’s attribution is the one that is required; crediting Hydrionis as well is welcome and is not something Hydrionis can require of you.

Recommended, not required. Hydrionis does not claim rights over unmodified public data it relayed, and doing arithmetic on somebody else’s measurements does not make them Hydrionis’s. Cite the version and the access date: the Hetch Hetchy Res A Hetch Hetchy CA record changes as publishers release and as methods are revised, and a citation without both cannot be checked later. Full guidance is on cite and reuse.

None of the contributing licences requires attribution. Crediting the publishers is still good practice and they are listed above.

Where this comes from

873 monthly observations 1930–2026 Mm³
Spatially aggregatedHigh confidenceSeptember 2026

Source: USGS — National Water Information System, 11275500. Licence: Open.

Method: Monthly mean of daily reservoir storage published by the USGS in acre-feet and converted to millions of cubic metres. Months with fewer than 20 days of record are not published rather than averaged from a partial month.

Reservoir storage is managed as well as observed: a drawdown may reflect an operating rule, flood-control rule curve or scheduled release rather than a shortage. Compare a month against the same month in other years, and do not read a fall as a supply signal without knowing the operating regime.

Retrieved 1 October 2026.

Monthly mean of daily reservoir storage published by the USGS in acre-feet and converted to millions of cubic metres. Months with fewer than 20 days of record are not published rather than averaged from a partial month.

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